- 由于Integer是不可变的，每个循环增加key的value时会创建一个新的对象
- 每次value+1时不需要重新创建Integer对象
Integer, Boolean 等 is immutable, you can only change the reference, but you can't change the object itself. 固有MutableInteger
class Holder<T> { public T value; }
The choice of these two types should not depend on the performance. The main choice for AtomicInteger is if
you want to achieve thread safety with the operations on the integer. However the performace difference might
strongly depend on the choosen operating system, as the detailed implementation of atomic operations depend
on the operating system.