Generics 在 Java 中传播

Question

I have a Java code with quite tricky generics propagation scenario:

public class Main {

  public static void parent(Parent<String> parent) {}

  public static void child(Child<String> parent) {}

  public static <T> Parent<T> makeParent() {
    return new Parent<>();
  }

  public static void main(String[] args) {
    // this works
    parent(makeParent());

    // this works
    Parent<String> objectParent = makeParent();
    child(objectParent.child());

    // this works
    child(Main.<String>makeParent().child());

    // this does not work
    child(makeParent().child());
  }

  static class Parent<T> {

    public Child<T> child() {
      return new Child<>();
    }

  }

  static class Child<T> {}

}

1. I am curious why and how this works parent(makeParent()); ? How replacement for T is inferred?

2. And also why this does not work child(makeParent().child()); ? As the same generic type is propagated.

Answer 1

The thing to remember about type inference in call chains is that the type inference is done as you move along the chain: if you have a().b() :

• Type inference is run for a() first;
• Then a type is inferred for a().b()

So:

1. parent(makeParent()) : the type of makeParent() is inferred according to what is compatible with the parameter of parent() . Since that will only accept a Parent<String> , that's the inferred type.

2. child(makeParent().child()) : the type of makeParent() is inferred; but there are no constraints on the type (it's not the parameter to the outer child ), so it's inferred to be Parent<Object> ; then the type of makeParent().child() is Child<Object> , which isn't compatible with the parameter type of child , hence the compiler error.

It's annoying that Java doesn't defer the type inference of makeParent() until it has more information, namely the fact that makeParent().child() has to be compatible with some particular type.

I suppose this is done to limit the complexity of the inference: I can imagine a situation where some obscenely complex set of type inference constraints could need to be solved.