Java参考分配与通用列表 [英] Java Reference assignment with generic lists

问题描述

我觉得很蠢，要求我这样。

I feel stupid asking this but I am.

List< HasId> ids = list 在下面的代码中给出一个编译错误：

The line List<HasId> ids = list is giving a compile error in the following code:

public class MyGarbageClass {

    public void myMethod(List<MyCompany> list){
        List<HasId> ids = list;
    }

    interface HasId {
        int getId();
    }
    class MyCompany implements HasId{
        private int id = 5;
        @Override
        public int getId() {
            return id;
        }
    }
}

MyCompany实现HasId我应该能够分配它。为什么不能我？ 更重要的是，将一个简单的方法分配给HasId对象列表。

MyCompany implements HasId so I thought I should be able to assign it. Why cant I? And more importantly, what is an easy way to assign this to HasId list of objects.

更新：列表

推荐答案

不允许这样的泛型赋值的原因是可以做转换，然后添加一些到 ids 这不是一个MyCompany（也许MyPerson也实现HasId ）。

The reason why generic assignment like this is disallowed is it possible to do the cast and then add something to ids which is not a MyCompany (perhaps MyPerson which also implements HasId). So if the cast was allowed then you could do this.

public void myMethod(List<MyCompany> list){
    List<HasId> ids = list;
    ids.add(new MyPerson());
}

现在列表已经破坏了通用保证，因为您的列表已声明为 < MyCompany>

Now the list has broken the generic guarantee because you have list that was declared as <MyCompany> with a MyPerson in it.

b
$ b

You could cast it like this.

public void myMethod(List<MyCompany> list){
    List<? extends HasId> ids = list;
}

但不允许add（）操作，如果你愿意，得到id。

But add() operations will not be permitted, but you can iterate it to get the id if you wish.

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