interface problem

[payal sharma]

//Contents of File AnInterface.java 
                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"); 
                    } 
                } 

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

here object a1 is of tyep AnInterface but reference of AnInterfaceImpl.
Call method on this object is giving compile time error methodOne do not catch or throw Exception.
But why

 

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

this compiles and with out and error or exception.

[George Pincharoen]

Hi,payal
In statement
AnInterface ai = new AnInterfaceImpl();
compile-time type of ai is AnInterface that methodOne() throws an
Exception if you not catch it compile-time error occur,
but statement
AnInterfaceImpl ai = new AnInterfaceImpl();
compile-time type is AnInterfaceImpl that methodOne() dosen't throws an Exception so it compiles without error

[Ashish Hareet]

To do away with the problem we could have a1.methodOne() in a try catch block or we could simply use a throws with the main() in ATest .
Thanx , it cleared my doubt aswell .
Rest w/ George

[Desai Sandeep]

Hi,
Would like to add that it is always better to program to an interface rather than program to a class.The advantage you get is you can change the stub without any requirement for client compilation.This cannot be possible when you program to a class.
Hence,

AnInterface ai = new AnInterfaceImpl();

is better than

AnInterfaceImpl aii = new AnInterfaceImpl();

It is bad programming to implement the methodOne() without throwing an exception.Remember that the interface signifies responsibilities which every implementing class must adhere to.

Hope this helps,
Sandeep
SCJP2, OCSD(Oracle JDeveloper), OCED(Oracle Internet Platform)
[This message has been edited by Desai Sandeep (edited July 25, 2001).]