Java泛型编译器错误

Question

当用Java做一些不太奇特的东西时,我遇到了泛型错误,我无法理解为什么它不起作用.代码是:

package test;
import java.util.*;
public class TestClass {
  public static class A extends C{}
  public static class B extends C{}
  public static class C{}
  public static class D<T>{}
  public static class E<T>{}

  public static void main(String args[]){
      E<D<? extends C>> a = new E<D<A>>();
      E<D<? extends Object>> b = new E<D<? extends C>>();
      E<D<? extends A>> c = new E<D<A>>();
      E<D<? super A>> d = new E<D<A>>();
      D<? extends C> e = new D<A>();
      D<? extends A> f = new D<A>();
      D<? extends A> g = new D<A>();
  }
}

编译时得到的错误是:

test/TestClass.java:11: incompatible types
found   : test.TestClass.E<test.TestClass.D<test.TestClass.A>>
required: test.TestClass.E<test.TestClass.D<? extends test.TestClass.C>>
      E<D<? extends C>> a = new E<D<A>>();
                            ^
test/TestClass.java:12: incompatible types
found   : test.TestClass.E<test.TestClass.D<? extends test.TestClass.C>>
required: test.TestClass.E<test.TestClass.D<? extends java.lang.Object>>
      E<D<? extends Object>> b = new E<D<? extends C>>();
                                 ^
test/TestClass.java:13: incompatible types
found   : test.TestClass.E<test.TestClass.D<test.TestClass.A>>
required: test.TestClass.E<test.TestClass.D<? extends test.TestClass.A>>
      E<D<? extends A>> c = new E<D<A>>();
                            ^
test/TestClass.java:14: incompatible types
found   : test.TestClass.E<test.TestClass.D<test.TestClass.A>>
required: test.TestClass.E<test.TestClass.D<? super test.TestClass.A>>
      E<D<? super A>> d = new E<D<A>>();
                          ^
4 errors

如果E<D<? extends C>>找到了,那肯定会匹配E<D<? extends Object>>,对吧？还是我错过了什么？

Answer 1

检查 Arrays.asList 调用返回的对象的类型。我猜它会返回一个 List<D<? 扩展 C>> 对象，该对象不能转换为 List<D<? 扩展对象>>。