[FIXED] What's the point of a write-only collection?

Issue

<? extends T> makes for a read-only collection

<? super T> makes for a write-only collection

I somehow get why use a read-only collection,for instance to use it in a multithreaded environment (any other cases?)

But why use a write-only collection? What's the point if you cannot read from it and use its values at some point? I know that you can get an Object out of it but that defies type safety.

Edit: @Thomas the linked question (Difference between <? super T> and <? extends T> in Java) does show how to make a write only collection but does not answer 'why' would you need one in the first place.So it's not a duplicate

Solution

Note that "write only collection" depends on the point of view.

Lets write a method that adds a bunch of numbers to a collection:

public static void addNumbers(List<? super Integer> target, int count) {
    for (int i = 0; i < count; i++) {
        target.add(i);
    }
}

For this method the list target is a write only list: the method can only add numbers to it, it can not use the values that it added to the list.

On the other side there is the caller:

public static void caller() {
    List<Number> myList = new ArrayList<>();
    addNumbers(myList, 10);
    double sum = 0;
    for (Number n: myList) {
        sum += n.doubleValue();
    }
    System.out.println(sum);
}

This method works with a specific list (myList) and therefore can read the values that addNumbers stuffed into it.

For this method the list is not a write only list, for this method it is an ordinary list.

Answered By - Thomas Kläger
Answer Checked By - Katrina (JavaFixing Volunteer)