question: public java . lang . String intern ( ) { int off = str . indexOf ( this ) ; if ( off > - 1 ) { return str . get ( off ) ; } str . add ( this ) ; return this ; }
target:  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 the equals(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 strings s and t, 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 Section 3.10.5 of the Java Language Specification