Interface questions

[Parameswaran Thangavel]

 
    public interface AnInterface 
                { 
                    public void methodOne() throws Exception; 
                } 
  
 
    //Contents of File AnInterfaceImpl.java 
 
                public class AnInterfaceImpl implements AnInterface 
                { 
                   public void methodOne() 
                    { 
                        System.out.println("I will never throw an exception"); 
                    } 
                } 
 

 
            public class ATest 
            { 
                public static void main(String args[]) 
                { 
                    AnInterface ai = new AnInterfaceImpl(); //(1)
                    ai.methodOne(); 
                } 
            } 
 

when i changed the reference variable type as AnInterfaceIml ai then there is no error...
why???

[vidya sagar]

Because baseclass or baseinterface reference can hold derived class object

[vinuharan haran]

Hi,
If the type of the object is AnInterface,the method declaration in that interface has throws clause.Hence the compiler complains.
If the type of the object is AnInterfaceImpl, the method declaration in that class doesn't have the throws clause.so the compiler doesn't complain.
-vinu.

[Parameswaran Thangavel]

hi vinu
i didn't get what u r telling..

here the methods r overridden.since the overridden methods are called based on the objcet type rather than the reference variable type. the overridding method didn't violate any contract for overridding. Then why it getting error..