【Java基础18_3】TreeMap类

分手后的思念是犯贱 2022-06-07 14:15 183阅读 0赞

> **概述**

*  底层是**二叉树** ( 红黑树 ) 的数据结构
 *  TreeMap能够对键进行**排序**  
    
    
     *  按照**自然顺序排序**
     *  按照**比较器排序**

> **构造方法**

<table> 
 <thead> 
  <tr> 
   <th>构造方法</th> 
   <th>作用</th> 
  </tr> 
 </thead> 
 <tbody> 
  <tr> 
   <td>public TreeMap ( )</td> 
   <td>使用自然顺序对键进行排序</td> 
  </tr> 
  <tr> 
   <td>public TreeMap ( Comparator comparator )</td> 
   <td>使用比较器对键进行排序</td> 
  </tr> 
 </tbody> 
</table>

> **TreeSet自然排序Demo**

import java.util.Map.Entry;
    import java.util.Set;
    import java.util.TreeMap;
    /* * TreeMep的简单Demo */
    public class TreeMapDemo {
    
        public static void main(String[] args) {
    
            //创建TreeMap集合
            TreeMap<Integer,String> ts =new TreeMap<>();
    
            //往集合里放数据
            ts.put(1, "田馥甄");
            ts.put(5, "银临");
            ts.put(8, "谢春花");
            ts.put(11, "Taylor");
    
            //获取键值对集合
            Set<Entry<Integer, String>> entrySet = ts.entrySet();
    
            System.out.println("序号\t\t歌手");
    
            //遍历键值对分别获取键和值
            for (Entry<Integer, String> entry : entrySet) {
                System.out.println(entry.getKey()+"\t\t"+entry.getValue());
            }
        }
    
        /* * 结果： * 序号 歌手 1 田馥甄 5 银临 8 谢春花 11 Taylor */
    
    }

> **TreeSet按照比较器排序Demo**

*  由于排序是按照键排序，当键是自定义对象时，就需要使用TreeMap的有参构方法public TreeMap ( Comparator

package com.itbull.treeset;
    
    import java.util.Comparator;
    import java.util.Map.Entry;
    import java.util.Set;
    import java.util.TreeMap;
    
    public class TreeMapDemo2 { 
    
        public static void main(String[] args) {
    
            //创建自定义类对象
            Singer s1 = new Singer("田馥甄", "小幸运", 28);
            Singer s2 = new Singer("银临", "牵丝戏", 20);
            Singer s4 = new Singer("孙燕姿", "天黑黑", 22);
            Singer s3 = new Singer("Taylor", "TikTok", 26);
    
            //创建键为自定义类Singer的TreeMap对象
            TreeMap<Singer,String> tm =new TreeMap<>(new Comparator<Singer>() {
    
                @Override
                public int compare(Singer o1, Singer o2) {
    
                    //先比较姓名长度
                    int num = o1.getName().length()-o2.getName().length();
                    //再比较年龄
                    int num2=(num==0)?o1.getAge()-o2.getAge():num;
    
                    return num2;
                }
            });
    
            //将数据放入TreeMap
            tm.put(s1, "台湾");
            tm.put(s2, "大陆");
            tm.put(s3, "美国");
            tm.put(s4, "台湾");
    
            //获取Entry键值对集合
            Set<Entry<Singer, String>> entrySet = tm.entrySet();
    
            //遍历键值对获取键和值的数据
            for (Entry<Singer, String> entry : entrySet) {
    
                Singer key = entry.getKey();
    
                //获取键中的歌手信息
                System.out.println(key.getName()+"---"+key.getAge()+"---"+key.getSongName()+"-----"+entry.getValue());
            }
        }
    
    
    }

package com.itbull.treemap;
    
    //歌手类
    public class Singer { 
    private String name;
    private String songName;
    private int age;
    
    public Singer() {
        super();
    }
    public Singer(String name, String songName, int age) {
        super();
        this.name = name;
        this.songName = songName;
        this.age = age;
    }
    public String getName() {
        return name;
    }
    public void setName(String name) {
        this.name = name;
    }
    public String getSongName() {
        return songName;
    }
    public void setSongName(String songName) {
        this.songName = songName;
    }
    public int getAge() {
        return age;
    }
    public void setAge(int age) {
        this.age = age;
    }
    @Override
    public String toString() {
        return "Singer [name=" + name + ", songName=" + songName + ", age=" + age + "]";
    }
    
    }

> 统计字符串中字符出现次数（理解）

[Demo传送门][Demo]

[Demo]: http://note.youdao.com/noteshare?id=d59a29e03db48642a99cd703ba0ae8f7&sub=70568AE9EA7F41BB8807B756D433390A