JDK API文档中，<E>、<T>、<?>分别代表什么意思？
Type Parameter
Conventions
You have already seen the angle bracket and single letter notation used to
represent a type parameter. By convention, a type parameter is a single,
uppercase letter — this allows easy identification and distinguishes a type
parameter from a class name. The most common type parameters you will see are:
* <T> — Type
* <S> — for Type, when T is already in use
* <E> — Element (used extensively by the Java Collections Framework)
* <K> — Key
* <V> — Value
* <N> — Number
You have already seen the angle bracket and single letter notation used to
represent a type parameter. By convention, a type parameter is a single,
uppercase letter — this allows easy identification and distinguishes a type
parameter from a class name. The most common type parameters you will see are:
* <T> — Type
* <S> — for Type, when T is already in use
* <E> — Element (used extensively by the Java Collections Framework)
* <K> — Key
* <V> — Value
* <N> — Number
