将泛型List强制转换为子类型(不安全的逆变)

Question

假设我有一个List<Base>，我知道它只包含Derived类型的元素，我如何强制转换为List<Derived> ?

Answer 1

我的解决方案:

  public static <T> List<T>
cast(List<? super T> list, Class<T> clazz) {
        if( list.stream().allMatch( clazz::isInstance ) )
            return (List<T>) list;
        else
            throw new IllegalArgumentException ("Not all inputs are of class " + clazz.getName());
    }

Answer 2

public static void main(String[] args) {
    List<Number> foo = new ArrayList<>();
    List<Integer> a = cast(foo);
    System.out.println("a = " + a);
}
public static <T, C extends T> List<C> cast( List<T> l ) {
    return (List<C>) l;
}