String类为什么要用final修饰? final修饰符的意义? https://www.cnblogs.com/loren-Yang/p/13380318.html String类被实现的目标是什么? 效率和安全 如何实现期望? 参考文献 https://www.jianshu.com/p/9c7f5daac283 https://www.zhihu.com/question/31345592 https://blog.csdn.net/qq_27093465/article/details/52190915