Java集合之NavigableMap与NavigableSet接口

一时失言乱红尘 2022-12-14 09:08 310阅读 0赞

本文接着上篇介绍SortedMap和SortedSet接口，介绍他们的扩展接口NavigableMap与NavigableSet接口，

提供了针对给定搜索目标返回最接近匹配项的导航方法。

SortedMap和SortedSet接口两个接口jdk1.2就已经提供，扩展的NavigableMap与NavigableSet接口jdk1.6才开始支持。

## 1.NavigableSet接口 ##

public interface NavigableSet<E> extends SortedSet<E> \{ 
所有已知实现类： 
ConcurrentSkipListSet, TreeSet

NavigableSet扩展了 SortedSet，具有了为给定搜索目标报告最接近匹配项的导航方法。方法 lower、floor、ceiling 和 higher 分别返回小于、小于等于、大于等于、大于给定元素的元素，如果不存在这样的元素，则返回 null。

类似地，方法 lowerKey、floorKey、ceilingKey 和 higherKey 只返回关联的键。所有这些方法是为查找条目而不是遍历条目而设计的。  
  
        可以按照键的升序或降序访问和遍历 NavigableMap。descendingMap 方法返回映射的一个视图，该视图表示的所有关系方法和方向方法都是逆向的。升序操作和视图的性能很可能比降序操作和视图的性能要好。subMap、headMap 和 tailMap 方法与名称相似的 SortedMap 方法的不同之处在于：可以接受用于描述是否包括（或不包括）下边界和上边界的附加参数。任何 NavigableMap 的 Submap 必须实现 NavigableMap 接口。  
  
此外，此接口还定义了 firstEntry、pollFirstEntry、lastEntry 和 pollLastEntry 方法，它们返回和/或移除最小和最大的映射关系（如果存在），否则返回 null。  
  
subMap(K, K)、headMap(K) 和 tailMap(K) 方法被指定为返回 SortedMap，以允许现有 SortedMap 实现能相容地改进为实现 NavigableMap，但鼓励此接口的扩展和实现重写这些方法以返回 NavigableMap。类似地，可以重写 SortedMap.keySet() 以返回 NavigableSet。

接口方法：

<table> 
 <tbody> 
 <tr> 
 <td colspan="2"> 方法摘要 </td> 
 </tr> 
 <tr> 
 <td> &nbsp;<a href="http://tool.oschina.net/uploads/apidocs/jdk-zh/java/util/NavigableSet.html" rel="nofollow">E</a> </td> 
 <td> <a href="https://blog.csdn.net/u010126792/article/details/62236367#ceiling%28E%29" rel="nofollow">ceiling</a>(<a href="http://tool.oschina.net/uploads/apidocs/jdk-zh/java/util/NavigableSet.html" rel="nofollow">E</a>&nbsp;e)&nbsp; &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;返回此&nbsp;set&nbsp;中大于等于给定元素的最小元素；如果不存在这样的元素，则返回&nbsp;null。 </td> 
 </tr> 
 <tr> 
 <td> &nbsp;<a href="http://tool.oschina.net/uploads/apidocs/jdk-zh/java/util/Iterator.html" rel="nofollow">Iterator</a>&lt;<a href="http://tool.oschina.net/uploads/apidocs/jdk-zh/java/util/NavigableSet.html" rel="nofollow">E</a>&gt; </td> 
 <td> <a href="https://blog.csdn.net/u010126792/article/details/62236367#descendingIterator%28%29" rel="nofollow">descendingIterator</a>()&nbsp; &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;以降序返回在此&nbsp;set&nbsp;的元素上进行迭代的迭代器。 </td> 
 </tr> 
 <tr> 
 <td> &nbsp;<a href="http://tool.oschina.net/uploads/apidocs/jdk-zh/java/util/NavigableSet.html" rel="nofollow">NavigableSet</a>&lt;<a href="http://tool.oschina.net/uploads/apidocs/jdk-zh/java/util/NavigableSet.html" rel="nofollow">E</a>&gt; </td> 
 <td> <a href="https://blog.csdn.net/u010126792/article/details/62236367#descendingSet%28%29" rel="nofollow">descendingSet</a>()&nbsp; &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;返回此&nbsp;set&nbsp;中所包含元素的逆序视图。 </td> 
 </tr> 
 <tr> 
 <td> &nbsp;<a href="http://tool.oschina.net/uploads/apidocs/jdk-zh/java/util/NavigableSet.html" rel="nofollow">E</a> </td> 
 <td> <a href="https://blog.csdn.net/u010126792/article/details/62236367#floor%28E%29" rel="nofollow">floor</a>(<a href="http://tool.oschina.net/uploads/apidocs/jdk-zh/java/util/NavigableSet.html" rel="nofollow">E</a>&nbsp;e)&nbsp; &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;返回此&nbsp;set&nbsp;中小于等于给定元素的最大元素；如果不存在这样的元素，则返回&nbsp;null。 </td> 
 </tr> 
 <tr> 
 <td> &nbsp;<a href="http://tool.oschina.net/uploads/apidocs/jdk-zh/java/util/SortedSet.html" rel="nofollow">SortedSet</a>&lt;<a href="http://tool.oschina.net/uploads/apidocs/jdk-zh/java/util/NavigableSet.html" rel="nofollow">E</a>&gt; </td> 
 <td> <a href="https://blog.csdn.net/u010126792/article/details/62236367#headSet%28E%29" rel="nofollow">headSet</a>(<a href="http://tool.oschina.net/uploads/apidocs/jdk-zh/java/util/NavigableSet.html" rel="nofollow">E</a>&nbsp;toElement)&nbsp; &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;返回此&nbsp;set&nbsp;的部分视图，其元素严格小于&nbsp;toElement。 </td> 
 </tr> 
 <tr> 
 <td> &nbsp;<a href="http://tool.oschina.net/uploads/apidocs/jdk-zh/java/util/NavigableSet.html" rel="nofollow">NavigableSet</a>&lt;<a href="http://tool.oschina.net/uploads/apidocs/jdk-zh/java/util/NavigableSet.html" rel="nofollow">E</a>&gt; </td> 
 <td> <a href="https://blog.csdn.net/u010126792/article/details/62236367#headSet%28E,%20boolean%29" rel="nofollow">headSet</a>(<a href="http://tool.oschina.net/uploads/apidocs/jdk-zh/java/util/NavigableSet.html" rel="nofollow">E</a>&nbsp;toElement,&nbsp;boolean&nbsp;inclusive)&nbsp; &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;返回此&nbsp;set&nbsp;的部分视图，其元素小于（或等于，如果&nbsp;inclusive&nbsp;为&nbsp;true）toElement。 </td> 
 </tr> 
 <tr> 
 <td> &nbsp;<a href="http://tool.oschina.net/uploads/apidocs/jdk-zh/java/util/NavigableSet.html" rel="nofollow">E</a> </td> 
 <td> <a href="https://blog.csdn.net/u010126792/article/details/62236367#higher%28E%29" rel="nofollow">higher</a>(<a href="http://tool.oschina.net/uploads/apidocs/jdk-zh/java/util/NavigableSet.html" rel="nofollow">E</a>&nbsp;e)&nbsp; &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;返回此&nbsp;set&nbsp;中严格大于给定元素的最小元素；如果不存在这样的元素，则返回&nbsp;null。 </td> 
 </tr> 
 <tr> 
 <td> &nbsp;<a href="http://tool.oschina.net/uploads/apidocs/jdk-zh/java/util/Iterator.html" rel="nofollow">Iterator</a>&lt;<a href="http://tool.oschina.net/uploads/apidocs/jdk-zh/java/util/NavigableSet.html" rel="nofollow">E</a>&gt; </td> 
 <td> <a href="https://blog.csdn.net/u010126792/article/details/62236367#iterator%28%29" rel="nofollow">iterator</a>()&nbsp; &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;以升序返回在此&nbsp;set&nbsp;的元素上进行迭代的迭代器。 </td> 
 </tr> 
 <tr> 
 <td> &nbsp;<a href="http://tool.oschina.net/uploads/apidocs/jdk-zh/java/util/NavigableSet.html" rel="nofollow">E</a> </td> 
 <td> <a href="https://blog.csdn.net/u010126792/article/details/62236367#lower%28E%29" rel="nofollow">lower</a>(<a href="http://tool.oschina.net/uploads/apidocs/jdk-zh/java/util/NavigableSet.html" rel="nofollow">E</a>&nbsp;e)&nbsp; &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;返回此&nbsp;set&nbsp;中严格小于给定元素的最大元素；如果不存在这样的元素，则返回&nbsp;null。 </td> 
 </tr> 
 <tr> 
 <td> &nbsp;<a href="http://tool.oschina.net/uploads/apidocs/jdk-zh/java/util/NavigableSet.html" rel="nofollow">E</a> </td> 
 <td> <a href="https://blog.csdn.net/u010126792/article/details/62236367#pollFirst%28%29" rel="nofollow">pollFirst</a>()&nbsp; &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;获取并移除第一个（最低）元素；如果此&nbsp;set&nbsp;为空，则返回&nbsp;null。 </td> 
 </tr> 
 <tr> 
 <td> &nbsp;<a href="http://tool.oschina.net/uploads/apidocs/jdk-zh/java/util/NavigableSet.html" rel="nofollow">E</a> </td> 
 <td> <a href="https://blog.csdn.net/u010126792/article/details/62236367#pollLast%28%29" rel="nofollow">pollLast</a>()&nbsp; &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;获取并移除最后一个（最高）元素；如果此&nbsp;set&nbsp;为空，则返回&nbsp;null。 </td> 
 </tr> 
 <tr> 
 <td> &nbsp;<a href="http://tool.oschina.net/uploads/apidocs/jdk-zh/java/util/NavigableSet.html" rel="nofollow">NavigableSet</a>&lt;<a href="http://tool.oschina.net/uploads/apidocs/jdk-zh/java/util/NavigableSet.html" rel="nofollow">E</a>&gt; </td> 
 <td> <a href="https://blog.csdn.net/u010126792/article/details/62236367#subSet%28E,%20boolean,%20E,%20boolean%29" rel="nofollow">subSet</a>(<a href="http://tool.oschina.net/uploads/apidocs/jdk-zh/java/util/NavigableSet.html" rel="nofollow">E</a>&nbsp;fromElement,&nbsp;boolean&nbsp;fromInclusive,&nbsp;<a href="http://tool.oschina.net/uploads/apidocs/jdk-zh/java/util/NavigableSet.html" rel="nofollow">E</a>&nbsp;toElement,&nbsp;boolean&nbsp;toInclusive)&nbsp; &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;返回此&nbsp;set&nbsp;的部分视图，其元素范围从&nbsp;fromElement&nbsp;到&nbsp;toElement。 </td> 
 </tr> 
 <tr> 
 <td> &nbsp;<a href="http://tool.oschina.net/uploads/apidocs/jdk-zh/java/util/SortedSet.html" rel="nofollow">SortedSet</a>&lt;<a href="http://tool.oschina.net/uploads/apidocs/jdk-zh/java/util/NavigableSet.html" rel="nofollow">E</a>&gt; </td> 
 <td> <a href="https://blog.csdn.net/u010126792/article/details/62236367#subSet%28E,%20E%29" rel="nofollow">subSet</a>(<a href="http://tool.oschina.net/uploads/apidocs/jdk-zh/java/util/NavigableSet.html" rel="nofollow">E</a>&nbsp;fromElement,&nbsp;<a href="http://tool.oschina.net/uploads/apidocs/jdk-zh/java/util/NavigableSet.html" rel="nofollow">E</a>&nbsp;toElement)&nbsp; &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;返回此&nbsp;set&nbsp;的部分视图，其元素从&nbsp;fromElement（包括）到&nbsp;toElement（不包括）。 </td> 
 </tr> 
 <tr> 
 <td> &nbsp;<a href="http://tool.oschina.net/uploads/apidocs/jdk-zh/java/util/SortedSet.html" rel="nofollow">SortedSet</a>&lt;<a href="http://tool.oschina.net/uploads/apidocs/jdk-zh/java/util/NavigableSet.html" rel="nofollow">E</a>&gt; </td> 
 <td> <a href="https://blog.csdn.net/u010126792/article/details/62236367#tailSet%28E%29" rel="nofollow">tailSet</a>(<a href="http://tool.oschina.net/uploads/apidocs/jdk-zh/java/util/NavigableSet.html" rel="nofollow">E</a>&nbsp;fromElement)&nbsp; &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;返回此&nbsp;set&nbsp;的部分视图，其元素大于等于&nbsp;fromElement。 </td> 
 </tr> 
 <tr> 
 <td> &nbsp;<a href="http://tool.oschina.net/uploads/apidocs/jdk-zh/java/util/NavigableSet.html" rel="nofollow">NavigableSet</a>&lt;<a href="http://tool.oschina.net/uploads/apidocs/jdk-zh/java/util/NavigableSet.html" rel="nofollow">E</a>&gt; </td> 
 <td> <a href="https://blog.csdn.net/u010126792/article/details/62236367#tailSet%28E,%20boolean%29" rel="nofollow">tailSet</a>(<a href="http://tool.oschina.net/uploads/apidocs/jdk-zh/java/util/NavigableSet.html" rel="nofollow">E</a>&nbsp;fromElement,&nbsp;boolean&nbsp;inclusive)&nbsp; &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;返回此&nbsp;set&nbsp;的部分视图，其元素大于（或等于，如果&nbsp;inclusive&nbsp;为&nbsp;true）fromElement。 </td> 
 </tr> 
 </tbody> 
</table>

代码实例：

1. //利用实现了NavigableSet的TreeSet做实验
 		NavigableSet<String> sortedTreeSet = new TreeSet<String>(); // SortedSet接收TreeSet的实例 
 		// 增加元素
 		sortedTreeSet.add("aa");
 		sortedTreeSet.add("bb");
 		sortedTreeSet.add("cc");
 		sortedTreeSet.add("dd");
 		sortedTreeSet.add("ee");
 		
 		System.out.println(sortedTreeSet.size());//5个元素：5
 		System.out.println( sortedTreeSet.ceiling("cc"));//大于等于cc的最小值，不存在返回null：cc
 		System.out.println(sortedTreeSet.descendingSet());//返回Set的逆序视图：[ee, dd, cc, bb, aa]
 		System.out.println(sortedTreeSet.floor("cc"));//返回小于等于cc的元素的最大值，不存在返回null：cc
 		System.out.println( sortedTreeSet.headSet("cc"));//返回元素小于cc的元素：[aa,bb]
 		System.out.println( sortedTreeSet.headSet("cc", true));//返回元素小于等于cc的元素视图:[aa,bb,cc]
 		
 		System.out.println(sortedTreeSet.higher("cc"));//返回大于给定元素的最小元素:dd
 		System.out.println( sortedTreeSet.lower("cc"));//返回小于cc的最大元素:bb
 		System.out.println(sortedTreeSet.pollFirst());//移除第一个元素:aa
 		System.out.println(sortedTreeSet.pollLast());//移除最后一个元素:ee
 		System.out.println( sortedTreeSet.subSet("aa",true,"dd",true));//返回部分视图，true表示包括当前元素:[bb,cc,dd]
 		System.out.println( sortedTreeSet.subSet("bb","dd"));//返回部分视图包括前面的，不包括后面的:[bb,cc]
 		System.out.println( sortedTreeSet.tailSet("cc"));//返回元素大于cc的元素视图,包括cc:[cc,dd]
 		System.out.println( sortedTreeSet.tailSet("cc", false));//返回元素大于等于cc的元素视图:[dd]
 		System.out.println( sortedTreeSet.iterator());//返回set上的升序排序的迭代器
 		System.out.println( sortedTreeSet.descendingIterator());//返回set上的降序排序的迭代器
 		
 		System.out.println("=====================================");
 [ee, dd, cc, bb, aa]
 		System.out.println(sortedTreeSet.floor("cc"));//返回小于等于cc的元素的最大值，不存在返回null：cc
 		System.out.println( sortedTreeSet.headSet("cc"));//返回元素小于cc的元素：[aa,bb]
 		System.out.println( sortedTreeSet.headSet("cc", true));//返回元素小于等于cc的元素视图:[aa,bb,cc]
 		
 		System.out.println(sortedTreeSet.higher("cc"));//返回大于给定元素的最小元素:dd
 		System.out.println( sortedTreeSet.lower("cc"));//返回小于cc的最大元素:bb
 		System.out.println(sortedTreeSet.pollFirst());//移除第一个元素:aa
 		System.out.println(sortedTreeSet.pollLast());//移除最后一个元素:ee
 		System.out.println( sortedTreeSet.subSet("aa",true,"dd",true));//返回部分视图，true表示包括当前元素:[bb,cc,dd]
 		System.out.println( sortedTreeSet.subSet("bb","dd"));//返回部分视图包括前面的，不包括后面的:[bb,cc]
 		System.out.println( sortedTreeSet.tailSet("cc"));//返回元素大于cc的元素视图,包括cc:[cc,dd]
 		System.out.println( sortedTreeSet.tailSet("cc", false));//返回元素大于等于cc的元素视图:[dd]
 		System.out.println( sortedTreeSet.iterator());//返回set上的升序排序的迭代器
 		System.out.println( sortedTreeSet.descendingIterator());//返回set上的降序排序的迭代器
 		
 		System.out.println("=====================================");
 
 结果：

结果：

5
    
    cc
    
    [ee, dd, cc, bb, aa]
    
    cc
    
    [aa, bb]
    
    [aa, bb, cc]
    
    dd
    
    bb
    
    aa
    
    ee
    
    [bb, cc, dd]
    
    [bb, cc]
    
    [cc, dd]
    
    [dd]
    
    java.util.TreeMap$KeyIterator@2a139a55
    
    java.util.TreeMap$NavigableSubMap$DescendingSubMapKeyIterator@15db9742
    
    =====================================

## 2 NavigableMap接口 ##

public interface NavigableMap<K,V> extends SortedMap<K,V> \{ 
所有已知实现类： 
ConcurrentSkipListMap, TreeMap

NavigableMap扩展了 SortedMap，具有了针对给定搜索目标返回最接近匹配项的导航方法。方法 lowerEntry、floorEntry、ceilingEntry 和 higherEntry 分别返回与小于、小于等于、大于等于、大于给定键的键关联的 Map.Entry 对象，如果不存在这样的键，则返回 null。类似地，方法 lowerKey、floorKey、ceilingKey 和 higherKey 只返回关联的键。所有这些方法是为查找条目而不是遍历条目而设计的。  
  
       可以按照键的升序或降序访问和遍历 NavigableMap。descendingMap 方法返回映射的一个视图，该视图表示的所有关系方法和方向方法都是逆向的。升序操作和视图的性能很可能比降序操作和视图的性能要好。subMap、headMap 和 tailMap 方法与名称相似的 SortedMap 方法的不同之处在于：可以接受用于描述是否包括（或不包括）下边界和上边界的附加参数。任何 NavigableMap 的 Submap 必须实现 NavigableMap 接口。  
  
       此接口还定义了 firstEntry、pollFirstEntry、lastEntry 和 pollLastEntry 方法，它们返回和/或移除最小和最大的映射关系（如果存在），否则返回 null。  
  
      subMap(K, K)、headMap(K) 和 tailMap(K) 方法被指定为返回 SortedMap，以允许现有 SortedMap 实现能相容地改进为实现 NavigableMap，但鼓励此接口的扩展和实现重写这些方法以返回 NavigableMap。类似地，可以重写 SortedMap.keySet() 以返回 NavigableSet。

接口方法：

<table> 
 <tbody> 
 <tr> 
 <td colspan="2"> 方法摘要 </td> 
 </tr> 
 <tr> 
 <td> &nbsp;<a href="http://tool.oschina.net/uploads/apidocs/jdk-zh/java/util/Map.Entry.html" rel="nofollow">Map.Entry</a>&lt;<a href="http://tool.oschina.net/uploads/apidocs/jdk-zh/java/util/NavigableMap.html" rel="nofollow">K</a>,<a href="http://tool.oschina.net/uploads/apidocs/jdk-zh/java/util/NavigableMap.html" rel="nofollow">V</a>&gt; </td> 
 <td> <a href="https://blog.csdn.net/u010126792/article/details/62236367#ceilingEntry%28K%29" rel="nofollow">ceilingEntry</a>(<a href="http://tool.oschina.net/uploads/apidocs/jdk-zh/java/util/NavigableMap.html" rel="nofollow">K</a>&nbsp;key)&nbsp; &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;返回一个键-值映射关系，它与大于等于给定键的最小键关联；如果不存在这样的键，则返回&nbsp;null。 </td> 
 </tr> 
 <tr> 
 <td> &nbsp;<a href="http://tool.oschina.net/uploads/apidocs/jdk-zh/java/util/NavigableMap.html" rel="nofollow">K</a> </td> 
 <td> <a href="https://blog.csdn.net/u010126792/article/details/62236367#ceilingKey%28K%29" rel="nofollow">ceilingKey</a>(<a href="http://tool.oschina.net/uploads/apidocs/jdk-zh/java/util/NavigableMap.html" rel="nofollow">K</a>&nbsp;key)&nbsp; &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;返回大于等于给定键的最小键；如果不存在这样的键，则返回&nbsp;null。 </td> 
 </tr> 
 <tr> 
 <td> &nbsp;<a href="http://tool.oschina.net/uploads/apidocs/jdk-zh/java/util/NavigableSet.html" rel="nofollow">NavigableSet</a>&lt;<a href="http://tool.oschina.net/uploads/apidocs/jdk-zh/java/util/NavigableMap.html" rel="nofollow">K</a>&gt; </td> 
 <td> <a href="https://blog.csdn.net/u010126792/article/details/62236367#descendingKeySet%28%29" rel="nofollow">descendingKeySet</a>()&nbsp; &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;返回此映射中所包含键的逆序&nbsp;<a href="http://tool.oschina.net/uploads/apidocs/jdk-zh/java/util/NavigableSet.html" rel="nofollow">NavigableSet</a>&nbsp;视图。 </td> 
 </tr> 
 <tr> 
 <td> &nbsp;<a href="http://tool.oschina.net/uploads/apidocs/jdk-zh/java/util/NavigableMap.html" rel="nofollow">NavigableMap</a>&lt;<a href="http://tool.oschina.net/uploads/apidocs/jdk-zh/java/util/NavigableMap.html" rel="nofollow">K</a>,<a href="http://tool.oschina.net/uploads/apidocs/jdk-zh/java/util/NavigableMap.html" rel="nofollow">V</a>&gt; </td> 
 <td> <a href="https://blog.csdn.net/u010126792/article/details/62236367#descendingMap%28%29" rel="nofollow">descendingMap</a>()&nbsp; &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;返回此映射中所包含映射关系的逆序视图。 </td> 
 </tr> 
 <tr> 
 <td> &nbsp;<a href="http://tool.oschina.net/uploads/apidocs/jdk-zh/java/util/Map.Entry.html" rel="nofollow">Map.Entry</a>&lt;<a href="http://tool.oschina.net/uploads/apidocs/jdk-zh/java/util/NavigableMap.html" rel="nofollow">K</a>,<a href="http://tool.oschina.net/uploads/apidocs/jdk-zh/java/util/NavigableMap.html" rel="nofollow">V</a>&gt; </td> 
 <td> <a href="https://blog.csdn.net/u010126792/article/details/62236367#firstEntry%28%29" rel="nofollow">firstEntry</a>()&nbsp; &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;返回一个与此映射中的最小键关联的键-值映射关系；如果映射为空，则返回&nbsp;null。 </td> 
 </tr> 
 <tr> 
 <td> &nbsp;<a href="http://tool.oschina.net/uploads/apidocs/jdk-zh/java/util/Map.Entry.html" rel="nofollow">Map.Entry</a>&lt;<a href="http://tool.oschina.net/uploads/apidocs/jdk-zh/java/util/NavigableMap.html" rel="nofollow">K</a>,<a href="http://tool.oschina.net/uploads/apidocs/jdk-zh/java/util/NavigableMap.html" rel="nofollow">V</a>&gt; </td> 
 <td> <a href="https://blog.csdn.net/u010126792/article/details/62236367#floorEntry%28K%29" rel="nofollow">floorEntry</a>(<a href="http://tool.oschina.net/uploads/apidocs/jdk-zh/java/util/NavigableMap.html" rel="nofollow">K</a>&nbsp;key)&nbsp; &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;返回一个键-值映射关系，它与小于等于给定键的最大键关联；如果不存在这样的键，则返回&nbsp;null。 </td> 
 </tr> 
 <tr> 
 <td> &nbsp;<a href="http://tool.oschina.net/uploads/apidocs/jdk-zh/java/util/NavigableMap.html" rel="nofollow">K</a> </td> 
 <td> <a href="https://blog.csdn.net/u010126792/article/details/62236367#floorKey%28K%29" rel="nofollow">floorKey</a>(<a href="http://tool.oschina.net/uploads/apidocs/jdk-zh/java/util/NavigableMap.html" rel="nofollow">K</a>&nbsp;key)&nbsp; &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;返回小于等于给定键的最大键；如果不存在这样的键，则返回&nbsp;null。 </td> 
 </tr> 
 <tr> 
 <td> &nbsp;<a href="http://tool.oschina.net/uploads/apidocs/jdk-zh/java/util/SortedMap.html" rel="nofollow">SortedMap</a>&lt;<a href="http://tool.oschina.net/uploads/apidocs/jdk-zh/java/util/NavigableMap.html" rel="nofollow">K</a>,<a href="http://tool.oschina.net/uploads/apidocs/jdk-zh/java/util/NavigableMap.html" rel="nofollow">V</a>&gt; </td> 
 <td> <a href="https://blog.csdn.net/u010126792/article/details/62236367#headMap%28K%29" rel="nofollow">headMap</a>(<a href="http://tool.oschina.net/uploads/apidocs/jdk-zh/java/util/NavigableMap.html" rel="nofollow">K</a>&nbsp;toKey)&nbsp; &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;返回此映射的部分视图，其键值严格小于&nbsp;toKey。 </td> 
 </tr> 
 <tr> 
 <td> &nbsp;<a href="http://tool.oschina.net/uploads/apidocs/jdk-zh/java/util/NavigableMap.html" rel="nofollow">NavigableMap</a>&lt;<a href="http://tool.oschina.net/uploads/apidocs/jdk-zh/java/util/NavigableMap.html" rel="nofollow">K</a>,<a href="http://tool.oschina.net/uploads/apidocs/jdk-zh/java/util/NavigableMap.html" rel="nofollow">V</a>&gt; </td> 
 <td> <a href="https://blog.csdn.net/u010126792/article/details/62236367#headMap%28K,%20boolean%29" rel="nofollow">headMap</a>(<a href="http://tool.oschina.net/uploads/apidocs/jdk-zh/java/util/NavigableMap.html" rel="nofollow">K</a>&nbsp;toKey,&nbsp;boolean&nbsp;inclusive)&nbsp; &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;返回此映射的部分视图，其键小于（或等于，如果&nbsp;inclusive&nbsp;为&nbsp;true）toKey。 </td> 
 </tr> 
 <tr> 
 <td> &nbsp;<a href="http://tool.oschina.net/uploads/apidocs/jdk-zh/java/util/Map.Entry.html" rel="nofollow">Map.Entry</a>&lt;<a href="http://tool.oschina.net/uploads/apidocs/jdk-zh/java/util/NavigableMap.html" rel="nofollow">K</a>,<a href="http://tool.oschina.net/uploads/apidocs/jdk-zh/java/util/NavigableMap.html" rel="nofollow">V</a>&gt; </td> 
 <td> <a href="https://blog.csdn.net/u010126792/article/details/62236367#higherEntry%28K%29" rel="nofollow">higherEntry</a>(<a href="http://tool.oschina.net/uploads/apidocs/jdk-zh/java/util/NavigableMap.html" rel="nofollow">K</a>&nbsp;key)&nbsp; &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;返回一个键-值映射关系，它与严格大于给定键的最小键关联；如果不存在这样的键，则返回&nbsp;null。 </td> 
 </tr> 
 <tr> 
 <td> &nbsp;<a href="http://tool.oschina.net/uploads/apidocs/jdk-zh/java/util/NavigableMap.html" rel="nofollow">K</a> </td> 
 <td> <a href="https://blog.csdn.net/u010126792/article/details/62236367#higherKey%28K%29" rel="nofollow">higherKey</a>(<a href="http://tool.oschina.net/uploads/apidocs/jdk-zh/java/util/NavigableMap.html" rel="nofollow">K</a>&nbsp;key)&nbsp; &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;返回严格大于给定键的最小键；如果不存在这样的键，则返回&nbsp;null。 </td> 
 </tr> 
 <tr> 
 <td> &nbsp;<a href="http://tool.oschina.net/uploads/apidocs/jdk-zh/java/util/Map.Entry.html" rel="nofollow">Map.Entry</a>&lt;<a href="http://tool.oschina.net/uploads/apidocs/jdk-zh/java/util/NavigableMap.html" rel="nofollow">K</a>,<a href="http://tool.oschina.net/uploads/apidocs/jdk-zh/java/util/NavigableMap.html" rel="nofollow">V</a>&gt; </td> 
 <td> <a href="https://blog.csdn.net/u010126792/article/details/62236367#lastEntry%28%29" rel="nofollow">lastEntry</a>()&nbsp; &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;返回与此映射中的最大键关联的键-值映射关系；如果映射为空，则返回&nbsp;null。 </td> 
 </tr> 
 <tr> 
 <td> &nbsp;<a href="http://tool.oschina.net/uploads/apidocs/jdk-zh/java/util/Map.Entry.html" rel="nofollow">Map.Entry</a>&lt;<a href="http://tool.oschina.net/uploads/apidocs/jdk-zh/java/util/NavigableMap.html" rel="nofollow">K</a>,<a href="http://tool.oschina.net/uploads/apidocs/jdk-zh/java/util/NavigableMap.html" rel="nofollow">V</a>&gt; </td> 
 <td> <a href="https://blog.csdn.net/u010126792/article/details/62236367#lowerEntry%28K%29" rel="nofollow">lowerEntry</a>(<a href="http://tool.oschina.net/uploads/apidocs/jdk-zh/java/util/NavigableMap.html" rel="nofollow">K</a>&nbsp;key)&nbsp; &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;返回一个键-值映射关系，它与严格小于给定键的最大键关联；如果不存在这样的键，则返回&nbsp;null。 </td> 
 </tr> 
 <tr> 
 <td> &nbsp;<a href="http://tool.oschina.net/uploads/apidocs/jdk-zh/java/util/NavigableMap.html" rel="nofollow">K</a> </td> 
 <td> <a href="https://blog.csdn.net/u010126792/article/details/62236367#lowerKey%28K%29" rel="nofollow">lowerKey</a>(<a href="http://tool.oschina.net/uploads/apidocs/jdk-zh/java/util/NavigableMap.html" rel="nofollow">K</a>&nbsp;key)&nbsp; &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;返回严格小于给定键的最大键；如果不存在这样的键，则返回&nbsp;null。 </td> 
 </tr> 
 <tr> 
 <td> &nbsp;<a href="http://tool.oschina.net/uploads/apidocs/jdk-zh/java/util/NavigableSet.html" rel="nofollow">NavigableSet</a>&lt;<a href="http://tool.oschina.net/uploads/apidocs/jdk-zh/java/util/NavigableMap.html" rel="nofollow">K</a>&gt; </td> 
 <td> <a href="https://blog.csdn.net/u010126792/article/details/62236367#navigableKeySet%28%29" rel="nofollow">navigableKeySet</a>()&nbsp; &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;返回此映射中所包含键的&nbsp;<a href="http://tool.oschina.net/uploads/apidocs/jdk-zh/java/util/NavigableSet.html" rel="nofollow">NavigableSet</a>&nbsp;视图。 </td> 
 </tr> 
 <tr> 
 <td> &nbsp;<a href="http://tool.oschina.net/uploads/apidocs/jdk-zh/java/util/Map.Entry.html" rel="nofollow">Map.Entry</a>&lt;<a href="http://tool.oschina.net/uploads/apidocs/jdk-zh/java/util/NavigableMap.html" rel="nofollow">K</a>,<a href="http://tool.oschina.net/uploads/apidocs/jdk-zh/java/util/NavigableMap.html" rel="nofollow">V</a>&gt; </td> 
 <td> <a href="https://blog.csdn.net/u010126792/article/details/62236367#pollFirstEntry%28%29" rel="nofollow">pollFirstEntry</a>()&nbsp; &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;移除并返回与此映射中的最小键关联的键-值映射关系；如果映射为空，则返回&nbsp;null。 </td> 
 </tr> 
 <tr> 
 <td> &nbsp;<a href="http://tool.oschina.net/uploads/apidocs/jdk-zh/java/util/Map.Entry.html" rel="nofollow">Map.Entry</a>&lt;<a href="http://tool.oschina.net/uploads/apidocs/jdk-zh/java/util/NavigableMap.html" rel="nofollow">K</a>,<a href="http://tool.oschina.net/uploads/apidocs/jdk-zh/java/util/NavigableMap.html" rel="nofollow">V</a>&gt; </td> 
 <td> <a href="https://blog.csdn.net/u010126792/article/details/62236367#pollLastEntry%28%29" rel="nofollow">pollLastEntry</a>()&nbsp; &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;移除并返回与此映射中的最大键关联的键-值映射关系；如果映射为空，则返回&nbsp;null。 </td> 
 </tr> 
 <tr> 
 <td> &nbsp;<a href="http://tool.oschina.net/uploads/apidocs/jdk-zh/java/util/NavigableMap.html" rel="nofollow">NavigableMap</a>&lt;<a href="http://tool.oschina.net/uploads/apidocs/jdk-zh/java/util/NavigableMap.html" rel="nofollow">K</a>,<a href="http://tool.oschina.net/uploads/apidocs/jdk-zh/java/util/NavigableMap.html" rel="nofollow">V</a>&gt; </td> 
 <td> <a href="https://blog.csdn.net/u010126792/article/details/62236367#subMap%28K,%20boolean,%20K,%20boolean%29" rel="nofollow">subMap</a>(<a href="http://tool.oschina.net/uploads/apidocs/jdk-zh/java/util/NavigableMap.html" rel="nofollow">K</a>&nbsp;fromKey,&nbsp;boolean&nbsp;fromInclusive,&nbsp;<a href="http://tool.oschina.net/uploads/apidocs/jdk-zh/java/util/NavigableMap.html" rel="nofollow">K</a>&nbsp;toKey,&nbsp;boolean&nbsp;toInclusive)&nbsp; &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;返回此映射的部分视图，其键的范围从&nbsp;fromKey&nbsp;到&nbsp;toKey。 </td> 
 </tr> 
 <tr> 
 <td> &nbsp;<a href="http://tool.oschina.net/uploads/apidocs/jdk-zh/java/util/SortedMap.html" rel="nofollow">SortedMap</a>&lt;<a href="http://tool.oschina.net/uploads/apidocs/jdk-zh/java/util/NavigableMap.html" rel="nofollow">K</a>,<a href="http://tool.oschina.net/uploads/apidocs/jdk-zh/java/util/NavigableMap.html" rel="nofollow">V</a>&gt; </td> 
 <td> <a href="https://blog.csdn.net/u010126792/article/details/62236367#subMap%28K,%20K%29" rel="nofollow">subMap</a>(<a href="http://tool.oschina.net/uploads/apidocs/jdk-zh/java/util/NavigableMap.html" rel="nofollow">K</a>&nbsp;fromKey,&nbsp;<a href="http://tool.oschina.net/uploads/apidocs/jdk-zh/java/util/NavigableMap.html" rel="nofollow">K</a>&nbsp;toKey)&nbsp; &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;返回此映射的部分视图，其键值的范围从&nbsp;fromKey（包括）到&nbsp;toKey（不包括）。 </td> 
 </tr> 
 <tr> 
 <td> &nbsp;<a href="http://tool.oschina.net/uploads/apidocs/jdk-zh/java/util/SortedMap.html" rel="nofollow">SortedMap</a>&lt;<a href="http://tool.oschina.net/uploads/apidocs/jdk-zh/java/util/NavigableMap.html" rel="nofollow">K</a>,<a href="http://tool.oschina.net/uploads/apidocs/jdk-zh/java/util/NavigableMap.html" rel="nofollow">V</a>&gt; </td> 
 <td> <a href="https://blog.csdn.net/u010126792/article/details/62236367#tailMap%28K%29" rel="nofollow">tailMap</a>(<a href="http://tool.oschina.net/uploads/apidocs/jdk-zh/java/util/NavigableMap.html" rel="nofollow">K</a>&nbsp;fromKey)&nbsp; &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;返回此映射的部分视图，其键大于等于&nbsp;fromKey。 </td> 
 </tr> 
 <tr> 
 <td> &nbsp;<a href="http://tool.oschina.net/uploads/apidocs/jdk-zh/java/util/NavigableMap.html" rel="nofollow">NavigableMap</a>&lt;<a href="http://tool.oschina.net/uploads/apidocs/jdk-zh/java/util/NavigableMap.html" rel="nofollow">K</a>,<a href="http://tool.oschina.net/uploads/apidocs/jdk-zh/java/util/NavigableMap.html" rel="nofollow">V</a>&gt; </td> 
 <td> <a href="https://blog.csdn.net/u010126792/article/details/62236367#tailMap%28K,%20boolean%29" rel="nofollow">tailMap</a>(<a href="http://tool.oschina.net/uploads/apidocs/jdk-zh/java/util/NavigableMap.html" rel="nofollow">K</a>&nbsp;fromKey,&nbsp;boolean&nbsp;inclusive)&nbsp; &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;返回此映射的部分视图，其键大于（或等于，如果&nbsp;inclusive&nbsp;为&nbsp;true）fromKey。 </td> 
 </tr> 
 </tbody> 
</table>

示例代码：

NavigableMap<String, Integer> navigatorTreeMap = new TreeMap<String, Integer>(); // SortedMap接收TreeMap的实例
 		// 增加元素
 		navigatorTreeMap.put("aa", 11);
 		navigatorTreeMap.put("bb", 22);
 		navigatorTreeMap.put("cc", 33);
 		navigatorTreeMap.put("dd", 44);
 		navigatorTreeMap.put("ee", 55);
 		navigatorTreeMap.put("ff", 55);
 		navigatorTreeMap.put("gg", 55);
 
 		System.out.println(navigatorTreeMap.size());// 7个元素：7
 		System.out.println(navigatorTreeMap.ceilingKey("cc"));// 返回大于等于cc的最小键：cc
 		System.out.println(navigatorTreeMap.ceilingEntry("c"));//  返回一个键-值映射关系，它与大于等于cc的最小键关联：cc=33		
 System.out.println(navigatorTreeMap.descendingMap());// 返回逆序视图:{gg=55, ff=55, ee=55, dd=44, cc=33, bb=22, aa=11}
 		System.out.println(navigatorTreeMap.firstKey());// 最小键:aa
 		System.out.println(navigatorTreeMap.firstEntry());// 最小键对应的k-v键值对：aa=11
 
 		System.out.println(navigatorTreeMap.floorEntry("c"));// 返回一个键-值映射关系，它与小于等于给定键的最大键关联:bb=22
 		System.out.println(navigatorTreeMap.floorKey("cc"));//   返回小于等于cc的最大键:cc
 		System.out.println(navigatorTreeMap.headMap("bb"));// 返回此映射的部分视图，其键值严格小于bb:{aa=11}
 		System.out.println(navigatorTreeMap.headMap("bb", true));// 同上小于等于（true）:{aa=11, bb=22}
 		System.out.println(navigatorTreeMap.higherEntry("c"));// 返回一个键-值映射关系，它与小于等于给定键的最大键关联:cc=33
 		System.out.println(navigatorTreeMap.higherKey("cc"));//   返回小于等于cc的最大键:dd
 		System.out.println(navigatorTreeMap.lastEntry());// 返回一个键-值映射关系，它与小于等于给定键的最大键关联:gg=55
 		System.out.println(navigatorTreeMap.lastKey());//   返回小于等于cc的最大键:gg
 		System.out.println(navigatorTreeMap.lowerEntry("c"));// 返回一个键-值映射关系，它与小于等于给定键的最大键关联:bb=22
 		System.out.println(navigatorTreeMap.lowerKey("cc"));//    返回严格小于cc的最大键:bb
 		System.out.println(navigatorTreeMap.pollFirstEntry());//  移除并返回与此映射中的最小键关联的键-值映射关系:aa=11
 		System.out.println(navigatorTreeMap.pollLastEntry());//  移除并返回与此映射中的最大键关联的键-值映射关系:gg=55
 		System.out.println(navigatorTreeMap.navigableKeySet());//   返回此映射中所包含键的
 														// NavigableSet 视图。:[bb, cc, dd, ee, ff]
 
 		System.out.println(navigatorTreeMap.subMap("aa", true, "dd", true));// 返回部分视图，true表示包括当前元素键值对:{bb=22, cc=33, dd=44}
 		System.out.println(navigatorTreeMap.subMap("bb", "dd"));// 返回部分视图包括前面的元素，不包括后面的：{bb=22, cc=33}
 		System.out.println(navigatorTreeMap.tailMap("cc"));// 返回元素大于cc的元素映射视图,包括cc：//{cc=33, dd=44, ee=55, ff=55}		
 System.out.println(navigatorTreeMap.tailMap("cc", false));// 返回元素大于等于cc的元素映射视图:{dd=44, ee=55, ff=55}
 		System.out.println(navigatorTreeMap.descendingMap());// 返回此映射中所包含映射关系的逆序：{ff=55, ee=55, dd=44, cc=33, bb=22}视图。:
 		System.out.println(navigatorTreeMap.descendingKeySet());// 返回此映射中所包含键的逆序
 														// NavigableSet视图:[ff, ee, dd, cc, bb]

原理简单说明：

之所以可以去到第一个最后一个元素，或者某个元素的前一个，后一个，是因为集合内部的元素是有序的。

举例TreeSet：

public class TreeSet<E> extends AbstractSet<E> 
    implements NavigableSet<E>, Cloneable, java.io.Serializable

实现了NavigableSet接口，NavigableSet又继承了SortedSet接口，SortedSet内部有 Comparator<? super E> comparator();

TreeSet内部用NavigableMap来保存数据，分析代码可以内部其实用的treeMap存储数据，所有添加数据使用的也是treeMap的功能，TreeMap.put();

public V put(K key, V value) {
 Entry<K,V> t = root;
 if (t == null) {
 compare(key, key); // type (and possibly null) check
 
 root = new Entry<>(key, value, null);
 size = 1;
 modCount++;
 return null;
 }
 int cmp;
 Entry<K,V> parent;
 // split comparator and comparable paths
 Comparator<? super K> cpr = comparator;
 if (cpr != null) {
 do {
 parent = t;
 cmp = cpr.compare(key, t.key);
 if (cmp < 0)
 t = t.left;
 else if (cmp > 0)
 t = t.right;
 else
 return t.setValue(value);
 } while (t != null);
 }
 else {
 if (key == null)
 throw new NullPointerException();
 @SuppressWarnings("unchecked")
 Comparable<? super K> k = (Comparable<? super K>) key;
 do {
 parent = t;
 cmp = k.compareTo(t.key);
 if (cmp < 0)
 t = t.left;
 else if (cmp > 0)
 t = t.right;
 else
 return t.setValue(value);
 } while (t != null);
 }
 Entry<K,V> e = new Entry<>(key, value, parent);
 if (cmp < 0)
 parent.left = e;
 else
 parent.right = e;
 fixAfterInsertion(e);
 size++;
 modCount++;
 return null;
 }

每次插入一个数据都会利用比较函数进行key的比较，重新对数据进行排序，保存的数据是有序的，按序取数据也就不足为奇了。

结束！