對內存不是特別了解，一直有個疑惑，String a ="a ";String b="b";String c="ab";那麼String c ==a+"b";為false，字符串常量池中有個a了，為什麼這裡的a地址變了？有哪位詳解一下！
String a ="a";String b="b";String c="ab".intern(); String d=(a+"b").intern(); System.out.println(c==d); 結果 true
必須調用intern() 方法才能把字符串放到常量池裡。
這是官方文檔對intern() 的描述
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 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.
