Question

我无法理解通配符捕获错误，或找到绕过它的方法：

import java.util.List;


class A<B extends A.AA<? extends C>, C> {

  List<C> list;

  public void foo(B inner) {
    C c = get();
    inner.nop(c);
  }

  private C get() {
    return list.get(0);
  }

  static public class AA<D> {
    public void nop(D d) {
        System.out.println(d);
    }
  }
}

这是javac输出：

Wildcards.java:10: error: method nop in class AA<D> cannot be applied to given types;                
      inner.nop(c);                                                                                
          ^                                                                                       
required: CAP#1                                                                                    
found: C                                                                                           
reason: actual argument C cannot be converted to CAP#1 by method invocation conversion             
where C,D are type-variables:                                                                      
  C extends Object declared in class A                                                             
  D extends Object declared in class AA                                                            
where CAP#1 is a fresh type-variable:                                                              
  CAP#1 extends C from capture of ? extends C                                                      
1 error

为什么我们不能将C的实例传递给B.nop（）？

Answer 1

假设您创建了A的实例：

A<A.AA<String>,Object> a = new A<A.AA<String>,Object>();
A.AA<String> inner = new A.AA<String>();
a.foo(inner);

inner.nop()应该预计String，但您传递Object。

一种可能的解决方法是：

class A<B extends A.AA<C>, C>

通配符捕获错误：为什么？

1 个答案: