<E>、<T>又是什么其它的含义呢？
E Element
T or S Type
K Key, as in <K, V>
V Value



* <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