java - 泛型的细微差别

Question

  class P {

   }
  class Q extends P {

   }
  class R extends Q { 

   }
  class S extends R {

  }

  public class Manager {

    public static void main(String[] args) {

      X<? super R> x4 = null;
      x4.test(new S());
      x4.test(new R());
      x4.test(new P());//compile time error

     }
}

    class X<A> {
      void test(A obj) {

       }
    }

Answer 1

以下是来自 JLS 的一些规则

A type argument T1 is said to contain another type argument T2, written T2 <= T1, if the set of types denoted by T2 is provably a subset of the set of types denoted by T1 under the reflexive and transitive closure of the following rules (where <: denotes subtyping (§4.10)):

? super T <= ? super S if S <: T

? super T <= ?

? super T <= ? extends Object

T <= ? super T

符号

X<? super R> x4 = null;

声明X使用下限为 R 的通用通配符类型参数.

根据上述规则和您对变量的声明，我们可以确定通配符涵盖了 R 的所有子类型。和 R本身。 S是 R 的子类型, R是R , P不是 R 的子类型.

困惑可能来自以下。你可以做

X<? super R> x4 = new X<P>();

自 P是 R 的父类(super class).但是，当你想使用类的类型参数时 X , 你所知道的是它必须至少是一个 R .

进一步阅读:

• What is PECS (Producer Extends Consumer Super)?
• Lower Bounded Wildcards
• java generics super keyword