1. 问题：很好奇为什么叫Join?
参考：
- https://blog.csdn.net/frankarmstrong/article/details/55504161
- https://stackoverflow.com/questions/15956231/what-does-this-thread-join-code-mean
- https://stackoverflow.com/questions/9366264/what-does-it-mean-to-join-a-thread
- https://en.wikipedia.org/wiki/Fork–join_model
- http://www.infoq.com/cn/articles/fork-join-introduction
2. 答案：就是一个线程（调用者）等待这个线程（调用了Join方法）的完成（正常或异常）。
2.1 The thread（一般是主线程main thread）calls t1.join() to wait for the t1 thread to finish.
2.2 就像是对t1说， "Come on, Join us!"