Java 通配符类型的上下限

Question

我环顾四周，到目前为止还没有看到在 java 中对同一通配符类型同时设置下限和上限的任何方法。我不确定这是否可能，我倾向于将其作为 java.

中通配符的当前限制

下面是我正在寻找的示例。

public class A {}
public class B extends A{}
public class C extends B{}
public class D extends C{}

public static void main(String[] args)
{
    List<A> a = Arrays.asList(new A());
    List<B> b = Arrays.asList(new B());
    List<C> c = Arrays.asList(new C());
    List<D> d = Arrays.asList(new D());

    extendsParam(a); // error as A is not assignable to B
    extendsParam(b);
    extendsParam(c); 
    extendsParam(d); // 3 previous ok as they can produce B

    superParam(a);
    superParam(b);
    superParam(c); // 3 previous ok as they can consume C
    superParam(d); // error as C is not assignable to D

    extendsSuperParam(a); // error as A is not assignable to B
    extendsSuperParam(b);
    extendsSuperParam(c); // 2 previous ok as they can consume C and produce B
    extendsSuperParam(d); // error as C is not assignable to D
}

public static void extendsParam(List<? extends B> blist)
{
    B b = blist.get(0);
    blist.add(new C()); // error
}

public static void superParam(List<? super C> clist)
{
    B b = clist.get(0); // error
    clist.add(new C());
}

public static void extendsSuperParam(List<???> bclist)
{
    B b = bclist.get(0); // ok
    bclist.add(new C()); // ok
}

查看 WildcardType.java class 的通用类型信息，它看起来支持使用该信息定义类型。但是，我找不到在语言中创建这种类型的方法。

有没有我遗漏的东西，或者这是目前无法用 java 语言描述的类型？

Answer 1

按照Oracle Docs是做不到的：

Note: You can specify an upper bound for a wildcard, or you can specify a lower bound, but you cannot specify both.

Java 通配符类型的上下限

Lower and Upper Bound for Java Wildcard Type

java

generics

wildcard