Question

I am trying to encapsulate 3 interfaces in one (ICountable) and make concrete class implement this encapsulating interface. `ICountable code:

public interface ICountable extends IAddable, Cloneable, Serializable {}

Addable interface definition is:

public interface IAddable<T> {
    T add(T other);
}

now when I am trying to implement ICountable in the concrete class (BaseCounter) I am getting the error that not all methods are implemented (complaining on add method). I fail to understand why. The BaseCounter code is as follows:

public class BaseCounter implements ICountable {
    @Override
    public BaseCounter add(BaseCounter other) {
        // TODO Auto-generated method stub
        return null;
    }
}

Change add method signature to

public ICountable add(ICountable other) 

fixes the situation.
However, what I wanted originally, was to make one interface (ICountable) that extends 3 others (2 markers and 1 interface that describes the ability of the implementing class to perform add on the object of the same type). What I need to change in order to make add method implementation in BaseCounter look like this:

public BaseCounter add(BaseCounter other)

I would appreciate your suggestions.

Answer 1

1 Add generic to ICountable

public interface ICountable<T> extends IAddable<T>, Cloneable, Serializable {}

2 Specify generic in implementation

public class BaseCounter implements ICountable<BaseCounter> {
    @Override
    public BaseCounter add(BaseCounter other) {
        // TODO Auto-generated method stub
        return null;
    }
}