为什么我不能添加到列表 <? extends String> 在Java中？

Question

In the snippet below, adding "Hulooo" to the list generates a compiler error since String doesnt extend String. However, typecasting an ArrayList of strings works. But typecasting an ArrayList of objects doesn't work.

Can someone explain why this is happening? What applies to String(in this context) that doesn't apply to Object?

public static void takeList(List<? extends String> list)
{
    list.add("Hulloo");//ERROR
    list=new ArrayList<String>();
    list=new ArrayList<Object>();//ERROR
}

Answer 1

The question mark ? is a so-called wild-card operator. List<? extends String> List<? extends String> means: any type List<T> where T is String or a sub-type of String . String is a final class, and thus there are no sub-types of String , but the compiler does not look at this. And thus, the compiler assumes that the list could be a list of some sub-type of String , and it would not be possible to add String s to such a list.

Let's simulate your example with a non-final class, A , which has a sub-class, B :

class A { 
    // empty
}

class B extends A {
    void f();
}

And now consider the equivalent to your method:

public static void takeList(List<? extends A> list) {
    list.add(new A());
}

This will not compile, for the following reason. Suppose you call this method as follows:

List<B> myList = new ArrayList<>();   // line 1
takeList(myList);                     // line 2
B element = myList.get(0);            // line 3
B.f();                                // line 4

Since B is a sub-class of A the function call in line 2 is legal. But the method takeList adds an A to the list which is not a B . An then the list of B s contains an element which is not a B , and line 3 and 4 break down.

The type system is there to prevent typing errors, so if there is one scenario where you could add an object of the wrong type to a list, the type system must forbid it.