Java interface design

Question

I had an interface initially as below.

public interface testMe {
      public Set<String> doSomething();
}
public class A implements testMe {
      public Set<String> doSomething() {
           return // Set<String>
      }
}

I had similar classes implementing testMe. Now I have to add one more class which returns Set<Some Object>

public class X implements testMe() {
     public Set<Some OBject> doSomething() {
     }
}

How could i add this method in the interface without breaking existing classes?

Answer 1

You can use

public interface testMe {
   public Set<?> doSomething();
}

Or

public interface testMe {
   public Set<? extends CommonSuperclass> doSomething();
}