Natural Ordering常在容器中被提到，和迭代器一起出現。
在Comparable接口的API規范中找到了描述。
（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.
即對於類C的任意實例對象e1和e2，表達式“e1.compareTo(e2) == 0”與另一個表達式“e1.equals(e2) ”返回相同的布爾值。