方法中的Java通配符泛型的返回类型

Question

This is perhaps the nth time I get confused about wildcards in generics so Im seeking your help. Please look at the code below.

public class MainClass {
    public static void main(String[] args) {
        ArrayList<Thread> l = new ArrayList<>();
        l.add(new Thread());
        l.add(new Thread());
        m1(l);
    }

     static <T extends Runnable> Collection<T> m1(ArrayList<T> l) {
        ArrayList<Thread> r = new ArrayList<>();
        return r;
    }
}

When I call m1, I can pass ArrayList<Runnable> or any of its implementing classes and it works. So far all is fine. Now within the method m1 , Im unable to return ArrayList<Thread> or ArrayList< Runnable> because the return type is Collection<T> . My confusion is, T is bounded by either Runnable or any of its implementing classes and that's why I can't send ArrayList<String> when calling m1. If the compiler is smart enough to know that, why would not it know that ArrayList<Thread> is a valid return argument for this method m1 ?

Can someone please help?

Thanks!

Answer 1

Let me explain, if compiler will allow you to return ArrayList<Thread> instead of ArrayList<T> it means you can call method with another subcalss of Runnable but still you will get ArrayList<Thread> .

For example lets say that you have another implementation of Runnable as MyThread and you are calling your method m1 as m1(new ArrayList<MyThread>) which means T=MyThread.class in the same time your method returns another implementation of Runnable ArrayList<Thread> which will lead to RuntimeException

So, generics that is why here to prevent you from future runtime exceptions

Answer 2

As said by @Andy and @Michael in comments, the type of the actual variable returned should be exactly the same as the the return type defined in the function signature.

It cannot be just a subtype of it as you wrote in your code, because if a user of your function was calling it with T defined as Runnable for example, your return type just won't match the actual return type which would be Collection<Runnable> .

So you should write your function like this:

    static <T extends Runnable> Collection<T> m1(ArrayList<T> l) {
        ArrayList<T> r = new ArrayList<>();
        return r;
    }