gistfile1.java

public class ChildClass {
  private List<Callback> callbacks;
  
  @UiHandler AcceptButton
  void onClick(){
    for (Callback callback : callbacks) {
      callback.Accept()
    }
  }
  
  @UiHandler DeclineButton
  void onClick() {
    for (Callback callback : callbacks) {
      callback.decline
    }
  }
  
  public void addCallback(Callback callback) {
    callbacks.add(callback);
  }
  
  public interface Callback {
    void accept();
    void decline();
  }
}


public class ParentClass() {
  private ChildClass mChildClass;
  
  private void init() {
    mChildClass.addCallback(acceptCallback);
    mChildClass.addCallback(declineCallback);
  }
  
  Callback acceptCallback = new Callback() {
    @Override
    void accept() {
      dosomething;
    }
    @Override
    void decline() {
      //do nothing
    }
  }
  
  Callback declineCallback = new Callback() {
    @Override
    void accept() {
      //do nothing
    }
    @Override
    void decline() {
      dosomethingelse();
    }
  }
}