常量池
1、java中直接使用双引号展示的字符串会在常量池中直接创建
2、string.intern会先尝试在常量池中查找对象，如果存在就返回该对象在常量池中
的地址找不到就会先放入到常量池再返回地址
3、java7及以上，常量池放到了heap存储当中
使用建议
1、stringbuilder vs +
一般情况下+的操作编译器会做优化，为stringbuilder操作
问题：
More importantly given we have only 3 properties it might not make a difference,
but at what point do you switch from concat to builder?
答案：
At the point where you're concatenating in a loop - that's usually when the
compiler can't substitute StringBuilder by itself.
