Java important question - 4

1) What is anonymous inner class ?

          A local inner class without name is known as anonymous inner class. An anonymous class is defined and instantiated in a single statement. Anonymous inner class always extend a class or implement an interface.

          Since an anonymous class has no name, it is not possible to define a constructor for an anonymous class. Anonymous inner classes are accessible only at the point where it is defined.

2) What is the difference between == and .equals() method in Java?

          In general both equals() and "==" operator in java are used to compare objects to check equality but here are some of the difference between the two:

1) Main difference between . equals() method and == operator is that one is method and other is operator.

2) We can use == operators for reference comparison and .equals() method for content comparison. In simple words, == checks if both objects point to the same memory location whereas . equals() evaluates to the comparison of values in the objects.

 Java program to understand the concept 

public class Test {

public static void main (String[] args )

     { 

String s1 = new String("HELLO");

String s2 = new String ("HELLO");

System.out.println(s1==s2);

System.out.println(s1.equals(s2));

     }

}

Output:

False

true

3) What is a package? Which package is imported by default ?

          A package is a collection of related classes and interfaces. A java class fully classified name contains package and class name. For example, java.lang.Object is the fully classified name of object class that is part of java.lang package.

          java.lang package is imported by default and we don't need to import any class from this package explicitly.

4) Can we have multiple public classes in a Java source file ?

          We can't have more than one public class in a single java source file. A single source file can have multiple classes that are not public.

5) What is this keyword ?

          this keyword provides the reference to the current object and it's mostly used to make sure that object variables are used, not the local variables having the same name.

 // Constructor

 public Point( int x, int y)

{

this.x= x;

this.y= y;

}

We can also used this keyword to invoke other constructors from a constructor.

public Rectangle ()

{

this(0,0,0,0);

}

public Rectangle (int width, int height)

{

this.x = x;

this.y = y;

this.width = width;

this.height = height;

}