"? extends ParentClass" makes Read only?

In the following code Java, I have created a list nums. I can assign the another list during the declaration. But new items cannot be added except the null. So, does it mean the nums is readonly? Why? Is it possible to add new items in that list?

List<Integer> ints = new ArrayList<Integer>();
ints.add(1);
ints.add(2);

List<? extends Number> nums = ints;
nums.add(3.14); //Generates error
nums.addAll(ints);  //Generates error

nums.add(null);     //works
System.out.println(nums.get(0));    //works

I have gone through this link. I can't get exact reason.

Jon Skeet
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Is it possible to add new items in that list?

Nope... because that code doesn't know what it's "actually" a list of. Imagine if you could:

List<String> strings = new ArrayList<>();
List<? extends Object> objects = strings; // This is fine
objects.add(new Object()); // Fortunately this *isn't* valid...
System.out.println(strings.get(0).length()); // Or what would this do?

Basically, when you use a wildcard like ? extends T you can only get values out via the API... and when you use a wildcard like ? super T, you can only put the values in via the API - because that's what's safe.

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