对内存不是特别了解，一直有个疑惑，String a ="a ";String b="b";String c="ab";那么String c ==a+"b";为false，字符串常量池中有个a了，为什么这里的a地址变了？有哪位详解一下！
String a ="a";String b="b";String c="ab".intern(); String d=(a+"b").intern(); System.out.println(c==d); 结果 true
必须调用intern() 方法才能把字符串放到常量池里。
这是官方文档对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.
String c ==a+"b";为false 其中a是对象，不是字符串，java没那么机智，会再次判断a的值
String c =="a"+"b";为true
final String a ="a";String b="b";String c="ab";
String d=a+"b";
System.out.println(c==d); //结果是true
final 会在编译期间时就把a替换成"a"
String不是基本类型，他是不可更改的，你给他赋值其实是new 了一个对象所以地址会变，StringBuffer很方便高效建议使用他
一个是对象,一个是常量
可以使用 intern,当调用 intern 方法时，如果池已经包含一个等于此 String 对象的字符串（该对象由 equals(Object) 方法确定），则返回池中的字符串。否则，将此 String 对象添加到池中，并且返回此 String 对象的引用。