Question

I have an interface that looks something like

public interface KeyRetriever {

public Object getKey(Object obj);

}

I want the implementation to be something like

CustomerTypeKeyRetriever (Implementation class)

public String getKey(Customer customer) {
    return null;
}

How can I achieve this. Currently it throws a compilation error – “The type CustomerTypeKeyRetriever must implement the inherited abstract method KeyRetriever.getKey(Object)”

Answer 1

Your implementation should have method like

@Override
    public Object getKey(Object obj) {
        // TODO Auto-generated method stub
        return null;
    }

I would also put a @Override annotation to on all my implementaion method so that compiler can catch any method changes/conflicts in future in case API’s change.

In case you want generified interface, below should work (this is what i think you might want)

public interface KeyRetriever<T> {
    public Object getKey(T obj);
}

public class CustomerTypeKeyRetriever implements KeyRetriever<Customer> {

    @Override
    public String getKey(Customer obj) {
        // TODO Auto-generated method stub
        return null;
    }

}

Here the return type can be any subclass of Object since java supports covariant return types.