Question

I have two interfaces. An interface A, and interface B, which extends interface A. Now, I have class which likes to keep reference of object that either implements A or B. And I would like to provide setter for it. I tried below, but getting type mis match exception.

public interface A {
}

public interface B extends A {
}

public class AB {
private Class<? extends A> object;

public void setObject(Class<? extends A> o){
   this.object = o;
}
}

So basically, I would like setObject method to accept an object that either implements interface A or B.

Answer 1

Simple answer:

Type it as A: setObject(A a).

A class that implements B also implements A. Full code:

public class UsesA {
  private A a;

  public void setObject(A a){
    this.a = a;
  }
}

Now, if you really want to work with B, you’d type it as B, which would also allow you to treat it as an A, since B inherits from it:

public class UsesB {
  private B b; // can call A's methods on this field

  public void setObject(B b) {
    this.b = b;
  }
}

But now you can’t pass an A (static) reference to setObject. If you really want to do this, then you’d need to first downcast it as a B, which could fail at runtime. Generics will not improve on this.