番石榴：合并两个多图

Question

I have tree classes eg ClassA , ClassB , ClassC . ClassA and ClassB extends ClassC . I have two Multimaps - Multimap<Integer, ClassA> and Multimap<Integer, ClassB> and I would like to merge this two multimaps into one. I have tried find some solution but unsuccessfully. I have tried sth. like Multimap<Integer, ? extends ClassC> Multimap<Integer, ? extends ClassC> but I don't know if I make it right and if I can merge two multimaps together. Can someone help me? Thank you for response, I appreciate every help.

Answer 1

Multimap<Integer, ? extends ClassC> Multimap<Integer, ? extends ClassC> would mean that the generic type can be any type that extends ClassC , but the type has to be fixed. ie, it can be either all ClassA or all ClassB . So you should use Multimap<Integer, ClassC> instead. It will accept both the types ClassA and ClassB .

Answer 2

Multimap<Integer, ClassC> combine(Multimap<Integer, ? extends ClassC> a, Multimap<Integer, ? extends ClassC> b) {
  Multimap<Integer, ClassC> combined = new SetMultimap<Integer, ClassC>();  // or whatever kind you'd like
  combined.putAll(a);
  combined.putAll(b);
  return combined;
}