Sunday, January 18, 2009

Java Interview Questions

Q1. What if the main method is declared as private?
The program compiles properly but at runtime it will give "Main method not public." message.

Q2. What is meant by pass by reference and pass by value in Java?
Pass by reference means, passing the address itself rather than passing the value. Pass by value means passing a copy of the value.

Q3. If you’re overriding the method equals() of an object, which other method you might also consider?

Q4.What is Byte Code?
What gives java it’s “write once and run anywhere” nature?
All Java programs are compiled into class files that contain bytecodes. These byte codes can be run in any platform and hence java is said to be platform independent.

Q5.Expain the reason for each keyword of public static void main(String args[])?
public- main(..) is the first method called by java environment when a program is executed so it has to accessible from java environment. Hence the access specifier has to be public.
static: Java environment should be able to call this method without creating an instance of the class , so this method must be declared as static.
void: main does not return anything so the return type must be void
The argument String indicates the argument type which is given at the command line and arg is an array for string given during command line.

Q6.What are the differences between == and .equals() ?
what is difference between == and equals
Difference between == and equals method
What would you use to compare two String variables - the operator == or the method equals()?
How is it possible for two String objects with identical values not to be equal under the == operator?
The == operator compares two objects to determine if they are the same object in memory i.e. present in the same memory location. It is possible for two String objects to have the same value, but located in different areas of memory.
== compares references while .equals compares contents. The method public boolean equals(Object obj) is provided by the Object class and can be overridden. The default implementation returns true only if the object is compared with itself, which is equivalent to the equality operator == being used to compare aliases to the object. String, BitSet, Date, and File override the equals() method. For two String objects, value equality means that they contain the same character sequence. For the Wrapper classes, value equality means that the primitive values are equal.
public class EqualsTest {

public static void main(String[] args) {

String s1 = "abc";
String s2 = s1;
String s5 = "abc";
String s3 = new String("abc");
String s4 = new String("abc");
System.out.println("== comparison : " + (s1 == s5));
System.out.println("== comparison : " + (s1 == s2));
System.out.println("Using equals method : " + s1.equals(s2));
System.out.println("== comparison : " + s3 == s4);
System.out.println("Using equals method : " + s3.equals(s4));

== comparison : true
== comparison : true
Using equals method : true
Using equals method : true

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