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POAn abstract class in Java need not implement any methods from its implementing interface. Why?
 

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    <p>Let's look at the following simple code snippet in Java.</p> <pre><code>interface Sum { abstract public void showSum(); } interface Mul { abstract public void showMul(); } abstract class Super implements Sum { protected int x; protected int y; public Super(int x, int y) { this.x=x; this.y=y; } //No error, though the method showSum() of the implementing iterface Sum is commented. Why? /*public void showSum() { System.out.println("Sum = "+(x+y)); }*/ } final class Calculation extends Super implements Mul { public Calculation(int x, int y) { super(x,y); } public void showSum() { System.out.println("Summation = "+(x+y)); } //If showMul() is commented , it would issue a compile-time error. Why? public void showMul() { System.out.println("Multiplication = "+(x*y)); } } final public class Main { public static void main(String[] args) throws IOException { Scanner s=new Scanner(System.in); System.out.print("\nEnter a number : "); int x=s.nextInt(); System.out.print("\nEnter another number : "); int y=s.nextInt(); Calculation c=new Calculation(x,y); c.showSum(); c.showMul(); } } </code></pre> <hr> <p>Since the interface <strong>Sum</strong> containing only one method <code>showSum();</code> is being implemented by the abstract class <strong>Super</strong> , there should be necessary for the abstract class <strong>Super</strong> to implement that method <code>showSum();</code>. The compiler however doesn't complain at all and the program is working well with no problem at all. Why?</p> <hr> <p>Similarly, the non-abstract final class <strong>Calculation</strong> is implementing the <strong>Mul</strong> interface and contains the actual implementation of the method <code>showMul();</code> presented in it's implementing interface. In case, if this method <code>showMul()</code> in the class <strong>Calculation</strong> is commented, it issues a compile-time error. Why is the same thing not applicable to that abstract class <strong>Super</strong>?</p>
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