How do HashMap.values() and HashMap.keySet() return values and keys?

Question

The source code of HashMap.values() is shown as follows

public Collection<V> values() {
    Collection<V> vs = values;
    return (vs != null ? vs : (values = new Values()));
}

As you can see, when the values() method first called, it just returns a Values object. The Values object is a subclass of AbstractCollection with no constructor, and of course contains no element. But when I called the method, it returned a collection rapidly

Collection<String> values = map.values();
System.out.println(values);

That's so weird. Not only values(), but also keySet() and entrySet() method return such empty objects. So, here is my question, when and how do these methods return objects with elements we need?

Answer 1

It's a misconception that the Values class is "of course empty". Just because there is no method called on it and its constructor doesn't have any arguments doesn't mean that the collection is empty.

The Values class is an "inner class" (a non-static nested class) of HashMap, which means that it has an implicit reference to the HashMap object that created it. It can therefore access all elements of the HashMap, either explicitly by using the HashMap.this reference or by just accessing the members directly. Since it is an inner class, it is even allowed to access the HashMap's private members.

You can see that for example in the Values class' implementation of the size method:

public int size() {
    return size;
}

The Values class doesn't have a size member, so that size refers to the HashMap's size. It's equivalent to:

public int size() {
    return HashMap.this.size;
}

EDIT: Note that this also means that the collection you receive is not a copy, but still refers to the original HashMap contents and therefore changes when you update the HashMap:

    // Getting the entry set (not a copy!)
    Set<Entry<String, String>> entries = map.entrySet();

    // Add elements to the map afterwards
    map.put("abc", "def");

    // Check out the entries in the collection
    // (magically containing the elements added after getting the collection)
    System.out.println(entries); // "[abc=def]"