java - What is meant by "the erasure of the static type of the expression on which it is called" in the getClass() docs?

Question

The documentation for the "public final Class<?> getClass()" method of Object says:

The actual result type is Class<? extends |X|> where |X| is the erasure of the static type of the expression on which getClass is called. For example, no cast is required in this code fragment:

I don't understand the explanation, particularly with regard to what |X| is said to be - "the erasure of the static type of the expression on which getClass is called".

What form of notation is |X| ? Or maybe, where else would |X| type notation be used?

Answer 1

Let's consider the following example:

List<Integer> test = new ArrayList<Integer>();
Class<? extends List> clazz = test.getClass();

the static type of test (the expression on which getClass() is called) is List<Integer> of which the erasure is List (see type erasure). Note that the dynamic (or runtime) type of test is ArrayList, and the runtime type of clazz will be Class<ArrayList>.

So, Class<? extends |X|> in this case is Class<? extends List>.