Natural Ordering常在容器中被提到，和迭代器一起出现。
在Comparable接口的API规范中找到了描述。
（https://docs.oracle.com/javase/8/docs/api/java/lang/Comparable.html）
The natural ordering for a class C is said to be consistent with equals if and only if e1.compareTo(e2) == 0 has the same boolean value as e1.equals(e2) for every e1 and e2 of class C.
Note that null is not an instance of any class, and e.compareTo(null) should throw a NullPointerException even though e.equals(null) returns false.
即对于类C的任意实例对象e1和e2，表达式“e1.compareTo(e2) == 0”与另一个表达式“e1.equals(e2) ”返回相同的布尔值。