About different quest

What is the difference between an Interface and an Abstract class?

A:        An abstract class can have instance methods that implement a default behavior. An Interface can only declare constants and instance methods, but cannot implement default behavior and all methods are implicitly abstract. An interface has all public members and no implementation. An abstract class is a class which may have the usual flavors of class members (private, protected, etc.), but has some abstract methods.

Q:        What is the purpose of garbage collection in Java, and when is it used?

A:        The purpose of garbage collection is to identify and discard objects that are no longer needed by a program so that their resources can be reclaimed and reused. A Java object is subject to garbage collection when it becomes unreachable to the program in which it is used.

           

Q:        Describe synchronization in respect to multithreading.

A:        With respect to multithreading, synchronization is the capability to control the access of multiple threads to shared resources. Without synchonization, it is possible for one thread to modify a shared variable while another thread is in the process of using or updating same shared variable. This usually leads to significant errors.

           

Q:        Explain different way of using thread?

A:        The thread could be implemented by using runnable interface or by inheriting from the Thread class. The former is more advantageous, 'cause when you are going for multiple inheritance..the only interface can help.

           

Q: What are pass by reference and passby value?

A:        Pass By Reference means the passing the address itself rather than passing the value. Passby Value means passing a copy of the value to be passed.

           

Q:       

What is HashMap and Map?

A:        Map is Interface and Hashmap is class that implements that.

           

Q:       

Difference between HashMap and HashTable?

A:        The HashMap class is roughly equivalent to Hashtable, except that it is unsynchronized and permits nulls. (HashMap allows null values as key and value whereas Hashtable doesnt allow). HashMap does not guarantee that the order of the map will remain constant over time. HashMap is unsynchronized and Hashtable is synchronized.

           

Q:        Difference between Vector and ArrayList?

A:        Vector is synchronized whereas arraylist is not.

Q:        Difference between Swing and Awt?

A:        AWT are heavy-weight componenets. Swings are light-weight components. Hence swing works faster than AWT.

           

Q: What is the difference between a constructor and a method?

A:        A constructor is a member function of a class that is used to create objects of that class. It has the same name as the class itself, has no return type, and is invoked using the new operator.

A method is an ordinary member function of a class. It has its own name, a return type (which may be void), and is invoked using the dot operator.

           

Q: What is an Iterator?

A:        Some of the collection classes provide traversal of their contents via a java.util.Iterator interface. This interface allows you to walk through a collection of objects, operating on each object in turn. Remember when using Iterators that they contain a snapshot of the collection at the time the Iterator was obtained; generally it is not advisable to modify the collection itself while traversing an Iterator.

           

Q: State the significance of public, private, protected, default modifiers both singly and in combination and state the effect of package relationships on declared items qualified by these modifiers.

A: public : Public class is visible in other packages, field is visible everywhere (class must be public too)

private : Private variables or methods may be used only by an instance of the same class that declares the variable or method, A private feature may only be accessed by the class that owns the feature.

protected : Is available to all classes in the same package and also available to all subclasses of the class that owns the protected feature.This access is provided even to subclasses that reside in a different package from the class that owns the protected feature.

default :What you get by default ie, without any access modifier (ie, public private or protected).It means that it is visible to all within a particular package.

           

Q:What is an abstract class?

A: Abstract class must be extended/subclassed (to be useful). It serves as a template. A class that is abstract may not be instantiated (ie, you may not call its constructor), abstract class may contain static data. Any class with an abstract method is automatically abstract itself, and must be declared as such.

A class may be declared abstract even if it has no abstract methods. This prevents it from being instantiated.

Q:       What is static in java?

A:        Static means one per class, not one for each object no matter how many instance of a class might exist. This means that you can use them without creating an instance of a class.Static methods are implicitly final, because overriding is done based on the type of the object, and static methods are attached to a class, not an object. A static method in a superclass can be shadowed by another static method in a subclass, as long as the original method was not declared final. However, you can't override a static method with a nonstatic method. In other words, you can't change a static method into an instance method in a subclass.

Q:        What is final?

A:        A final class can't be extended ie., final class may not be subclassed. A final method can't be overridden when its class is inherited. You can't change value of a final variable (is a constant).

Q:        What if the main method is declared as private?

A:        The program compiles properly but at runtime it will give "Main method not public." message.

           

Q:        What if the static modifier is removed from the signature of the main method?

A:        Program compiles. But at runtime throws an error "NoSuchMethodError".

           

Q:        What if I write static public void instead of public static void?

A:        Program compiles and runs properly.

           

Q:        What if I do not provide the String array as the argument to the method?

A:        Program compiles but throws a runtime error "NoSuchMethodError".

           

Q:        What is the first argument of the String array in main method?

A:        The String array is empty. It does not have any element. This is unlike C/C++ where the first element by default is the program name.

Q:        If I do not provide any arguments on the command line, then the String array of Main method will be empty or null?

A:        It is empty. But not null.

Q:        How can one prove that the array is not null but empty using one line of code?

A:        Print args.length. It will print 0. That means it is empty. But if it would have been null then it would have thrown a NullPointerException on attempting to print args.length.

Q:        What environment variables do I need to set on my machine in order to be able to run Java programs?

A:        CLASSPATH and PATH are the two variables.

           

Q:        Can an application have multiple classes having main method?

A:        Yes it is possible. While starting the application we mention the class name to be run. The JVM will look for the Main method only in the class whose name you have mentioned. Hence there is not conflict amongst the multiple classes having main method.

           

Q:        Can I have multiple main methods in the same class?

A:        No the program fails to compile. The compiler says that the main method is already defined in the class.

           

Q: Do I need to import java.lang package any time? Why ?

A:        No. It is by default loaded internally by the JVM.

           

Q: Can I import same package/class twice? Will the JVM load the package twice at runtime?

A:        One can import the same package or same class multiple times. Neither compiler nor JVM complains abt it. And the JVM will internally load the class only once no matter how many times you import the same class.

           

Q:        What are Checked and UnChecked Exception?

A:        A checked exception is some subclass of Exception (or Exception itself), excluding class RuntimeException and its subclasses.

Making an exception checked forces client programmers to deal with the possibility that the exception will be thrown. eg, IOException thrown by java.io.FileInputStream's read() method·

Unchecked exceptions are RuntimeException and any of its subclasses. Class Error and its subclasses also are unchecked. With an unchecked exception, however, the compiler doesn't force client programmers either to catch the

exception or declare it in a throws clause. In fact, client programmers may not even know that the exception could be thrown. eg, StringIndexOutOfBoundsException thrown by String's charAt() method· Checked exceptions must be caught at compile time. Runtime exceptions do not need to be. Errors often cannot be.

           

Q: What is Overriding?

A:        When a class defines a method using the same name, return type, and arguments as a method in its superclass, the method in the class overrides the method in the superclass.

When the method is invoked for an object of the class, it is the new definition of the method that is called, and not the method definition from superclass. Methods may be overridden to be more public, not more private.

Q:       What are different types of inner classes?

A:        Nested -level classes, Member classes, Local classes, Anonymous classes

Nested -level classes- If you declare a class within a class and specify the static modifier, the compiler treats the class just like any other -level class.

Any class outside the declaring class accesses the nested class with the declaring class name acting similarly to a package. eg, outer.inner. -level inner classes implicitly have access only to static variables.There can also be inner interfaces. All of these are of the nested -level variety.

Member classes - Member inner classes are just like other member methods and member variables and access to the member class is restricted, just like methods and variables. This means a public member class acts similarly to a nested -level class. The primary difference between member classes and nested -level classes is that member classes have access to the specific instance of the enclosing class.

Local classes - Local classes are like local variables, specific to a block of code. Their visibility is only within the block of their declaration. In order for the class to be useful beyond the declaration block, it would need to implement a

more publicly available interface.Because local classes are not members, the modifiers public, protected, private, and static are not usable.

Anonymous classes - Anonymous inner classes extend local inner classes one level further. As anonymous classes have no name, you cannot provide a constructor.

Are the imports checked for validity at compile time? e.g. will the code containing an import such as java.lang.ABCD compile?

A: Yes the imports are checked for the semantic validity at compile time. The code containing above line of import will not compile. It will throw an error saying,can not resolve symbol

symbol : class ABCD

location: package io

import java.io.ABCD;

Q:        Does importing a package imports the subpackages as well? e.g. Does importing com.MyTest.* also import com.MyTest.UnitTests.*?

A:        No you will have to import the subpackages explicitly. Importing com.MyTest.* will import classes in the package MyTest only. It will not import any class in any of it's subpackage.

           

Q:        What is the difference between declaring a variable and defining a variable?

A:        In declaration we just mention the type of the variable and it's name. We do not initialize it. But defining means declaration + initialization.

e.g String s; is just a declaration while String s = new String ("abcd"); Or String s = "abcd"; are both definitions.

           

Q:        What is the default value of an object reference declared as an instance variable?

A:        null unless we define it explicitly.

           

Q:        Can a  level class be private or protected?

A:        No. A  level class can not be private or protected. It can have either "public" or no modifier. If it does not have a modifier it is supposed to have a default access.If a  level class is declared as private the compiler will complain that the "modifier private is not allowed here". This means that a  level class can not be private. Same is the case with protected.

 Q:       What type of parameter passing does Java support?

 A:       In Java the arguments are always passed by value .

           

Q:        Primitive data types are passed by reference or pass by value?

A:        Primitive data types are passed by value.

           

Q:        Objects are passed by value or by reference?

A:        Java only supports pass by value. With objects, the object reference itself is passed by value and so both the original reference and parameter copy both refer to the same object .

Q:        What is serialization?

A:        Serialization is a mechanism by which you can save the state of an object by converting it to a byte stream.

           

Q:        How do I serialize an object to a file?

A:        The class whose instances are to be serialized should implement an interface Serializable. Then you pass the instance to the ObjectOutputStream which is connected to a fileoutputstream. This will save the object to a file.

           

Q:        Which methods of Serializable interface should I implement?

A:        The serializable interface is an empty interface, it does not contain any methods. So we do not implement any methods.

           

Q:        How can I customize the seralization process? i.e. how can one have a control over the serialization process?

A:        Yes it is possible to have control over serialization process. The class should implement Externalizable interface. This interface contains two methods namely readExternal and writeExternal. You should implement these methods and write the logic for customizing the serialization process.

           

Q:        What is the common usage of serialization?

A:        Whenever an object is to be sent over the network, objects need to be serialized. Moreover if the state of an object is to be saved, objects need to be serilazed.

           

Q:        What is Externalizable interface?

A:        Externalizable is an interface which contains two methods readExternal and writeExternal. These methods give you a control over the serialization mechanism. Thus if your class implements this interface, you can customize the serialization process by implementing these methods.

           

Q: When you serialize an object, what happens to the object references included in the object?

A:        The serialization mechanism generates an object graph for serialization. Thus it determines whether the included object references are serializable or not. This is a recursive process. Thus when an object is serialized, all the included objects are also serialized alongwith the original obect.

Q:What one should take care of while serializing the object?

A:        One should make sure that all the included objects are also serializable. If any of the objects is not serializable then it throws a NotSerializableException.

           

Q:What happens to the static fields of a class during serialization?

A:There are three exceptions in which serialization doesnot necessarily read and write to the stream. These are

1. Serialization ignores static fields, because they are not part of ay particular state state.

2. Base class fields are only hendled if the base class itself is serializable.

3. Transient fields.

Does Java provide any construct to find out the size of an object?

A:        No there is not sizeof operator in Java. So there is not direct way to determine the size of an object directly in Java.

           

Q:       

Give a simplest way to find out the time a method takes for execution without using any profiling tool?

A:        Read the system time just before the method is invoked and immediately after method returns. Take the time difference, which will give you the time taken by a method for execution.

To put it in code...

long start = System.currentTimeMillis ();

method ();

long end = System.currentTimeMillis ();

System.out.println ("Time taken for execution is " + (end - start));

Remember that if the time taken for execution is too small, it might show that it is taking zero milliseconds for execution. Try it on a method which is big enough, in the sense the one which is doing considerable amout of processing.

Q:        What are wrapper classes?

A:        Java provides specialized classes corresponding to each of the primitive data types. These are called wrapper classes. They are e.g. Integer, Character, Double etc.

Q:        Why do we need wrapper classes?

A:        It is sometimes easier to deal with primitives as objects. Moreover most of the collection classes store objects and not primitive data types. And also the wrapper classes provide many utility methods also. Because of these resons we need wrapper classes. And since we create instances of these classes we can store them in any of the collection classes and pass them around as a collection. Also we can pass them around as method parameters where a method expects an object.

Q: What are checked exceptions?

A:        Checked exception are those which the Java compiler forces you to catch. e.g. IOException are checked Exceptions.

           

Q:What are runtime exceptions?

A:        Runtime exceptions are those exceptions that are thrown at runtime because of either wrong input data or because of wrong business logic etc. These are not checked by the compiler at compile time.

           

Q:What is the difference between error and an exception?

A:        An error is an irrecoverable condition occurring at runtime. Such as OutOfMemory error. These JVM errors and you can not repair them at runtime. While exceptions are conditions that occur because of bad input etc. e.g. FileNotFoundException will be thrown if the specified file does not exist. Or a NullPointerException will take place if you try using a null reference. In most of the cases it is possible to recover from an exception (probably by giving user a feedback for entering proper values etc.).

           

Q:        How to create custom exceptions?

A:        Your class should extend class Exception, or some more specific type thereof.

           

Q:        If I want an object of my class to be thrown as an exception object, what should I do?

A:        The class should extend from Exception class. Or you can extend your class from some more precise exception type also.

           

Q:       If my class already extends from some other class what should I do if I want an instance of my class to be thrown as an exception object?

A:        One can not do anytihng in this scenarion. Because Java does not allow multiple inheritance and does not provide any exception interface as well.

Q: How does an exception permeate through the code?

A:        An unhandled exception moves up the method stack in search of a matching When an exception is thrown from a code which is wrapped in a try block followed by one or more catch blocks, a search is made for matching catch block. If a matching type is found then that block will be invoked. If a matching type is not found then the exception moves up the method stack and reaches the caller method. Same procedure is repeated if the caller method is included in a try catch block. This process continues until a catch block handling the appropriate type of exception is found. If it does not find such a block then finally the program terminates.

Q: What are the different ways to handle exceptions?

A: There are two ways to handle exceptions,

1. By wrapping the desired code in a try block followed by a catch block to catch the exceptions. and

2. List the desired exceptions in the throws clause of the method and let the caller of the method hadle those exceptions.

Q: What is the basic difference between the 2 approaches to exception handling.

1> try catch block and

2> specifying the candidate exceptions in the throws clause?

When should you use which approach?

A: In the first approach as a programmer of the method, you urself are dealing with the exception. This is fine if you are in a best position to decide should be done in case of an exception. Whereas if it is not the responsibility of the method to deal with it's own exceptions, then do not use this approach. In this case use the second approach. In the second approach we are forcing the caller of the method to catch the exceptions, that the method is likely to throw. This is often the approach library creators use. They list the exception in the throws clause and we must catch them. You will find the same approach throughout the java libraries we use.

 Q:       Is it necessary that each try block must be followed by a catch block?

A:        It is not necessary that each try block must be followed by a catch block. It should be followed by either a catch block OR a finally block. And whatever exceptions are likely to be thrown should be declared in the throws clause of the method.

           

Q:        If I write return at the end of the try block, will the finally block still execute?

A:        Yes even if you write return as the last statement in the try block and no exception occurs, the finally block will execute. The finally block will execute and then the control return.

           

Q:If I write System.exit (0); at the end of the try block, will the finally block still execute?

A:        No in this case the finally block will not execute because when you say System.exit (0); the control immediately goes out of the program, and thus finally never executes.

           

Q: How are Observer and Observable used?

A:  Objects that subclass the Observable class maintain a list of observers. When an Observable object is updated it invokes the update() method of each of its observers to notify the observers that it has changed state. The Observer interface is implemented by objects that observe Observable objects.

Q: What is synchronization and why is it important?

A: With respect to multithreading, synchronization is the capability to control

the access of multiple threads to shared resources. Without synchronization, it is possible for one thread to modify a shared object while another thread is in the process of using or updating that object's value. This often leads to significant errors.

Q: How does Java handle integer overflows and underflows?

A:        It uses those low order bytes of the result that can fit into the size of the type allowed by the operation.

Q:        Does garbage collection guarantee that a program will not run out of memory?

A:        Garbage collection does not guarantee that a program will not run out of memory. It is possible for programs to use up memory resources faster than they are garbage collected. It is also possible for programs to create objects that are not subject to garbage collection

.

 Q:       What is the difference between preemptive scheduling and time slicing?

A:        Under preemptive scheduling, the highest priority task executes until it enters the waiting or dead states or a higher priority task comes into existence. Under time slicing, a task executes for a predefined slice of time and then reenters the pool of ready tasks. The scheduler then determines which task should execute next, based on priority and other factors.

           

Q:        When a thread is created and started, what is its initial state?

A:        A thread is in the ready state after it has been created and started.

Q:        What is the purpose of finalization?

A:        The purpose of finalization is to give an unreachable object the opportunity to perform any cleanup processing before the object is garbage collected.

Q:        What is the Locale class?

A:        The Locale class is used to tailor program output to the conventions of a particular geographic, political, or cultural region.

Q: What is the difference between a while statement and a do statement?

A:        A while statement checks at the beginning of a loop to see whether the next loop iteration should occur. A do statement checks at the end of a loop to see whether the next iteration of a loop should occur. The do statement will always execute the body of a loop at least once.

Q:        What is the difference between static and non-static variables?

A:        A static variable is associated with the class as a whole rather than with specific instances of a class. Non-static variables take on unique values with each object instance.

           

Q:        How are this() and super() used with constructors?

A:        This() is used to invoke a constructor of the same class. super() is used to invoke a superclass constructor.

Q:        What are synchronized methods and synchronized statements?

A:        Synchronized methods are methods that are used to control access to an object. A thread only executes a synchronized method after it has acquired the lock for the method's object or class. Synchronized statements are similar to synchronized methods. A synchronized statement can only be executed after a thread has acquired the lock for the object or class referenced in the synchronized statement.

Q: What is daemon thread and which method is used to create the daemon thread?

A:        Daemon thread is a low priority thread which runs intermittently in the back ground doing the garbage collection operation for the java runtime system. setDaemon method is used to create a daemon thread.

           

Q:        Can applets communicate with each other?

A:        At this point in time applets may communicate with other applets running in the same virtual machine. If the applets are of the same class, they can communicate via shared static variables. If the applets are of different classes, then each will need a reference to the same class with static variables. In any case the basic idea is to pass the information back and forth through a static variable.

An applet can also get references to all other applets on the same page using the getApplets() method of java.applet.AppletContext. Once you get the reference to an applet, you can communicate with it by using its public members.

It is conceivable to have applets in different virtual machines that talk to a server somewhere on the Internet and store any data that needs to be serialized there. Then, when another applet needs this data, it could connect to this same server. Implementing this is non-trivial.

Q:        What are the steps in the JDBC connection?

A:          While making a JDBC connection we go through the following steps :

Step 1 : Register the database driver by using :

Class.forName(\" driver classs for that specific database\" );

Step 2 : Now create a database connection using :

Connection con = DriverManager.getConnection(url,username,password);

Step 3: Now Create a query using :

Statement stmt = Connection.Statement(\"select * from TABLE NAME\");

Step 4 : Exceute the query :

stmt.exceuteUpdate();

Q:        How does a try statement determine which catch clause should be used to handle an exception?

A:        When an exception is thrown within the body of a try statement, the catch clauses of the try statement are examined in the order in which they appear. The first catch clause that is capable of handling the exceptionis executed. The remaining catch clauses are ignored.

Q:        Can an unreachable object become reachable again?

A:        An unreachable object may become reachable again. This can happen when the object's finalize() method is invoked and the object performs an operation which causes it to become accessible to reachable objects.

Q: What method must be implemented by all threads?

A:        All tasks must implement the run() method, whether they are a subclass of Thread or implement the Runnable interface.

Q:What are synchronized methods and synchronized statements?

A:        Synchronized methods are methods that are used to control access to an object. A thread only executes a synchronized method after it has acquired the lock for the method's object or class. Synchronized statements are similar to synchronized methods. A synchronized statement can only be executed after a thread has acquired the lock for the object or class referenced in the synchronized statement.

Q: What is Externalizable?

A:        Externalizable is an Interface that extends Serializable Interface. And sends data into Streams in Compressed Format. It has two methods, writeExternal(ObjectOuput out) and readExternal(ObjectInput in)

Q:        What modifiers are allowed for methods in an Interface?

A:        Only public and abstract modifiers are allowed for methods in interfaces.

Q:        What are some alternatives to inheritance?

A:        Delegation is an alternative to inheritance. Delegation means that you include an instance of another class as an instance variable, and forward messages to the instance. It is often safer than inheritance because it forces you to think about each message you forward, because the instance is of a known class, rather than a new class, and because it doesn't force you to accept all the methods of the super class: you can provide only the methods that really make sense. On the other hand, it makes you write more code, and it is harder to re-use (because it is not a subclass).

           

Q:What does it mean that a method or field is "static"?

A:        Static variables and methods are instantiated only once per class. In other words they are class variables, not instance variables. If you change the value of a static variable in a particular object, the value of that variable changes for all instances of that class.

Static methods can be referenced with the name of the class rather than the name of a particular object of the class (though that works too). That's how library methods like System.out.println() work out is a static field in the java.lang.System class.

Q:       What is the difference between preemptive scheduling and time slicing?

A:        Under preemptive scheduling, the highest priority task executes until it enters the waiting or dead states or a higher priority task comes into existence. Under time slicing, a task executes for a predefined slice of time and then reenters the pool of ready tasks. The scheduler then determines which task should execute next, based on priority and other factors.

Q:        What is the catch or declare rule for method declarations?

A:        If a checked exception may be thrown within the body of a method, the method must either catch the exception or declare it in its throws clause.

           

Q: Is Empty .java file a valid source file?

A:        Yes, an empty .java file is a perfectly valid source file.

           

Q: Can a .java file contain more than one java classes?

A:        Yes, a .java file contain more than one java classes, provided at the most one of them is a public class.

Q:        Is String a primitive data type in Java?

A:        No String is not a primitive data type in Java, even though it is one of the most extensively used object. Strings in Java are instances of String class defined in java.lang package.

Q:        Is main a keyword in Java?

A:        No, main is not a keyword in Java.

           

Q:        Is next a keyword in Java?

A:        No, next is not a keyword.

           

Q:        Is delete a keyword in Java?

A:        No, delete is not a keyword in Java. Java does not make use of explicit destructors the way C++ does.

           

Q:        Is exit a keyword in Java?

A:        No. To exit a program explicitly you use exit method in System object.

           

Q:        What happens if you dont initialize an instance variable of any of the primitive types in Java?

A:        Java by default initializes it to the default value for that primitive type. Thus an int will be initialized to 0, a boolean will be initialized to false.

 Q:       What will be the initial value of an object reference which is defined as an instance variable?

A:        The object references are all initialized to null in Java. However in order to do anything useful with these references, you must set them to a valid object, else you will get NullPointerExceptions everywhere you try to use such default initialized references.

           

Q:        What are the different scopes for Java variables?

A:        The scope of a Java variable is determined by the context in which the variable is declared. Thus a java variable can have one of the three scopes at any given point in time.

1. Instance : - These are typical object level variables, they are initialized to default values at the time of creation of object, and remain accessible as long as the object accessible.

2. Local : - These are the variables that are defined within a method. They remain accessbile only during the course of method excecution. When the method finishes execution, these variables fall out of scope.

3. Static: - These are the class level variables. They are initialized when the class is loaded in JVM for the first time and remain there as long as the class remains loaded. They are not tied to any particular object instance.

           

Q:        What is the default value of the local variables?

A:        The local variables are not initialized to any default value, neither primitives nor object references. If you try to use these variables without initializing them explicitly, the java compiler will not compile the code. It will complain abt the local varaible not being initilized..

           

Q:How many objects are created in the following piece of code?

MyClass c1, c2, c3;

c1 = new MyClass ();

c3 = new MyClass ();

A:        Only 2 objects are created, c1 and c3. The reference c2 is only declared and not initialized.

          

Q:        Can a public class MyClass be defined in a source file named YourClass.java?

A:        No the source file name, if it contains a public class, must be the same as the public class name itself with a .java extension.

Q:        Can main method be declared final?

A:        Yes, the main method can be declared final, in addition to being public static.

           

Q:        What will be the output of the following statement?

System.out.println ("1" + 3);

A:        It will print 13.

           

Q:        What will be the default values of all the elements of an array defined as an instance variable?

A:        If the array is an array of primitive types, then all the elements of the array will be initialized to the default value corresponding to that primitive type. e.g. All the elements of an array of int will be initialized to 0, while that of boolean type will be initialized to false. Whereas if the array is an array of references (of any type), all the elements will be initialized to null.

What is the Collections API?

           

            The Collections API is a set of classes and interfaces that support operations on collections of objects

What is the List interface?

           

            The List interface provides support for ordered collections of objects.

What is the Vector class?

           

            The Vector class provides the capability to implement a growable array of objects

What is an Iterator interface?

           

            The Iterator interface is used to step through the elements of a Collection

Which java.util classes and interfaces support event handling?

           

            The EventObject class and the EventListener interface support event processing

What is the GregorianCalendar class?

           

            The GregorianCalendar provides support for traditional Western calendars

What is the Locale class?

           

            The Locale class is used to tailor program output to the conventions of a particular geographic, political, or cultural region

What is the SimpleTimeZone class?

           

            The SimpleTimeZone class provides support for a Gregorian calendar

What is the Map interface?

           

            The Map interface replaces the JDK 1.1 Dictionary class and is used associate keys with values

What is the highest-level event class of the event-delegation model?

           

            The java.util.EventObject class is the highest-level class in the event-delegation class hierarchy

What is the Collection interface?

           

            The Collection interface provides support for the implementation of a mathematical bag - an unordered collection of objects that may contain duplicates

What is the Set interface?

           

            The Set interface provides methods for accessing the elements of a finite mathematical set. Sets do not allow duplicate elements

What is the purpose of the enableEvents() method?

           

            The enableEvents() method is used to enable an event for a particular object. Normally, an event is enabled when a listener is added to an object for a particular event. The enableEvents() method is used by objects that handle events by overriding their event-dispatch methods.

What is the ResourceBundle class?

           

            The ResourceBundle class is used to store locale-specific resources that can be loaded by a program to tailor the program’s appearance to the particular locale in which it is being run.

What is the difference between yielding and sleeping?

           

            When a task invokes its yield() method, it returns to the ready state. When a task invokes its sleep() method, it returns to the waiting state.

When a thread blocks on I/O, what state does it enter?

           

            A thread enters the waiting state when it blocks on I/O.

When a thread is created and started, what is its initial state?

           

            A thread is in the ready state after it has been created and started.

What invokes a thread’s run() method?

           

            After a thread is started, via its start() method or that of the Thread class, the JVM invokes the thread’s run() method when the thread is initially executed.

What method is invoked to cause an object to begin executing as a separate thread?

           

            The start() method of the Thread class is invoked to cause an object to begin executing as a separate thread.

What is the purpose of the wait(), notify(), and notifyAll() methods?

           

            The wait(),notify(), and notifyAll() methods are used to provide an efficient way for threads to wait for a shared resource. When a thread executes an object’s wait() method, it enters the waiting state. It only enters the ready state after another thread invokes the object’s notify() or notifyAll() methods.

What are the high-level thread states?

           

            The high-level thread states are ready, running, waiting, and dead

What happens when a thread cannot acquire a lock on an object?

           

            If a thread attempts to execute a synchronized method or synchronized statement and is unable to acquire an object’s lock, it enters the waiting state until the lock becomes available.

How does multithreading take place on a computer with a single CPU?

           

            The operating system’s task scheduler allocates execution time to multiple tasks. By quickly switching between executing tasks, it creates the impression that tasks execute sequentially.

What happens when you invoke a thread’s interrupt method while it is sleeping or waiting? - When a task’s interrupt() method is executed, the task enters the ready state. The next time the task enters the running state, an InterruptedException is thrown.

What state is a thread in when it is executing?

           

            An executing thread is in the running state

What are three ways in which a thread can enter the waiting state?

           

            A thread can enter the waiting state by invoking its sleep() method, by blocking on I/O, by unsuccessfully attempting to acquire an object’s lock, or by invoking an object’s wait() method. It can also enter the waiting state by invoking its (deprecated) suspend() method.

What method must be implemented by all threads?

           

            All tasks must implement the run() method, whether they are a subclass of Thread or implement the Runnable interface.

What are the two basic ways in which classes that can be run as threads may be defined?

            A thread class may be declared as a subclass of Thread, or it may implement the Runnable interface.

How can you store international / Unicode characters into a cookie? 

            One way is, before storing the cookie URLEncode it. URLEnocder.encoder(str); And use URLDecoder.decode(str) when you get the stored cookie.

JAVA COLLECTIONS:

Q:        What is the Collections API?

A:        The Collections API is a set of classes and interfaces that support operations on collections of objects.

           

Q:        What is the List interface?

A:        The List interface provides support for ordered collections of objects.

           

Q:        What is the Vector class?

A:        The Vector class provides the capability to implement a growable array of objects.

           

Q:        What is an Iterator interface?

A:        The Iterator interface is used to step through the elements of a Collection .

           

Q:        Which java.util classes and interfaces support event handling?

A:        The EventObject class and the EventListener interface support event processing.

           

Q:        What is the GregorianCalendar class?

A:        The GregorianCalendar provides support for traditional Western calendars

           

Q:        What is the Locale class?

A:        The Locale class is used to tailor program output to the conventions of a particular geographic, political, or cultural region .

           

Q:        What is the SimpleTimeZone class?

A:        The SimpleTimeZone class provides support for a Gregorian calendar .

           

Q:        What is the Map interface?

A:        The Map interface replaces the JDK 1.1 Dictionary class and is used associate keys with values.

           

Q:        What is the highest-level event class of the event-delegation model?

A:        The java.util.EventObject class is the highest-level class in the event-delegation class hierarchy.

           

Q:        What is the Collection interface?

A:        The Collection interface provides support for the implementation of a mathematical bag - an unordered collection of objects that may contain duplicates.

           

Q:        What is the Set interface?

A:        The Set interface provides methods for accessing the elements of a finite mathematical set. Sets do not allow duplicate elements.

           

Q:        What is the typical use of Hashtable?

A:        Whenever a program wants to store a key value pair, one can use Hashtable.

           

Q:        I am trying to store an object using a key in a Hashtable. And some other object already exists in that location, then what will happen? The existing object will be overwritten? Or the new object will be stored elsewhere?

A:        The existing object will be overwritten and thus it will be lost.

           

Q:        What is the difference between the size and capacity of a Vector?

A:        The size is the number of elements actually stored in the vector, while capacity is the maximum number of elements it can store at a given instance of time.

           

Q:        Can a vector contain heterogenous objects?

A:        Yes a Vector can contain heterogenous objects. Because a Vector stores everything in terms of Object.

           

Q:        Can a ArrayList contain heterogenous objects?

A:        Yes a ArrayList can contain heterogenous objects. Because a ArrayList stores everything in terms of Object.

           

Q:        What is an enumeration?

A:        An enumeration is an interface containing methods for accessing the underlying data structure from which the enumeration is obtained. It is a construct which collection classes return when you request a collection of all the objects stored in the collection. It allows sequential access to all the elements stored in the collection.

           

Q:        Considering the basic properties of Vector and ArrayList, where will you use Vector and where will you use ArrayList?

A:        The basic difference between a Vector and an ArrayList is that, vector is synchronized while ArrayList is not. Thus whenever there is a possibility of multiple threads accessing the same instance, one should use Vector. While if not multiple threads are going to access the same instance then use ArrayList. Non synchronized data structure will give better performance than the synchronized one.

           

Q:        Can a vector contain heterogenous objects?

A:        Yes a Vector can contain heterogenous objects. Because a Vector stores everything in terms of Object.

--

What is HashMap and Map?

Map is Interface and Hashmap is class that implements this interface.

What is the significance of ListIterator?

Or

What is the difference b/w Iterator and ListIterator?

Iterator : Enables you to cycle through a collection in the forward direction only, for obtaining or removing elements

ListIterator : It extends Iterator, allow bidirectional traversal of list and the modification of elements

Difference between HashMap and HashTable? Can we make hashmap synchronized?

1. The HashMap class is roughly equivalent to Hashtable, except that it is unsynchronized and permits nulls. (HashMap allows null values as key and value whereas Hashtable doesn’t allow nulls).

2. HashMap does not guarantee that the order of the map will remain constant over time.

3. HashMap is non synchronized whereas Hashtable is synchronized.

4. Iterator in the HashMap is fail-safe while the enumerator for the Hashtable isn't.

Note on Some Important Terms

1)Synchronized means only one thread can modify a hash table at one point of time. Basically, it means that any thread before performing an update on a hashtable will have to acquire a lock on the object while others will wait for lock to be released.

2)Fail-safe is relevant from the context of iterators. If an iterator has been created on a collection object and some other thread tries to modify the collection object "structurally”, a concurrent modification exception will be thrown. It is possible for other threads though to invoke "set" method since it doesn’t modify the collection "structurally”. However, if prior to calling "set", the collection has been modified structurally, "IllegalArgumentException" will be thrown.

HashMap can be synchronized by

Map m = Collections.synchronizeMap(hashMap);

What is the difference between set and list?

A Set stores elements in an unordered way and does not contain duplicate elements, whereas a list stores elements in an ordered way but may contain duplicate elements.

Difference between Vector and ArrayList? What is the Vector class?

Vector is synchronized whereas ArrayList is not. The Vector class provides the capability to implement a growable array of objects. ArrayList and Vector class both implement the List interface. Both classes are implemented using dynamically resizable arrays, providing fast random access and fast traversal. In vector the data is retrieved using the elementAt() method while in ArrayList, it is done using the get() method. ArrayList has no default size while vector has a default size of 10. when you want programs to run in multithreading environment then use concept of vector because it is synchronized. But ArrayList is not synchronized so, avoid use of it in a multithreading environment.

What is an Iterator interface? Is Iterator a Class or Interface? What is its use?

The Iterator is an interface, used to traverse through the elements of a Collection. It is not advisable to modify the collection itself while traversing an Iterator.

What is the Collections API?

The Collections API is a set of classes and interfaces that support operations on collections of objects.

Example of classes: HashSet, HashMap, ArrayList, LinkedList, TreeSet and TreeMap.

Example of interfaces: Collection, Set, List and Map.

What is the List interface?

The List interface provides support for ordered collections of objects.

How can we access elements of a collection?

We can access the elements of a collection using the following ways:

1.Every collection object has get(index) method to get the element of the object. This method will return Object.

2.Collection provide Enumeration or Iterator object so that we can get the objects of a collection one by one.

What is the Set interface?

The Set interface provides methods for accessing the elements of a finite mathematical set. Sets do not allow duplicate elements.

What’s the difference between a queue and a stack?

Stack is a data structure that is based on last-in-first-out rule (LIFO), while queues are based on First-in-first-out (FIFO) rule.

What is the Map interface?

The Map interface is used associate keys with values.

What is the Properties class?

The properties class is a subclass of Hashtable that can be read from or written to a stream. It also provides the capability to specify a set of default values to be used.

Which implementation of the List interface provides for the fastest insertion of a new element into the middle of the list?

a. Vector

b. ArrayList

c. LinkedList

d. None of the above

ArrayList and Vector both use an array to store the elements of the list. When an element is inserted into the middle of the list the elements that follow the insertion point must be shifted to make room for the new element. The LinkedList is implemented using a doubly linked list; an insertion requires only the updating of the links at the point of insertion. Therefore, the LinkedList allows for fast insertions and deletions.

How can we use hashset in collection interface?

This class implements the set interface, backed by a hash table (actually a HashMap instance). It makes no guarantees as to the iteration order of the set; in particular, it does not guarantee that the order will remain constant over time. This class permits the Null element.

This class offers constant time performance for the basic operations (add, remove, contains and size), assuming the hash function disperses the elements properly among the buckets.

What are differences between Enumeration, ArrayList, Hashtable and Collections and Collection?

Enumeration: It is series of elements. It can be use to enumerate through the elements of a vector, keys or values of a hashtable. You can not remove elements from Enumeration.

ArrayList: It is re-sizable array implementation. Belongs to 'List' group in collection. It permits all elements, including null. It is not thread -safe.

Hashtable: It maps key to value. You can use non-null value for key or value. It is part of group Map in collection.

Collections: It implements Polymorphic algorithms which operate on collections.

Collection: It is the root interface in the collection hierarchy.

What is difference between array & arraylist?

An ArrayList is resizable, where as, an array is not. ArrayList is a part of the Collection Framework. We can store any type of objects, and we can deal with only objects. It is growable. Array is collection of similar data items. We can have array of primitives or objects. It is of fixed size. We can have multi dimensional arrays.

Array: can store primitive            ArrayList: Stores object only

Array: fix size                            ArrayList: resizable

Array: can have multi dimensional

Array: lang                                ArrayList: Collection framework

Can you limit the initial capacity of vector in java?

Yes you can limit the initial capacity. We can construct an empty vector with specified initial capacity

public vector(int initialcapacity)

What method should the key class of Hashmap override?

The methods to override are equals() and hashCode().

What is the difference between Enumeration and Iterator?

The functionality of Enumeration interface is duplicated by the Iterator interface. Iterator has a remove() method while Enumeration doesn't. Enumeration acts as Read-only interface, because it has the methods only to traverse and fetch the objects, where as using Iterator we can manipulate the objects also like adding and removing the objects.

So Enumeration is used when ever we want to make Collection objects as Read-only.

1. What is garbage collection? What is the process that is responsible for doing that in java? - Reclaiming the unused memory by the invalid objects. Garbage collector is responsible for this process
2. What kind of thread is the Garbage collector thread? - It is a daemon thread.
3. What is a daemon thread? - These are the threads which can run without user intervention. The JVM can exit when there are daemon thread by killing them abruptly.
4. How will you invoke any external process in Java? - Runtime.getRuntime().exec(….)
5. What is the finalize method do? - Before the invalid objects get garbage collected, the JVM give the user a chance to clean up some resources before it got garbage collected.
6. What is mutable object and immutable object? - If a object value is changeable then we can call it as Mutable object. (Ex., StringBuffer, …) If you are not allowed to change the value of an object, it is immutable object. (Ex., String, Integer, Float, …)
7. What is the basic difference between string and stringbuffer object? - String is an immutable object. StringBuffer is a mutable object.
8. What is the purpose of Void class? - The Void class is an uninstantiable placeholder class to hold a reference to the Class object representing the primitive Java type void.
9. What is reflection? - Reflection allows programmatic access to information about the fields, methods and constructors of loaded classes, and the use reflected fields, methods, and constructors to operate on their underlying counterparts on objects, within security restrictions.
10. What is the base class for Error and Exception? - Throwable
11. What is the byte range? -128 to 127
12. What is the implementation of destroy method in java.. is it native or java code? - This method is not implemented.
13. What is a package? - To group set of classes into a single unit is known as packaging. Packages provides wide namespace ability.
14. What are the approaches that you will follow for making a program very efficient? - By avoiding too much of static methods avoiding the excessive and unnecessary use of synchronized methods Selection of related classes based on the application (meaning synchronized classes for multiuser and non-synchronized classes for single user) Usage of appropriate design patterns Using cache methodologies for remote invocations Avoiding creation of variables within a loop and lot more.
15. What is a DatabaseMetaData? - Comprehensive information about the database as a whole.
16. What is Locale? - A Locale object represents a specific geographical, political, or cultural region
17. How will you load a specific locale? - Using ResourceBundle.getBundle(…);
18. What is JIT and its use? - Really, just a very fast compiler… In this incarnation, pretty much a one-pass compiler — no offline computations. So you can’t look at the whole method, rank the expressions according to which ones are re-used the most, and then generate code. In theory terms, it’s an on-line problem.
19. Is JVM a compiler or an interpreter? - Interpreter
20. When you think about optimization, what is the best way to findout the time/memory consuming process? - Using profiler
21. What is the purpose of assert keyword used in JDK1.4.x? - In order to validate certain expressions. It effectively replaces the if block and automatically throws the AssertionError on failure. This keyword should be used for the critical arguments. Meaning, without that the method does nothing.
22. How will you get the platform dependent values like line separator, path separator, etc., ? - Using Sytem.getProperty(…) (line.separator, path.separator, …)
23. What is skeleton and stub? what is the purpose of those? - Stub is a client side representation of the server, which takes care of communicating with the remote server. Skeleton is the server side representation. But that is no more in use… it is deprecated long before in JDK.
24. What is the final keyword denotes? - final keyword denotes that it is the final implementation for that method or variable or class. You can’t override that method/variable/class any more.
25. What is the significance of ListIterator? - You can iterate back and forth.
26. What is the major difference between LinkedList and ArrayList? - LinkedList are meant for sequential accessing. ArrayList are meant for random accessing.
27. What is nested class? - If all the methods of a inner class is static then it is a nested class.
28. What is inner class? - If the methods of the inner class can only be accessed via the instance of the inner class, then it is called inner class.
29. What is composition? - Holding the reference of the other class within some other class is known as composition.
30. What is aggregation? - It is a special type of composition. If you expose all the methods of a composite class and route the method call to the composite method through its reference, then it is called aggregation.
31. What are the methods in Object? - clone, equals, wait, finalize, getClass, hashCode, notify, notifyAll, toString
32. Can you instantiate the Math class? - You can’t instantiate the math class. All the methods in this class are static. And the constructor is not public.
33. What is singleton? - It is one of the design pattern. This falls in the creational pattern of the design pattern. There will be only one instance for that entire JVM. You can achieve this by having the private constructor in the class. For eg., public class Singleton { private static final Singleton s = new Singleton(); private Singleton() { } public static Singleton getInstance() { return s; } // all non static methods … }
34. What is DriverManager? - The basic service to manage set of JDBC drivers.
35. What is Class.forName() does and how it is useful? - It loads the class into the ClassLoader. It returns the Class. Using that you can get the instance ( “class-instance”.newInstance() ).
36. Inq adds a question: Expain the reason for each keyword of

public static void main(String args[])

1. What is a Marker Interface? - An interface with no methods. Example: Serializable, Remote, Cloneable
2. What interface do you implement to do the sorting? - Comparable
3. What is the eligibility for a object to get cloned? - It must implement the Cloneable interface
4. What is the purpose of abstract class? - It is not an instantiable class. It provides the concrete implementation for some/all the methods. So that they can reuse the concrete functionality by inheriting the abstract class.
5. What is the difference between interface and abstract class? - Abstract class defined with methods. Interface will declare only the methods. Abstract classes are very much useful when there is a some functionality across various classes. Interfaces are well suited for the classes which varies in functionality but with the same method signatures.
6. What do you mean by RMI and how it is useful? - RMI is a remote method invocation. Using RMI, you can work with remote object. The function calls are as though you are invoking a local variable. So it gives you a impression that you are working really with a object that resides within your own JVM though it is somewhere.
7. What is the protocol used by RMI? - RMI-IIOP
8. What is a hashCode? - hash code value for this object which is unique for every object.
9. What is a thread? - Thread is a block of code which can execute concurrently with other threads in the JVM.
10. What is the algorithm used in Thread scheduling? - Fixed priority scheduling.
11. What is hash-collision in Hashtable and how it is handled in Java? - Two different keys with the same hash value. Two different entries will be kept in a single hash bucket to avoid the collision.
12. What are the different driver types available in JDBC? - 1. A JDBC-ODBC bridge 2. A native-API partly Java technology-enabled driver 3. A net-protocol fully Java technology-enabled driver 4. A native-protocol fully Java technology-enabled driver For more information: Driver Description
13. Is JDBC-ODBC bridge multi-threaded? - No
14. Does the JDBC-ODBC Bridge support multiple concurrent open statements per connection? - No
15. What is the use of serializable? - To persist the state of an object into any perminant storage device.
16. What is the use of transient? - It is an indicator to the JVM that those variables should not be persisted. It is the users responsibility to initialize the value when read back from the storage.
17. What are the different level lockings using the synchronization keyword? - Class level lock Object level lock Method level lock Block level lock
18. What is the use of preparedstatement? - Preparedstatements are precompiled statements. It is mainly used to speed up the process of inserting/updating/deleting especially when there is a bulk processing.
19. What is callable statement? Tell me the way to get the callable statement? - Callablestatements are used to invoke the stored procedures. You can obtain the callablestatement from Connection using the following methods prepareCall(String sql) prepareCall(String sql, int resultSetType, int resultSetConcurrency)
20. In a statement, I am executing a batch. What is the result of the execution? - It returns the int array. The array contains the affected row count in the corresponding index of the SQL.
21. Can a abstract method have the static qualifier? - No
22. What are the different types of qualifier and what is the default qualifier? - public, protected, private, package (default)
23. What is the super class of Hashtable? - Dictionary
24. What is a lightweight component? - Lightweight components are the one which doesn’t go with the native call to obtain the graphical units. They share their parent component graphical units to render them. Example, Swing components
25. What is a heavyweight component? - For every paint call, there will be a native call to get the graphical units. Example, AWT.
26. What is an applet? - Applet is a program which can get downloaded into a client environment and start executing there.
27. What do you mean by a Classloader? - Classloader is the one which loads the classes into the JVM.
28. What are the implicit packages that need not get imported into a class file? - java.lang
29. What is the difference between lightweight and heavyweight component? - Lightweight components reuses its parents graphical units. Heavyweight components goes with the native graphical unit for every component. Lightweight components are faster than the heavyweight components.
30. What are the ways in which you can instantiate a thread? - Using Thread class By implementing the Runnable interface and giving that handle to the Thread class.
31. What are the states of a thread? - 1. New 2. Runnable 3. Not Runnable 4. Dead
32. What is a socket? - A socket is an endpoint for communication between two machines.
33. How will you establish the connection between the servlet and an applet? - Using the URL, I will create the connection URL. Then by openConnection method of the URL, I will establish the connection, through which I can be able to exchange data.
34. What are the threads will start, when you start the java program? - Finalizer, Main, Reference Handler, Signal Dispatcher
35. Can there be an abstract class with no abstract methods in it? - Yes
36. Can an Interface be final? - No
37. Can an Interface have an inner class? - Yes.

38.     public interface abc

39.     {

40.                     static int i=0; void dd();

41.                     class a1

42.                     {

43.                                    a1()

44.                                    {

45.                                                   int j;

46.                                                   System.out.println("inside");

47.                                    };

48.                                    public static void main(String a1[])

49.                                    {

50.                                                   System.out.println("in interfia");

51.                                    }

52.                     }

53.     }

54. Can we define private and protected modifiers for variables in interfaces? - No
55. What is Externalizable? - Externalizable is an Interface that extends Serializable Interface. And sends data into Streams in Compressed Format. It has two methods, writeExternal(ObjectOuput out) and readExternal(ObjectInput in)
56. What modifiers are allowed for methods in an Interface? - Only public and abstract modifiers are allowed for methods in interfaces.
57. What is a local, member and a class variable? - Variables declared within a method are “local” variables. Variables declared within the class i.e not within any methods are “member” variables (global variables). Variables declared within the class i.e not within any methods and are defined as “static” are class variables
58. What are the different identifier states of a Thread? - The different identifiers of a Thread are: R - Running or runnable thread, S - Suspended thread, CW - Thread waiting on a condition variable, MW - Thread waiting on a monitor lock, MS - Thread suspended waiting on a monitor lock
59. What are some alternatives to inheritance? - Delegation is an alternative to inheritance. Delegation means that you include an instance of another class as an instance variable, and forward messages to the instance. It is often safer than inheritance because it forces you to think about each message you forward, because the instance is of a known class, rather than a new class, and because it doesn’t force you to accept all the methods of the super class: you can provide only the methods that really make sense. On the other hand, it makes you write more code, and it is harder to re-use (because it is not a subclass).
60. Why isn’t there operator overloading? - Because C++ has proven by example that operator overloading makes code almost impossible to maintain. In fact there very nearly wasn’t even method overloading in Java, but it was thought that this was too useful for some very basic methods like print(). Note that some of the classes like DataOutputStream have unoverloaded methods like writeInt() and writeByte().
61. What does it mean that a method or field is “static”? - Static variables and methods are instantiated only once per class. In other words they are class variables, not instance variables. If you change the value of a static variable in a particular object, the value of that variable changes for all instances of that class. Static methods can be referenced with the name of the class rather than the name of a particular object of the class (though that works too). That’s how library methods like System.out.println() work. out is a static field in the java.lang.System class.
62. How do I convert a numeric IP address like 192.18.97.39 into a hostname like java.sun.com?

63.     String hostname = InetAddress.getByName("192.18.97.39").getHostName();

64. Difference between JRE/JVM/JDK?
65. Why do threads block on I/O? - Threads block on i/o (that is enters the waiting state) so that other threads may execute while the I/O operation is performed.
66. What is synchronization and why is it important? - With respect to multithreading, synchronization is the capability to control the access of multiple threads to shared resources. Without synchronization, it is possible for one thread to modify a shared object while another thread is in the process of using or updating that object’s value. This often leads to significant errors.
67. Is null a keyword? - The null value is not a keyword.
68. Which characters may be used as the second character of an identifier,but not as the first character of an identifier? - The digits 0 through 9 may not be used as the first character of an identifier but they may be used after the first character of an identifier.
69. What modifiers may be used with an inner class that is a member of an outer class? - A (non-local) inner class may be declared as public, protected, private, static, final, or abstract.
70. How many bits are used to represent Unicode, ASCII, UTF-16, and UTF-8 characters? - Unicode requires 16 bits and ASCII require 7 bits. Although the ASCII character set uses only 7 bits, it is usually represented as 8 bits. UTF-8 represents characters using 8, 16, and 18 bit patterns. UTF-16 uses 16-bit and larger bit patterns.
71. What are wrapped classes? - Wrapped classes are classes that allow primitive types to be accessed as objects.
72. What restrictions are placed on the location of a package statement within a source code file? - A package statement must appear as the first line in a source code file (excluding blank lines and comments).
73. What is the difference between preemptive scheduling and time slicing? - Under preemptive scheduling, the highest priority task executes until it enters the waiting or dead states or a higher priority task comes into existence. Under time slicing, a task executes for a predefined slice of time and then reenters the pool of ready tasks. The scheduler then determines which task should execute next, based on priority and other factors.
74. What is a native method? - A native method is a method that is implemented in a language other than Java.
75. What are order of precedence and associativity, and how are they used? - Order of precedence determines the order in which operators are evaluated in expressions. Associatity determines whether an expression is evaluated left-to-right or right-to-left
76. What is the catch or declare rule for method declarations? - If a checked exception may be thrown within the body of a method, the method must either catch the exception or declare it in its throws clause.
77. Can an anonymous class be declared as implementing an interface and extending a class? - An anonymous class may implement an interface or extend a superclass, but may not be declared to do both.
78. What is the range of the char type? - The range of the char type is 0 to 2^16 - 1

1. What is the diffrence between an Abstract class and Interface ?

2. What is user defined exception ?

3. What do you know about the garbate collector ?

4. What is the difference between C++ & Java ?

5. Explain RMI Architecture?

6. How do you communicate in between Applets & Servlets ?

7. What is the use of Servlets ?

8. What is JDBC? How do you connect to the Database ?

9. In an HTML form I have a Button which makes us to open another page in 15 seconds. How will do you that ?

10. What is the difference between Process and Threads ?

11. What is the difference between RMI & Corba ?

12. What are the services in RMI ?

13. How will you initialize an Applet ?

14. What is the order of method invocation in an Applet ?

15. When is update method called ?

16. How will you pass values from HTML page to the Servlet ?

17. Have you ever used HashTable and Dictionary ?

18. How will you communicate between two Applets ?

19. What are statements in JAVA ?

20. What is JAR file ?

21. What is JNI ?

22. What is the base class for all swing components ?

23. What is JFC ?

24. What is Difference between AWT and Swing ?

25. Considering notepad/IE or any other thing as process, What > will happen if you start notepad or IE 3 times? Where 3 processes are started or 3 threads are started ?

26. How does thread synchronization occurs inside a monitor ?

27. How will you call an Applet using a Java Script function ?

28. Is there any tag in HTML to upload and download files ?

29. Why do you Canvas ?

30. How can you push data from an Applet to Servlet ?

31. What are 4 drivers available in JDBC ?

32. How you can know about drivers and database information ?

33. If you are truncated using JDBC, How can you know ..that how much > > > data is truncated ?

34. And What situation , each of the 4 drivers used ?

35. How will you perform transaction using JDBC ?

36. In RMI, server object first loaded into the memory and then the stub reference is sent to the client ? or whether a stub reference is directly sent to the client ?

37. Suppose server object is not loaded into the memory, and theclient request for it , what will happen?

38. What is serialization ?

39. Can you load the server object dynamically? If so, what are the major 3 steps involved in it ?

40. What is difference RMI registry and OSAgent ?

41. To a server method, the client wants to send a value 20, with this value exceeds to 20,. a message

should be sent to the client ? What will you do for achieving for this ?

42. What are the benefits of Swing over AWT ?

43. Where the CardLayout is used ?

44. What is the Layout for ToolBar ?

45. What is the difference between Grid and GridbagLayout ?

46. How will you add panel to a Frame ?

47. What is the corresponding Layout for Card in Swing ?

48. What is light weight component ?

49. Can you run the product development on all operating systems ?

50. What is the webserver used for running the Servlets ?

51. What is Servlet API used for conneting database ?

52. What is bean ? Where it can be used ?

53. What is difference in between Java Class and Bean ?

54. Can we send object using Sockets ?

55. What is the RMI and Socket ?

56. How to communicate 2 threads each other ?

57. What are the files generated after using IDL to Java Compilet ?

58. What is the protocol used by server and client ?

59. Can I modify an object in CORBA ?

60. What is the functionality stubs and skeletons ?

61. What is the mapping mechanism used by Java to identify IDL language ?

62. Diff between Application and Applet ?

63. What is serializable Interface ?

64. What is the difference between CGI and Servlet ?

65. What is the use of Interface ?

66. Why Java is not fully objective oriented ?

67. Why does not support multiple Inheritance ?

68. What it the root class for all Java classes ?

69. What is polymorphism ?

70. Suppose If we have variable ' I ' in run method, If I can create one or more thread each thread will

occupy a separate copy or same variable will be shared ?

71. In servlets, we are having a web page that is invoking servlets username and password ? which is cheks in the database ? Suppose the second page also If we want to verify the same information whether it will connect to the database or it will be used previous information?

72. What are virtual functions ?

73. Write down how will you create a binary Tree ?

74. What are the traverses in Binary Tree ?

75. Write a program for recursive Traverse ?

76. What are session variable in Servlets ?

77. What is client server computing ?

78. What is Constructor and Virtual function? Can we call Virtual funciton in a constructor ?

79. Why we use OOPS concepts? What is its advantage ?

80. What is the middleware ? What is the functionality of Webserver ?

81. Why Java is not 100 % pure OOPS ? ( EcomServer )

82. When we will use an Interface and Abstract class ?

83. What is an RMI?

84. How will you pass parameters in RMI ? Why u serialize?

85. What is the exact difference in between Unicast and Multicast object ? Where we will use ?

86. What is the main functionality of the Remote Reference Layer ?

87. How do you download stubs from a Remote place ?

88. What is the difference in between C++ and Java ? can u explain in detail ?

89. I want to store more than 10 objects in a remote server ? Which methodology will follow ?

90. What is the main functionality of the Prepared Statement ?

91. What is meant by static query and dynamic query ?

92. What are the Normalization Rules ? Define the Normalization ?

93. What is meant by Servelet? What are the parameters of the service method ?

94. What is meant by Session ? Tell me something about HTTPSession Class ?

95. How do you invoke a Servelt? What is the difference in between doPost and doGet methods ?

96. What is the difference in between the HTTPServlet and Generic Servlet ? Expalin their methods ? Tell me their parameter names also ?

97. Have you used threads in Servelet ?

98. Write a program on RMI and JDBC using StoredProcedure ?

99. How do you sing an Applet ?

100. In a Container there are 5 components. I want to display the all the components names, how will you do that one ?

101. Why there are some null interface in java ? What does it mean ? Give me some null interfaces in

JAVA ?

102. Tell me the latest versions in JAVA related areas ?

103. What is meant by class loader ? How many types are there? When will we use them ?

104. How do you load an Image in a Servlet ?

105. What is meant by flickering ?

106. What is meant by distributed Application ? Why we are using that in our applications ?

107. What is the functionality of the stub ?

108. Have you used any version control ?

109. What is the latest version of JDBC ? What are the new features are added in that ?

110. Explain 2 tier and 3 -tier Architecture ?

111. What is the role of the webserver ?

112. How have you done validation of the fileds in your project ?

113. What is the main difficulties that you are faced in your project ?

114. What is meant by cookies ? Explain ?

Make sure you have a copy of your resume in front of you. OK to have a cheat sheet or two - just don't let anyone hear papers shuffling.

Know your OOA&D definitions, such as polymorphism, inheritance, etc.

Know the difference between an interface and an abstract class.

Know that Java does not support multiple inheritance the way C++ does.

Know that you implement an interface (can implement more than one).

Know that you extend an abstract class (can only extend more than one).

Know about the access modifiers: public/friendly(default)/protected/private. Be able to explain in one or two sentences for each case. This is where you can get tangled up in a phone conversation and confuse the check out of the interviewer and yourself.

Know AWT Event Model - tough to do over the phone - but you may get hit on a question.

Know the two ways to start a thread - "extending Thread" or "implementing Runnable".

Know that the method is "run" but to run a thread you use "start".

1. How do I instantiate a bean whose constructor accepts parameters using the useBean tag?
2. Replacing Characters in a String?
3. Searching a String?
4. Connecting to a Database and Strings Handling?
5. What is a transient variable?
6. What is the difference between Serializalble and Externalizable interface?
7. How many methods in the Externalizable interface?
8. How many methods in the Serializable interface?
9. How to make a class or a bean serializable?
10. What is the serialization?
11. What are synchronized methods and synchronized statements?
12. What is synchronization and why is it important?
13. What is the purpose of finalization?
14. What classes of exceptions may be caught by a catch clause?
15. What is the difference between the Reader/Writer class hierarchy and the InputStream/OutputStream class hierarchy?
16. What happens when a thread cannot acquire a lock on an object?
17. What restrictions are placed on method overriding?
18. What restrictions are placed on method overloading?
19. How does multithreading take place on a computer with a single CPU?
20. How is it possible for two String objects with identical values not to be equal under the == operator?
21. How are this() and super() used with constructors?
22. What class allows you to read objects directly from a stream?
23. What is the ResourceBundle class?
24. What interface must an object implement before it can be written to a stream as an object?
25. What is Serialization and deserialization?
26. What are the Object and Class classes used for?
27. Can you write Java code for declaration of multiple inheritance in Java ?
28. What do you mean by multiple inheritance in C++ ?
29. Write the Java code to declare any constant (say gravitational constant) and to get its value.
30. What are the disadvantages of using threads?
31. Given two tables Student(SID, Name, Course) and Level(SID, level) write the SQL statement to get the name and SID of the student who are taking course = 3 and at freshman level.
32. What do you mean by virtual methods?
33. What do you mean by static methods?
34. What do mean by polymorphism, inheritance, encapsulation?
35. What are the advantages of OOPL?
36. How many methods do u implement if implement the Serializable Interface?
37. Are there any other 'marker' interfaces?
38. What is the difference between instanceof and isInstance?
39. Why do you create interfaces, and when MUST you use one?
40. What's the difference between the == operator and the equals() method? What test does Object.equals() use, and why?
41. Discuss the differences between creating a new class, extending a class and implementing an interface; and when each would be appropriate.
42. Given a text file, input.txt, provide the statement required
43. Name four methods every Java class will have.
44. What does the "abstract" keyword mean in front of a method? A class?
45. Does Java have destructors?
46. Are constructors inherited? Can a subclass call the parent's class constructor? When?
47. What synchronization constructs does Java provide? How do they work?
48. Why "bytecode"? Can you reverse-engineer the code from bytecode?
49. Does Java have "goto"?
50. What does the "final" keyword mean in front of a variable? A method? A class?
51. Access specifiers: "public", "protected", "private", nothing?
52. What is JDBC? Describe the steps needed to execute a SQL query using JDBC.
53. What is RMI?
54. What are native methods? How do you use them?
55. What does the keyword "synchronize" mean in java. When do you use it? What are the disadvantages of synchronization?
56. How many different types of JDBC drivers are present? Discuss them.
57. What is the difference between a Vector and an Array. Discuss the advantages and disadvantages of both?
58. Describe java's security model.
59. Java says "write once, run anywhere". What are some ways this isn't quite true?
60. What is the difference between an Applet and an Application?
61. How can you force all derived classes to implement a method present in the base class?
62. What are abstract classes, abstract methods?
63. What's the difference between == and equals method?
64. Describe, in general, how java's garbage collector works?
65. What is the difference between StringBuffer and String class?
66. How can you achieve Multiple Inheritance in Java?
67. What are interfaces?
68. What are the main differences between Java and C++?
69. In Java, You can create a String object as: String str = "abc"; & String str = new String("abc"); Why cant a button object be created as : Button bt = "abc" Why is it compulsory to create a button object as: Button bt = new Button("abc"); Why is this not compulsory in String's case?
70. Why are Java ARchive (JAR) files important?
71. What is it reflection (introspection) ? Why is reflection possible in the Java language?
72. How to make application thread-safe ?
73. What is it object serialization?
74. What do you know about networking support in Java?
75. What you know about Corba implementation in Java?
76. What is Java Beans?
77. Compare SWING components to standard AWT.
78. What is layout manager ? How does it work?
79. What is the purpose of the toolkit in the Abstract Window Toolkit (AWT)? How does AWT work?
80. What are the differences between Java & C++ in terms of its features?
81. Is the ternary operator written x : y ? z or x ? y : z ?
82. What is the List interface?
83. What is the difference between an if statement and a switch statement?
84. What are the problems faced by Java programmers who don't use layoutmanagers?
85. What are the two basic ways in which classes that can be run as threads may be defined?
86. What are synchronized methods and synchronized statements?
87. Which Component subclass is used for drawing and painting?
88. What methods are used to get and set the text label displayed by a Buttonobject?
89. What method must be implemented by all threads?
90. When is an object subject to garbage collection?
91. Can an unreachable object become reachable again?
92. How does a try statement determine which catch clause should be used tohandle an exception?
93. What are the Object and Class classes used for?
94. What modifiers may be used with a top-level class?
95. What is a Java package and how is it used?
96. What is the purpose of a statement block?
97. What is the difference between the prefix and postfix forms of the ++ operator?
98. Can try statements be nested?
99. To what value is a variable of the boolean type automatically initialized?
100. What is the difference between a public and a non-public class?

101.                         

102.                        1. What is J2EE?J2EE is an environment for developing and deploying enterprise applications. The J2EE platform consists of a set of services, application programming interfaces (APIs), and protocols that provide the functionality for developing multitiered, web-based applications.

103.                         

104.                         

105.                         

106.                        2. What is the J2EE module?

107.                        A J2EE module consists of one or more J2EE components for the same container type and one component deployment descriptor of that type.

108.                         

109.                        3. What are the components of J2EE application? 

110.                        A J2EE component is a self-contained functional software unit that is assembled into a J2EE application with its related classes and files and communicates with other components. The J2EE specification defines the following J2EE components:

111.                        Application clients and applets are client components.

112.                        Java Servlet and JavaServer PagesTM (JSPTM) technology components are web components.

113.                        Enterprise JavaBeansTM (EJBTM) components (enterprise beans) are business components.

114.                        Resource adapter components provided by EIS and tool vendors.4. What are the four types of J2EE modules?

115.                         

116.                         

117.                         

118.                        1. Application client module

119.                         

120.                        2. Web module

121.                        3. Enterprise JavaBeans module

122.                        4. Resource adapter module

123.                         

124.                        5. What does application client module contain? 

125.                        The application client module contains:

126.                        --class files,

127.                        --an application client deployment descriptoor.

128.                        Application client modules are packaged as JAR files with a .jar extension.6. What does web module contain?The web module contains:

129.                        --JSP files,

130.                        --class files for servlets,

131.                        --GIF and HTML files, and

132.                        --a Web deployment descriptor.

133.                        Web modules are packaged as JAR files with a .war (Web ARchive) extension.

134.                         

135.                        7. What are the differences between Ear, Jar and War files? Under what circumstances should we use each one? 

136.                        There are no structural differences between the files; they are all archived using zip-jar compression. However, they are intended for different purposes.

137.                        --Jar files (files with a .jar extension) arre intended to hold generic libraries of Java classes, resources, auxiliary files, etc.

138.                        --War files (files with a .war extension) arre intended to contain complete Web applications. In this context, a Web application is defined as a single group of files, classes, resources, .jar files that can be packaged and accessed as one servlet context.

139.                        --Ear files (files with a .ear extension) arre intended to contain complete enterprise applications. In this context, an enterprise application is defined as a collection of .jar files, resources, classes, and multiple Web applications.

140.                        Each type of file (.jar, .war, .ear) is processed uniquely by application servers, servlet containers, EJB containers, etc.8. What is the difference between Session bean and Entity bean?one?The Session bean and Entity bean are two main parts of EJB container.

141.                        Session Bean

142.                        --represents a workflow on behalf of a cliennt

143.                        --one-to-one logical mapping to a client. --created and destroyed by a client

144.                        --not permanent objects

145.                        --lives its EJB container(generally) does noot survive system shut down

146.                        --two types: stateless and stateful beans Entity Bean

147.                        --represents persistent data and behavior off this data

148.                        --can be shared among multiple clients

149.                        --persists across multiple invocations

150.                        --findable permanent objects

151.                        --outlives its EJB container, survives systeem shutdown

152.                        --two types: container managed persistence(CCMP) and bean managed persistence(BMP)

153.                         

154.                        9. What is "applet" ?

155.                        A J2EE component that typically executes in a Web browser but can execute in a variety of other applications or devices that support the applet programming model.10. What is "applet container" ?A container that includes support for the applet programming model.

156.                         

157.                        11. What is "application assembler" ?

158.                        A person who combines J2EE components and modules into deployable application units.

159.                         

160.                        12. What is "application client" ?

161.                        A first-tier J2EE client component that executes in its own Java virtual machine. Application clients have access to some J2EE platform APIs.

162.                         

163.                        13. What is "application client container" ?

164.                        A container that supports application client components.

165.                         

166.                        14. What is "application client module" ?

167.                        A software unit that consists of one or more classes and an application client deployment descriptor.15. What is "application component provider" ?A vendor that provides the Java classes that implement components' methods, JSP page definitions, and any required deployment descriptors.

168.                         

169.                        16. What is "application configuration resource file" ?

170.                        An XML file used to configure resources for a JavaServer Faces application, to define navigation rules for the application, and to register converters, validators, listeners, renderers, and components with the application.

171.                         

172.                        17. What is "archiving" ?

173.                        The process of saving the state of an object and restoring it.18. What is "asant" ?A Java-based build tool that can be extended using Java classes. The configuration files are XML-based, calling out a target tree where various tasks get executed.

174.                         

175.                        19. What is "attribute"What is "asant" ?

176.                        A qualifier on an XML tag that provides additional information.20. What is authentication ?The process that verifies the identity of a user, device, or other entity in a computer system, usually as a prerequisite to allowing access to resources in a system. The Java servlet specification requires three types of authentication-basic, form-based, and mutual-and supports digest authentication.21. What is authorization?The process by which access to a method or resource is determined. Authorization depends on the determination of whether the principal associated with a request through authentication is in a given security role. A security role is a logical grouping of users defined by the person who assembles the application. A deployer maps security roles to security identities. Security identities may be principals or groups in the operational environment.

177.                         

178.                        22. What is authorization constraint ?

179.                        An authorization rule that determines who is permitted to access a Web resource collection.

180.                         

181.                        23. What is B2B ?

182.                        B2B stands for Business-to-business.

183.                         

184.                        24. What is backing bean ?

185.                        A JavaBeans component that corresponds to a JSP page that includes JavaServer Faces components. The backing bean defines properties for the components on the page and methods that perform processing for the component. This processing includes event handling, validation, and processing associated with navigation.

186.                         

187.                        25. What is basic authentication ?

188.                        An authentication mechanism in which a Web server authenticates an entity via a user name and password obtained using the Web application's built-in authentication mechanism.

189.                         

190.                        26. What is bean-managed persistence ?

191.                        The mechanism whereby data transfer between an entity bean's variables and a resource manager is managed by the entity bean.

192.                         

193.                        27. What is bean-managed transaction ?

194.                        A transaction whose boundaries are defined by an enterprise bean.

195.                         

196.                        28. What is binding (XML) ?

197.                        Generating the code needed to process a well-defined portion of XML data.

198.                         

199.                        29. What is binding (JavaServer Faces technology) ?

200.                        Wiring UI components to back-end data sources such as backing bean properties.

201.                         

202.                        30. What is build file ?

203.                        The XML file that contains one or more asant targets. A target is a set of tasks you want to be executed. When starting asant, you can select which targets you want to have executed. When no target is given, the project's default target is executed.

204.                         

205.                        31. What is business logic ?

206.                        The code that implements the functionality of an application. In the Enterprise JavaBeans architecture, this logic is implemented by the methods of an enterprise bean.

207.                         

208.                        32.What is business method ?

209.                        A method of an enterprise bean that implements the business logic or rules of an application.

210.                         

211.                        33. What is callback methods ?

212.                        Component methods called by the container to notify the component of important events in its life cycle.

213.                         

214.                        34. What is caller ?

215.                        Same as caller principal.

216.                         

217.                        35. What is caller principal ?

218.                        The principal that identifies the invoker of the enterprise bean method.

219.                         

220.                        36. What is cascade delete ?

221.                        A deletion that triggers another deletion. A cascade delete can be specified for an entity bean that has container-managed persistence.

222.                         

223.                        37. What is CDATA ?

224.                        A predefined XML tag for character data that means "don't interpret these characters," as opposed to parsed character data (PCDATA), in which the normal rules of XML syntax apply. CDATA sections are typically used to show examples of XML syntax.

225.                         

226.                        38. What is certificate authority ?

227.                        A trusted organization that issues public key certificates and provides identification to the bearer.39. What is client-certificate authentication ?An authentication mechanism that uses HTTP over SSL, in which the server and, optionally, the client authenticate each other with a public key certificate that conforms to a standard that is defined by X.509 Public Key Infrastructure.