You must implement all of an interface's methods or mark your implementing class abstract.  If you do not, you will receive a compile error reminding you to do so.  i.e.

This will not compile because BigInterface's method3(  ) is not implemented:


interface BigInterface {

public void method1( ) ;

public void method2( ) ;

public void method3( ) ;



public class Big implements BigInterface {              // making the class abstract would fix things

public void method1( ) { }

public void method2( ) { }

}                                                                       // implementing method3( ) here would fix things


  A class implementing an interface becomes an object of that type interface. It will return true from an instanceof check.   i.e.   Using the BigInterface code example above, with the Big class compiling and not abstract:


Big B = new Big( ); 

if ( B instanceof BigInterface ) returns true.