Java Programing Questions And Answers-1

Q1. What is the difference between an Abstract class and Interface?

Abstract classes may have some executable methods and methods left

unimplemented. Interfaces contain no implementation code.

An class can implement any number of interfaces, but subclass at most one abstract class.

An abstract class can have nonabstract methods. All methods of an interface are

abstract.

An abstract class can have instance variables. An interface cannot.

An abstract class can define constructor. An interface cannot.

An abstract class can have any visibility: public, protected, private or none

(package). An interface's visibility must be public or none (package).

An abstract class inherits from Object and includes methods such as clone() and

equals().

Q2.What are checked and unchecked exceptions?

Java defines two kinds of exceptions :

Checked exceptions : Exceptions that inherit from the Exception class are

checked exceptions. Client code has to handle the checked exceptions thrown by

the API, either in a catch clause or by forwarding it outward with the throws clause.

Examples - SQLException, IOxception.

Unchecked exceptions : RuntimeException also extends from Exception. However,all of the exceptions that inherit from RuntimeException get special treatment.

There is no requirement for the client code to deal with them, and hence they are called unchecked exceptions. Example Unchecked exceptions are NullPointerException, OutOfMemoryError, DivideByZeroException typically, programming errors.

Q3.What is a user defined exception?

User-defined exceptions may be implemented by

defining a class to respond to the exception and

embedding a throw statement in the try block where the exception can occur or

declaring that the method throws the exception (to another method where it is

handled).

The developer can define a new exception by deriving it from the Exception class as follows:

public class MyException extends Exception {

/* class definition of constructors (but NOT the exception handling code) goes here public MyException() {

super();

}

public MyException( String errorMessage ) {

super( errorMessage );

}

}

The throw statement is used to signal the occurance of the exception within a try block. Often, exceptions are instantiated in the same statement in which they are thrown using the syntax.

throw new MyException("I threw my own exception.")

To handle the exception within the method where it is thrown, a catch statement that handles MyException, must follow the try block. If the developer does not want to handle the exception in the method itself, the method must pass the exception using the syntax:

public myMethodName() throws MyException

Q4.What is the difference between C++ & Java?

Well as Bjarne Stroustrup says "..despite the syntactic similarities, C++ and Java are very different languages. In many ways, Java seems closer to Smalltalk than to C++..". Here are few I discovered:

Java is multithreaded

Java has no pointers

Java has automatic memory management (garbage collection)

Java is platform independent (Stroustrup may differ by saying "Java is a platform"

Java has built-in support for comment documentation

Java has no operator overloading

Java doesn’t provide multiple inheritance

There are no destructors in Java

Q5.What are statements in JAVA ?

Statements are equivalent to sentences in natural languages. A statement forms a complete

unit of execution. The following types of expressions can be made into a statement by

terminating the expression with a semicolon

Assignment expressions

Any use of ++ or --

Method calls

Object creation expressions

These kinds of statements are called expression statements. In addition to these kinds of expression statements, there are two other kinds of statements. A declaration statement

declares a variable. A control flow statement regulates the order in which statements get executed. The for loop and the if statement are both examples of control flow statements.

Q6.What is JAR file?

JavaARchive files are a big glob of Java classes, images, audio, etc., compressed to make one simple, smaller file to ease Applet downloading. Normally when a browser encounters an applet, it goes and downloads all the files, images, audio, used by the Applet separately.

This can lead to slower downloads.

Q7.What is JNI?

JNI is an acronym of Java Native Interface. Using JNI we can call functions which are written in other languages from Java. Following are its advantages and disadvantages.

Advantages:

You want to use your existing library which was previously written in other

language.

You want to call Windows API function.

For the sake of execution speed.

You want to call API function of some server product which is in c or c++ from java

client.

Disadvantages:

You can’t say write once run anywhere.

Difficult to debug runtime error in native code.

Potential security risk.

You can’t call it from Applet.