java中的通用集合和通配符

发布时间：2020-12-15 05:06:56 所属栏目：Java 来源：网络整理

导读：在以下情况下我遇到问题,请参阅下面的内联评论： 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 com

在以下情况下我遇到问题,请参阅下面的内联评论：

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?  
}

解决方法

//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.你和编制者都没有办法知道那是什么？是的,所以你无法向该套装添加任何东西.

您可以将集合的值分配给AbstractGroup类型的变量,但不能相反.

相反,你需要这个：

Set<? super AbstractGroup> test;

这个原则有时被称为PECS并且很好地解释了in this answer.

（编辑：李大同）

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