SortedMap概述
A {@link Map} that further provides a total ordering on its keys. 提供了key的排序；
The map is ordered according to the {@linkplain Comparable natural ordering} of its keys, or by a {@link Comparator} typically provided at sorted map creation time.
根据 key的自然排序 或 创建SortedMap时提供的Comparator 进行排序；
This order is reflected when iterating over the sorted map's collection views (returned by the {@code entrySet}, {@code keySet} and {@code values} methods).
排序结果 可以根据 entrySet(), keySet(), values() 方法 迭代展示；
All keys inserted into a sorted map must implement the {@code Comparable} interface.
所有的key需要实现Comparable接口；
All general-purpose sorted map implementation classes should provide four "standard" constructors：
所有实现SortedMap的class 应该提供标准构造器：
A void (no arguments) constructor, which creates an empty sorted map sorted according to the natural ordering of its keys.
无参构造，key自然排序；
A constructor with a single argument of type {@code Comparator}, which creates an empty sorted map sorted according to the specified comparator.
Comparator参数，key根据指定Comparator排序；
A constructor with a single argument of type {@code Map}, which creates a new map with the same key-value mappings as its argument, sorted according to the keys' natural ordering.
Map参数，key自然排序；
A constructor with a single argument of type {@code SortedMap}, which creates a new sorted map with the same key-value mappings and the same ordering as the input sorted map.
SortedMap参数，key排序规则和SortedMap参数排序规则一致；
实现类
TreeMap
概述
A Red-Black tree based {@link NavigableMap} implementation. 基于NavigableMap的红黑树实现；
The map is sorted according to the {@linkplain Comparable natural ordering} of its keys, or by a {@link Comparator} provided at map creation time, depending on which constructor is used.
key自然排序 或 创建Map的Comparator排序；
This implementation provides guaranteed log(n) time cost for the {@code containsKey}, {@code get}, {@code put} and {@code remove} operations.
containsKey(), get(), put(), remove() 复杂度 log(n) ；
Note that this implementation is not synchronized. TreeMap是非同步的；
If multiple threads access a map concurrently, and at least one of the threads modifies the map structurally, it must be synchronized externally.
如果多个线程并发访问，需要外部支持同步；
If no such object exists, the map should be "wrapped" using the {@link Collections.synchronizedSortedMap Collections. synchronizedSortedMap} method.
可以使用Collections.synchronizedSortedMap()， Collections. synchronizedSortedMap() 获取同步TreeMap；