Java中的通用集合和通配符

Question

在以下情况下，我遇到了仿制药的问题，请参见以下内联注释，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？扩展了AbstractGroup。而且您和编译器都无法知道那吗？是的，因此您无法向该Set添加任何东西。

您可以将集合的值分配给AbstractGroup类型的变量，但不能相反。

相反，您需要这样做：

Set<? super AbstractGroup> test;

该原理有时称为PECS，并且在此答案中得到了很好的解释。