返回List的方法 <T> 带参数列表 <S1> ，列表 <S2> ，其中S1和S2延伸T.

Question

I have this method:

public static <T> List<T> flat(List<T>... lists){
    return Stream.of(lists)
        .filter(Objects::nonNull)
        .flatMap(List::stream)
        .collect(toList());
}

The idea is to flat a list of lists and return a list with the same type.

Suppose that A and B extends C . This does not work:

List<A> listOfA;
List<B> listOfB;
List<C> listOfC = flat(listOfA, listOfB);

Because List<A> is not a List<C> !

Anyone know how to write a flat method signature with generics that admits what I'm trying to do?

Thanks!

Answer 1

If A extends C , B extends C , then List<A> and List<B> are assignable to List<? extends C> List<? extends C> .

So you can declare it this way:

public static <T> List<T> flat(List<? extends T>... lists)

Test:

public static void main(String[] args) {
    List<String> listOfString = List.of("sss"); // String extends from Object
    List<Integer> listOfInt = List.of(1); // Integer extends from Object
    List<Object> list = flat(listOfString, listOfInt);
    System.out.println(list); // [sss, 1]
}