所以总的来说：

      a线程和b线程会操作同一个 number 中 value，那么输出的结果是不可预测的，因为当前线程修改变量之后但是还没输出的时候，变量有可能被另外一个线程修改.

当如如果要保证输出我当前线程的值呢？

     其实也很简单：在 increase() 和 decrease() 

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