Extending an inner interface?

Question

I got a simple question:

Why does Eclipse scream about implementing these two interfaces?

public abstract class Gateway implements IPlayerity, IItemity {
    public interface IPlayerity { ... }
    public interface IItemity { ... }
    // I...ity
}

I get this error message:

IPlayerity cannot be resolved to a type

@jason IPlayerity cannot be resolved to a type. – imacake Jul 15 '11 at 18:16 — imacake, Jul 15 '11 at 18:16

score 5 · Answer 1 · answered Jul 15 '11 at 18:37

5

Declare the interfaces in another file. Its apparent to me that a top-level class cannot implement the interfaces nested within itself ( although I'm not sure why ).

If you want to keep the interfaces within the same file, then you have to change the modifier from public to default and declare them after the class definition.

answered Jul 15 '11 at 18:37

Kal

24,724
7
65
65

1

I *think* the reason is how class loading works. Normal references to other classes are loaded lazily, but outer classes, superclasses and implemented interfaces are loaded automatically before the class itself is loaded. When your outer class implements an inner interface and that construct would have been valid, you'd get a StackOverflowError in the class loader when loading the class at runtime... – mihi Oct 19 '12 at 23:42

score 5 · Accepted Answer · answered Jul 15 '11 at 19:02

You have a cyclic dependency that can't be resolved given the way the JLS works (although I'm not sure where in the JLS this is documented).

The interfaces IPlayerity and IItemity are not visible to the NestedInterfaces class header definition, since they are inside it. I can fix this by changing your program to

public class NestedInterfaces implements 
      NestedInterfaces.IPlayerity, NestedInterfaces.IItemity 
{
    public interface IPlayerity {}
    public interface IItemity {}
}

but then Eclipse gives me this error, which is much more clear:

 Multiple markers at this line
 - Cycle detected: the type NestedInterfaces cannot extend/implement itself or one of its own member types
 - Cycle detected: the type NestedInterfaces cannot extend/implement itself or one of its own member types

cycle detected solved by defining the interface outside the class implementing it. — Ahmed Hegazy, Feb 17 '15 at 12:07

score 0 · Answer 3 · answered Jul 15 '11 at 18:33

0

I see that these two interface is not default java interface, right? So, if you get the error "IPlayerity cannot be resolved to a type.", check whether you import the right packages yet. I should comment for this, but I just cant. Have fun.

answered Jul 15 '11 at 18:33

Khoa Le

372
2
9

score -1 · Answer 4 · answered Jul 15 '11 at 18:43

If it cannot be resolved as a type, then it cannot be "seen". You should add an import for the interface:

import package.Gateway.IPlayerity
import package.Gateway.IItemity

still, I have a feeling that then the compiler will throw some other weird cyclic errors..

It's better if you define the interfaces in another file.

Extending an inner interface?

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