String>
    String|StringBuffer|StringBuilder
    immutable|mutable|mutable  <-- depends on the char[] value is final or not;
    thread-safe|thread-safe|single thread

1, compile phase:
    constance will be directly written. OuerClass.constance not refer to it during runtime.
    + =after compiled=> StringBuilder

2, Performance:
    usually, StringBuilder>StringBuffer>+; but need  to make sure the real generated class file.
    String.intern() is better if too many duplicated string instance.

3, String <--> bytes
decode: String(byte bytes[], int offset, int length, Charset charset)
encode: String.getBytes(Charset charset)

4, StringTokenizer | String.split
    better performance | RegEx