# Comparator 返回值总结

1. p1 后一个数据
2. p2 前一个数据

1. 返回 1 保持 p1 ,p2 相对位置关系
2. 返回 -1 需要交换 p1, p2
3. 返回 0 不关心两者关系，保持现状

1. 升序排序（自然序列） asceding，也就是后一个比前一个大就保持相对位置，也就是返回 1.
2. 降序排序 descending， 反之后一个比前一个大，就要交换位置，返回 -1

a1 > a2 -> 1
a1 < a2 -> -1

1. 如果后者是 true 前者是 false，就是交换 -1
2. 如果后者是 false 前者是 true，就要保持相对顺序，返回1

# 参考

https://grokonez.com/kotlin/kotlin-array-sort-sortby-sortwith

## public interface Comparator

A comparison function, which imposes a total ordering on some collection of objects.

1. Comparators can be passed to a sort method (such as Collections.sort or Arrays.sort) to allow precise control over the sort order.
2. Comparators can also be used to control the order of certain data structures (such as sorted sets or sorted maps), or to provide an ordering for collections of objects that don’t have a natural ordering.

The ordering imposed by a comparator c on a set of elements S is said to be consistent with equals if and only if c.compare(e1, e2)==0 has the same boolean value as e1.equals(e2) for every e1 and e2 in S.

Caution should be exercised when using a comparator capable of imposing an ordering inconsistent with equals to order a sorted set (or sorted map). Suppose a sorted set (or sorted map) with an explicit comparator c is used with elements (or keys) drawn from a set S. If the ordering imposed by c on S is inconsistent with equals, the sorted set (or sorted map) will behave “strangely.” In particular the sorted set (or sorted map) will violate the general contract for set (or map), which is defined in terms of equals.

For example, suppose one adds two elements a and b such that (a.equals(b) && c.compare(a, b) != 0) to an empty TreeSet with comparator c. The second add operation will return true (and the size of the tree set will increase) because a and b are not equivalent from the tree set’s perspective, even though this is contrary to the specification of the Set.add method.
Note: It is generally a good idea for comparators to also implement java.io.Serializable, as they may be used as ordering methods in serializable data structures (like TreeSet, TreeMap). In order for the data structure to serialize successfully, the comparator (if provided) must implement Serializable.

For the mathematically inclined, the relation that defines the imposed ordering that a given comparator c imposes on a given set of objects S is:
{(x, y) such that c.compare(x, y) <= 0}.

The quotient for this total order is:
{(x, y) such that c.compare(x, y) == 0}.

It follows immediately from the contract for compare that the quotient is an equivalence relation on S, and that the imposed ordering is a total order on S. When we say that the ordering imposed by c on S is consistent with equals, we mean that the quotient for the ordering is the equivalence relation defined by the objects’ equals(Object) method(s):
{(x, y) such that x.equals(y)}.
Unlike Comparable, a comparator may optionally permit comparison of null arguments, while maintaining the requirements for an equivalence relation.
This interface is a member of the Java Collections Framework.
Since:
1.2