interface inter{
	int aMethod();
}
class One{
	public void aMethod(){}
}
public class Two extends One implements inter{
	public static void main(String[] args){
	}
}

The error was:

2: Method redefined with different return type.

I have two little questions about this:

1) why is the error called in the interface and not the public class?
2) is there a good reason why I didn't get the "interface collision" Eckel talks about in TIJ?

Thanks

And what is the 'interface collision' eckel is talking about? Can you give us some details?

You can give the implemetation of the methods in many ways.

1. The usual way is the implemention class (here class Two) itself can give the implemntation fot the abstract method aMethod() from the interface inter.

2.You can inherit the implementation from its parent class. THis is very much legal. This is what happens in your sample code. The class Two has an inherited method called aMethod() from its parent class One but its return type is NOT 'int' instead it is 'void'

3. The inference is in any class you can't have 2 methods with same name and parameter list (basically signature) and different signatures.

4. So when you say that class Two is extending class One and at the same time implementing inter, the compiler is very much confused. Where in the world are you going to give me a method of 'public int aMethod()' when you already inherited a method from your parent class with different type ? like that.

5. Well, the compiler is not exactly saying the same as Maha's words. Instead it is naive and thinks you are trying to implement the method from interface inter but you have given the WRONG return type like that.

In fact this is possible. A class can give the implementation for a method of the interface it implements by extending a class which happened to implement another totally different interface which happened to have the same method as our first class's implemented interface, and the compiler is happy to accecpt this kind of interface implementation. See the code foll.
This code will compile and run with the output of 'Hello Eric'
regds
maha anna

interface inter{
	int aMethod();
}
class One{
	public void aMethod(){}
}
/*public class Two extends One implements inter{
	public static void main(String[] args){
	}
}*/
public class Two extends Another implements inter{
	public static void main(String[] args){
		new Two().aMethod();
	}
}
interface inter1 {
	int aMethod();
}
class Another implements inter1 {
  public int aMethod() {
	System.out.println("Hello Eric");
	return 1;
   }
}

yes, that's more like the error I was expecting. I was surprised to get mine! I was using JDK1.1.7A at work on NT. I tried it again elsewhere in the Code Warrior IDE and got exactly the error you got. So that must be a quirk of that JDK.

Bruce Eckel cites an error code that uses the term "interface collision". I'll paste the code here:

The difficulty occurs because overriding, implementation, and
overloading get unpleasantly mixed together, and overloaded functions
cannot differ only by return type. When the last two lines are
uncommented, the error messages say it all:

InterfaceCollision.java:23: f() in C cannot
implement f() in I1; attempting to use
incompatible return type
found : int
required: void
InterfaceCollision.java:24: interfaces I3 and I1 are
incompatible; both define f
(), but with different return type

If anyone knows why I would get one error and not the other, i'd be curious to know - I may be missing a concept here.