java - Java中的通用集合和通配符

Question

在以下情况下，我无法理解泛型，请参阅下面的内联评论以了解我的问题:

public void exampleMethod() {
    //Intuitively I would expect this to mean that test is set containing objects 
    //that subclass AbstractGroup
    Set<? extends AbstractGroup> test;

    //Yet the compiler complains here and I do not understand why?

    test.add(new AnyAbstractGroupSubGroup());

    //I would guess that a method call such as this at runtime

    test = new HashSet<SubGroupA>()

    //would mean that only objects of subgroupA can be added to the collection, but then
    //what is the point in using the wildcard in the first place?  
}

Answer 1

//Intuitively I would expect this to mean that test is set containing objects 
//that subclass AbstractGroup
Set<? extends AbstractGroup> test;

不，这意味着它是一组特定的？它扩展了 AbstractGroup。你和编译器都不知道那是什么？是，所以您无法向该 Set 添加任何内容。

您可以将集合的值分配给 AbstractGroup 类型的变量，但反之则不行。

相反，你需要这个:

Set<? super AbstractGroup> test;

这个原则有时被称为PECS并解释得很好in this answer .