NOTE：intern方法是native方法

String java.lang.String.intern()

Returns a canonical representation for the string object. 

A pool of strings, initially empty, is maintained privately by the class String. 

When the intern method is invoked, if the pool already contains a string equal to this String object as determined by theequals(Object)method, then the string from the pool is returned. Otherwise, this String object is added to the pool and a reference to this String object is returned.

It follows that for any two stringssandt,s.intern() == t.intern() is true if and only if s.equals(t) is true. 

All literal strings and string-valued constant expressions are interned. String literals are defined in §3.10.5 of the Java Language Specification Returns:a string that has the same contents as this string, but is guaranteed to be from a pool of unique strings.