Java为什么我不能在函数接口中使用多重继承

Question

I was making a small library for Java, and part of the api was a functional interface (and thus, lambda expressions) that took 3 arguments. I realized that two of them might often be unwanted. So I thought to make something like this:

class Main {

  public static void main(String[] args) {
    call((s) -> System.out.println(s));
  }

  static void call(A a) {
    a.call(7, 42, "Hello World!");
  }

  static interface A {

    void call(int x, int y, String s);

  }

  static interface B extends A {

    void call(String s);

    default void call(int x, int y, String s) {
      call(s);
    }

  }

}

I expected that users could then insert the first two parameters if they wanted, but if they didn't they could leave them out and they would be ignored. However, when I tried to compile the above code, I got a compile-time exception with the message "incompatible types: incompatible parameter types in lambda expression". Why does this not work?

Answer 1

Your static void call(A a) method requires a parameter of type A , or a lambda expression that satisfies the SAM method signature that you have defined on it. That's why it will complain that you are just passing 1 parameter when it is actually expecting 3. If you want to pass a lambda expression of type B you must cast it first like this:

call((B)(s) -> System.out.println(s));

Or you can edit your static void call method to accept a parameter of type B .

Answer 2

The method call(A a) takes instance of interface A as an argument and in interface A the method call is taking 3 arguments: void call(int x, int y, String s); so it looks like you want to declare your argument as an instance of interface B - not A

So modifying:

static void call(A a) {
    a.call(7, 42, "Hello World!");
 }

to:

static void call(B b) {
    b.call(7, 42, "Hello World!");
 }

will solve the issue