Java SE规范| S | 符号

Question

I'm reading the Java Specification for subtyping and casting I know S <: T means S is a subtype of T. However, I don't understand what does |S| <: |T| means? For example in this context:

One type argument is a type variable or wildcard, with an upper bound (from capture conversion, if necessary) of S; and the other type argument T is not a type variable or wildcard; and neither |S| <: |T| nor |T| <: |S|.

|T| is the type of T reference? Does it refer to the class of T?

Answer 1

The notation "|T|" means the type erasure of type "T", according to the definition of type erasure in the JLS, Section 4.6 .

Type erasure is a mapping from types (possibly including parameterized types and type variables) to types (that are never parameterized types or type variables). We write |T| for the erasure of type T. The erasure mapping is defined as follows:

• The erasure of a parameterized type (§4.5) G is |G|.

• The erasure of a nested type TC is |T|.C.

• The erasure of an array type T[] is |T|[].

• The erasure of a type variable (§4.4) is the erasure of its leftmost bound.

• The erasure of every other type is the type itself.

So, "|S| <: |T|" means that the erasure of S is a subtype of the erasure of T.