Map集合的五种遍历方式及TreeMap方法

最新推荐文章于 2024-04-27 09:13:51 发布

学习微站公众平台

最新推荐文章于 2024-04-27 09:13:51 发布

阅读量1k

点赞数

分类专栏： SpringCloud 文章标签： java 开发语言后端

原文链接：https://blog.csdn.net/qq_30225725/article/details/88019622

版权

SpringCloud 专栏收录该内容

72 篇文章 2 订阅

订阅专栏

Map集合：链接: Map集合的五种遍历方式及Treemap方法
Set集合：链接: Java中遍历Set集合的三种方法
TreeSet集合：链接: Java深入了解TreeSet，和迭代器遍历方法
LIst集合：链接: Java中List集合的三种遍历方式(全网最详)
集合区别：链接: java中list，set，map集合的区别，及面试要点

//循环遍历map的方法
public class MapF {
 public static void main(String[] args) {
  Map<String, Integer> tempMap = new HashMap<String, Integer>();
  tempMap.put("a","12");
  tempMap.put("b","34");
  tempMap.put("c","56");
  // JDK1.4中
  // 遍历方法一 hashmap entrySet() 遍历
  Iterator it = tempMap.entrySet().iterator();
  while (it.hasNext()) {
   Map.Entry entry = (Map.Entry) it.next();
   Object key = entry.getKey();
   Object value = entry.getValue();
   System.out.println("key=" + key + " value=" + value);
  }
  System.out.println("");
  // JDK1.5中,应用新特性For-Each循环
  // 遍历方法二
  for (Map.Entry<String, Integer> entry : tempMap.entrySet()) {
   String key = entry.getKey().toString();
   String value = entry.getValue().toString();
   System.out.println("key=" + key + " value=" + value);
  }
  System.out.println("");
  // 遍历方法三 hashmap keySet() 遍历
  for (Iterator i = tempMap.keySet().iterator(); i.hasNext();) {
   Object obj = i.next();
   System.out.println(obj);// 循环输出key
   System.out.println("key=" + obj + " value=" + tempMap.get(obj));
  }
  for (Iterator i = tempMap.values().iterator(); i.hasNext();) {
   Object obj = i.next();
   System.out.println(obj);// 循环输出value
  }
  // 遍历方法四 treemap keySet()遍历
  for (Object o : tempMap.keySet()) {
   System.out.println("key=" + o + " value=" + tempMap.get(o));
  }
  System.out.println("11111");
  // java如何遍历Map <String, ArrayList> map = new HashMap <String,
  // ArrayList>();
  System.out.println("java  遍历Map <String, ArrayList> map = new HashMap<String, ArrayList>();");
  Map<String, ArrayList> map = new HashMap<String, ArrayList>();
  Set<String> keys = map.keySet();
  Iterator<String> iterator = keys.iterator();
  while (iterator.hasNext()) {
   String key = iterator.next();
   ArrayList arrayList = map.get(key);
   for (Object o : arrayList) {
    System.out.println(o);
   }
  }
  Map<String, List> map = new HashMap<String, List>();
  for (Map.Entry entry : map.entrySet()) {
   String key = entry.getKey().toString();
   List<String> list= (List) entry.getValue();
   for (String value : list) {
    System.out.println(key + "====" + value);
   }
  }
 }
}

 
 
 
 1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63

Map接口简介

今天来看一看map集合，map映射接口，用于存放键值对，<key,value>，通过key来查找value,顾名思义key不能为空，唯一且不重复，不然底层怎么查呢！

可以从图中看出Map为单独的接口，他和Collection有什么区别呢？

Map和Collection在集合中并列存在。　
Map集合是双列的，键值对，而Collection是单列集合
Map存储元素使用put方法，Collection使用Put方法。
Map遍历没有直接取出元素的方法，而是先转成Set集合，再通过迭代获取元素。
　--Map常用方法
　　
–Map应用
添加：使用HashMap。立了学生姓名和年龄之间的映射关系。并试图添加重复的键

复制代码
   public static void main(String[] args) {
        // 定义一个Map的容器对象  
        Map<String, Integer > map1 = new HashMap<String, Integer >();  
        map1.put("jack", 20);  
        map1.put("rose", 18);  
        map1.put("lucy", 17);  
        map1.put("java", 25);
      // map1.put("jack", 30)； 在没有hashCode和equals方式   添加重复的键值（值不同）,会覆盖掉前面key值相同的值
        System.out.println(map1);  
    Map<span class="token operator">&lt;</span>String<span class="token punctuation">,</span> Integer<span class="token operator">&gt;</span> map2 <span class="token operator">=</span> <span class="token keyword">new</span> <span class="token class-name">HashMap</span><span class="token operator">&lt;</span>String<span class="token punctuation">,</span> Integer<span class="token operator">&gt;</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>  
    map2<span class="token punctuation">.</span><span class="token function">put</span><span class="token punctuation">(</span><span class="token string">"张三丰"</span><span class="token punctuation">,</span> <span class="token number">100</span><span class="token punctuation">)</span><span class="token punctuation">;</span>  
    map2<span class="token punctuation">.</span><span class="token function">put</span><span class="token punctuation">(</span><span class="token string">"虚竹"</span><span class="token punctuation">,</span> <span class="token number">20</span><span class="token punctuation">)</span><span class="token punctuation">;</span>  
    System<span class="token punctuation">.</span>out<span class="token punctuation">.</span><span class="token function">println</span><span class="token punctuation">(</span><span class="token string">"map2:"</span> <span class="token operator">+</span> map2<span class="token punctuation">)</span><span class="token punctuation">;</span>  
    <span class="token comment">// 从指定映射中将所有映射关系复制到此映射中。  </span>
    map1<span class="token punctuation">.</span><span class="token function">putAll</span><span class="token punctuation">(</span>map2<span class="token punctuation">)</span><span class="token punctuation">;</span>  
    System<span class="token punctuation">.</span>out<span class="token punctuation">.</span><span class="token function">println</span><span class="token punctuation">(</span><span class="token string">"map1:"</span> <span class="token operator">+</span> map1<span class="token punctuation">)</span><span class="token punctuation">;</span>  
<span class="token punctuation">}</span>

复制代码

删除:

复制代码
   public static void main(String[] args) {   
　　 // 删除：  
        // remove() 删除关联对象，指定key对象  
        // clear() 清空集合对象  
    Map<span class="token operator">&lt;</span>String<span class="token punctuation">,</span> Integer<span class="token operator">&gt;</span> map1 <span class="token operator">=</span> <span class="token keyword">new</span> <span class="token class-name">HashMap</span><span class="token operator">&lt;</span>String<span class="token punctuation">,</span> Integer<span class="token operator">&gt;</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>  
    map1<span class="token punctuation">.</span><span class="token function">put</span><span class="token punctuation">(</span><span class="token string">"jack"</span><span class="token punctuation">,</span> <span class="token number">20</span><span class="token punctuation">)</span><span class="token punctuation">;</span>  
    map1<span class="token punctuation">.</span><span class="token function">put</span><span class="token punctuation">(</span><span class="token string">"rose"</span><span class="token punctuation">,</span> <span class="token number">18</span><span class="token punctuation">)</span><span class="token punctuation">;</span>  
    map1<span class="token punctuation">.</span><span class="token function">put</span><span class="token punctuation">(</span><span class="token string">"lucy"</span><span class="token punctuation">,</span> <span class="token number">17</span><span class="token punctuation">)</span><span class="token punctuation">;</span>  
    map1<span class="token punctuation">.</span><span class="token function">put</span><span class="token punctuation">(</span><span class="token string">"java"</span><span class="token punctuation">,</span> <span class="token number">25</span><span class="token punctuation">)</span><span class="token punctuation">;</span>  
    System<span class="token punctuation">.</span>out<span class="token punctuation">.</span><span class="token function">println</span><span class="token punctuation">(</span>map1<span class="token punctuation">)</span><span class="token punctuation">;</span>                 
    <span class="token comment">// 指定key，返回删除的键值对映射的值。  </span>
    map1<span class="token punctuation">.</span><span class="token function">remove</span><span class="token punctuation">(</span><span class="token string">"java"</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    System<span class="token punctuation">.</span>out<span class="token punctuation">.</span><span class="token function">println</span><span class="token punctuation">(</span>map1<span class="token punctuation">)</span><span class="token punctuation">;</span>  
    map1<span class="token punctuation">.</span><span class="token function">clear</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>  
    System<span class="token punctuation">.</span>out<span class="token punctuation">.</span><span class="token function">println</span><span class="token punctuation">(</span><span class="token string">"map1:"</span> <span class="token operator">+</span> map1<span class="token punctuation">)</span><span class="token punctuation">;</span>  
<span class="token punctuation">}</span>

复制代码

获取:

复制代码
public static void main(String[] args) {
     // 获取：  
        // V get(Object key) 通过指定的key对象获取value对象  
        // int size() 获取容器的大小  
        Map<String, Integer> map1 = new HashMap<String, Integer>();  
        map1.put("jack", 20);  
        map1.put("rose", 18);  
        map1.put("lucy", 17);  
        map1.put("java", 25);  
        System.out.println(map1);  
        // V get(Object key) 通过指定的key对象获取value对象  
        System.out.println("value:" + map1.get("jack"));  
        // int size() 获取容器的大小
        System.out.println("map.size:" + map1.size()); 
    } 
复制代码

判断:

复制代码
public static void main(String[] args) {
        // 判断：  
        // boolean isEmpty() 判断集合是否为空   长度为0返回true否则false  
        // boolean containsKey(Object key) 判断集合中是否包含指定的key  
        // boolean containsValue(Object value)  
    Map<span class="token operator">&lt;</span>String<span class="token punctuation">,</span> Integer<span class="token operator">&gt;</span> map1 <span class="token operator">=</span> <span class="token keyword">new</span> <span class="token class-name">HashMap</span><span class="token operator">&lt;</span>String<span class="token punctuation">,</span> Integer<span class="token operator">&gt;</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>  
    map1<span class="token punctuation">.</span><span class="token function">put</span><span class="token punctuation">(</span><span class="token string">"jack"</span><span class="token punctuation">,</span> <span class="token number">20</span><span class="token punctuation">)</span><span class="token punctuation">;</span>  
    map1<span class="token punctuation">.</span><span class="token function">put</span><span class="token punctuation">(</span><span class="token string">"rose"</span><span class="token punctuation">,</span> <span class="token number">18</span><span class="token punctuation">)</span><span class="token punctuation">;</span>  
    map1<span class="token punctuation">.</span><span class="token function">put</span><span class="token punctuation">(</span><span class="token string">"lucy"</span><span class="token punctuation">,</span> <span class="token number">17</span><span class="token punctuation">)</span><span class="token punctuation">;</span>  
    map1<span class="token punctuation">.</span><span class="token function">put</span><span class="token punctuation">(</span><span class="token string">"java"</span><span class="token punctuation">,</span> <span class="token number">25</span><span class="token punctuation">)</span><span class="token punctuation">;</span>  
    System<span class="token punctuation">.</span>out<span class="token punctuation">.</span><span class="token function">println</span><span class="token punctuation">(</span>map1<span class="token punctuation">)</span><span class="token punctuation">;</span>  
    System<span class="token punctuation">.</span>out<span class="token punctuation">.</span><span class="token function">println</span><span class="token punctuation">(</span><span class="token string">"isEmpty:"</span> <span class="token operator">+</span> map1<span class="token punctuation">.</span><span class="token function">isEmpty</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">)</span><span class="token punctuation">;</span>  
    System<span class="token punctuation">.</span>out<span class="token punctuation">.</span><span class="token function">println</span><span class="token punctuation">(</span><span class="token string">"containskey:"</span> <span class="token operator">+</span> map1<span class="token punctuation">.</span><span class="token function">containsKey</span><span class="token punctuation">(</span><span class="token string">"jack"</span><span class="token punctuation">)</span><span class="token punctuation">)</span><span class="token punctuation">;</span>  
    System<span class="token punctuation">.</span>out<span class="token punctuation">.</span><span class="token function">println</span><span class="token punctuation">(</span><span class="token string">"containsvalues:"</span> <span class="token operator">+</span> map1<span class="token punctuation">.</span><span class="token function">containsValue</span><span class="token punctuation">(</span><span class="token number">100</span><span class="token punctuation">)</span><span class="token punctuation">)</span><span class="token punctuation">;</span>  
<span class="token punctuation">}</span>

复制代码

遍历Map的4中方式：

第一种：

复制代码

public static void main(String[] args) {
        //遍历Map 第一种方式
        Map<String, Integer> map1 = new HashMap<String, Integer>();  
        map1.put("jack", 20);  
        map1.put("rose", 18);  
        map1.put("lucy", 17);  
        map1.put("java", 25);  
    <span class="token comment">//通过 map1.keySet() 获取key  通过key 找到value</span>
    <span class="token keyword">for</span> <span class="token punctuation">(</span>String key <span class="token punctuation">:</span> map1<span class="token punctuation">.</span><span class="token function">keySet</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">)</span> <span class="token punctuation">{<!-- --></span>
        Integer value <span class="token operator">=</span> map1<span class="token punctuation">.</span><span class="token keyword">get</span><span class="token punctuation">(</span>key<span class="token punctuation">)</span><span class="token punctuation">;</span>
        System<span class="token punctuation">.</span>out<span class="token punctuation">.</span><span class="token function">println</span><span class="token punctuation">(</span><span class="token string">"key : "</span><span class="token operator">+</span>key<span class="token operator">+</span><span class="token string">" value : "</span><span class="token operator">+</span>value<span class="token punctuation">)</span><span class="token punctuation">;</span>
    <span class="token punctuation">}</span>
<span class="token punctuation">}</span>

复制代码

第二种：

复制代码
public static void main(String[] args) {
        //遍历Map 第二种方式
        Map<String, Integer> map1 = new HashMap<String, Integer>();  
        map1.put("jack", 20);  
        map1.put("rose", 18);  
        map1.put("lucy", 17);  
        map1.put("java", 25);  
   <span class="token comment">//通过Map.Entry(String,Integer) 获取，然后使用entry.getKey()获取到键，通过entry.getValue()获取到值</span>
   <span class="token keyword">for</span><span class="token punctuation">(</span>Map<span class="token punctuation">.</span>Entry<span class="token operator">&lt;</span>String<span class="token punctuation">,</span> Integer<span class="token operator">&gt;</span> entry <span class="token punctuation">:</span> map1<span class="token punctuation">.</span><span class="token function">entrySet</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">)</span><span class="token punctuation">{<!-- --></span>
       System<span class="token punctuation">.</span>out<span class="token punctuation">.</span><span class="token function">println</span><span class="token punctuation">(</span><span class="token string">"键 key ："</span><span class="token operator">+</span>entry<span class="token punctuation">.</span><span class="token function">getKey</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token operator">+</span><span class="token string">" 值value ："</span><span class="token operator">+</span>entry<span class="token punctuation">.</span><span class="token function">getValue</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
   <span class="token punctuation">}</span>
<span class="token punctuation">}</span>

复制代码

第三种：

复制代码
//遍历Map 第三种方式
        Map<String, Integer> map1 = new HashMap<String, Integer>();  
        map1.put("jack", 20);  
        map1.put("rose", 18);  
        map1.put("lucy", 17);  
        map1.put("java", 25);  
        //第三种只遍历键或者值，通过加强for循环
        for(String s1:map1.keySet()){//遍历map的键
           System.out.println("键key ："+s1);
        }
        for(Integer s2:map1.values()){//遍历map的值
            System.out.println("值value ："+s2);
        }
           System.out.println("====================================");    
    }
复制代码

 
 
 
 1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17

第四种：

复制代码
public static void main(String[] args) {
        //遍历Map 第一种方式
        Map<String, Integer> map1 = new HashMap<String, Integer>();  
        map1.put("jack", 20);  
        map1.put("rose", 18);  
        map1.put("lucy", 17);  
        map1.put("java", 25);  
    <span class="token comment">//第四种Iterator遍历获取，然后获取到Map.Entry&lt;String, String&gt;，再得到getKey()和getValue()</span>
    Iterator<span class="token operator">&lt;</span>Map<span class="token punctuation">.</span>Entry<span class="token operator">&lt;</span>String<span class="token punctuation">,</span> Integer<span class="token operator">&gt;&gt;</span> it<span class="token operator">=</span>map1<span class="token punctuation">.</span><span class="token function">entrySet</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">.</span><span class="token function">iterator</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    <span class="token keyword">while</span><span class="token punctuation">(</span>it<span class="token punctuation">.</span><span class="token function">hasNext</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">)</span><span class="token punctuation">{<!-- --></span>
        Map<span class="token punctuation">.</span>Entry<span class="token operator">&lt;</span>String<span class="token punctuation">,</span> Integer<span class="token operator">&gt;</span> entry<span class="token operator">=</span>it<span class="token punctuation">.</span><span class="token function">next</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span> 
        System<span class="token punctuation">.</span>out<span class="token punctuation">.</span><span class="token function">println</span><span class="token punctuation">(</span><span class="token string">"键key ："</span><span class="token operator">+</span>entry<span class="token punctuation">.</span><span class="token function">getKey</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token operator">+</span><span class="token string">" value ："</span><span class="token operator">+</span>entry<span class="token punctuation">.</span><span class="token function">getValue</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    <span class="token punctuation">}</span>
    
<span class="token punctuation">}</span>

复制代码

HashMap

底层是哈希表数据结构，线程是不同步的，可以存入null键，null值。要保证键的唯一性，需要覆盖hashCode方法，和equals方法。
　　案例：自定义对象作为Map的键。

复制代码
    public class Demo3 {  
        public static void main(String[] args) {  
            HashMap<Person, String> hm = new HashMap<Person, String>();  
            hm.put(new Person("jack", 20), "1001");  
            hm.put(new Person("rose", 18), "1002");  
            hm.put(new Person("lucy", 19), "1003");  
            hm.put(new Person("hmm", 17), "1004");  
            hm.put(new Person("ll", 25), "1005");  
            System.out.println(hm);  
            System.out.println(hm.put(new Person("rose", 18), "1006"));  //重写hashCode和equalse后key相同不会覆盖
        Set<span class="token operator">&lt;</span>Entry<span class="token operator">&lt;</span>Person<span class="token punctuation">,</span> String<span class="token operator">&gt;&gt;</span> entrySet <span class="token operator">=</span> hm<span class="token punctuation">.</span><span class="token function">entrySet</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>  
        Iterator<span class="token operator">&lt;</span>Entry<span class="token operator">&lt;</span>Person<span class="token punctuation">,</span> String<span class="token operator">&gt;&gt;</span> it <span class="token operator">=</span> entrySet<span class="token punctuation">.</span><span class="token function">iterator</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>  
        <span class="token keyword">while</span> <span class="token punctuation">(</span>it<span class="token punctuation">.</span><span class="token function">hasNext</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">)</span> <span class="token punctuation">{<!-- --></span>  
            Entry<span class="token operator">&lt;</span>Person<span class="token punctuation">,</span> String<span class="token operator">&gt;</span> next <span class="token operator">=</span> it<span class="token punctuation">.</span><span class="token function">next</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>  
            Person key <span class="token operator">=</span> next<span class="token punctuation">.</span><span class="token function">getKey</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>  
            String value <span class="token operator">=</span> next<span class="token punctuation">.</span><span class="token function">getValue</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>  
            System<span class="token punctuation">.</span>out<span class="token punctuation">.</span><span class="token function">println</span><span class="token punctuation">(</span>key <span class="token operator">+</span> <span class="token string">" = "</span> <span class="token operator">+</span> value<span class="token punctuation">)</span><span class="token punctuation">;</span>  
        <span class="token punctuation">}</span>  
    <span class="token punctuation">}</span>  
<span class="token punctuation">}</span>  
  
<span class="token keyword">class</span> <span class="token class-name">Person</span> <span class="token punctuation">{<!-- --></span>  
    <span class="token keyword">private</span> String name<span class="token punctuation">;</span>  
    <span class="token keyword">private</span> int age<span class="token punctuation">;</span>  
  
    <span class="token function">Person</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token punctuation">{<!-- --></span>  
  
    <span class="token punctuation">}</span>  
  
    <span class="token keyword">public</span> <span class="token function">Person</span><span class="token punctuation">(</span>String name<span class="token punctuation">,</span> int age<span class="token punctuation">)</span> <span class="token punctuation">{<!-- --></span>  
        <span class="token keyword">this</span><span class="token punctuation">.</span>name <span class="token operator">=</span> name<span class="token punctuation">;</span>  
        <span class="token keyword">this</span><span class="token punctuation">.</span>age <span class="token operator">=</span> age<span class="token punctuation">;</span>  
    <span class="token punctuation">}</span>  
  
    <span class="token keyword">public</span> String <span class="token function">getName</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token punctuation">{<!-- --></span>  
        <span class="token keyword">return</span> name<span class="token punctuation">;</span>  
    <span class="token punctuation">}</span>  
  
    <span class="token keyword">public</span> <span class="token keyword">void</span> <span class="token function">setName</span><span class="token punctuation">(</span>String name<span class="token punctuation">)</span> <span class="token punctuation">{<!-- --></span>  
        <span class="token keyword">this</span><span class="token punctuation">.</span>name <span class="token operator">=</span> name<span class="token punctuation">;</span>  
    <span class="token punctuation">}</span>  
  
    <span class="token keyword">public</span> int <span class="token function">getAge</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token punctuation">{<!-- --></span>  
        <span class="token keyword">return</span> age<span class="token punctuation">;</span>  
    <span class="token punctuation">}</span>  
  
    <span class="token keyword">public</span> <span class="token keyword">void</span> <span class="token function">setAge</span><span class="token punctuation">(</span>int age<span class="token punctuation">)</span> <span class="token punctuation">{<!-- --></span>  
        <span class="token keyword">this</span><span class="token punctuation">.</span>age <span class="token operator">=</span> age<span class="token punctuation">;</span>  
    <span class="token punctuation">}</span>  
  
    @Override  
    <span class="token keyword">public</span> int <span class="token function">hashCode</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token punctuation">{<!-- --></span>  
  
        <span class="token keyword">return</span> <span class="token keyword">this</span><span class="token punctuation">.</span>name<span class="token punctuation">.</span><span class="token function">hashCode</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token operator">+</span> age <span class="token operator">*</span> <span class="token number">37</span><span class="token punctuation">;</span>  
    <span class="token punctuation">}</span>  
  
    @Override  
    <span class="token keyword">public</span> boolean <span class="token function">equals</span><span class="token punctuation">(</span>Object obj<span class="token punctuation">)</span> <span class="token punctuation">{<!-- --></span>  
        <span class="token keyword">if</span> <span class="token punctuation">(</span>obj <span class="token keyword">instanceof</span> <span class="token class-name">Person</span><span class="token punctuation">)</span> <span class="token punctuation">{<!-- --></span>  
            Person p <span class="token operator">=</span> <span class="token punctuation">(</span>Person<span class="token punctuation">)</span> obj<span class="token punctuation">;</span>  
            <span class="token keyword">return</span> <span class="token keyword">this</span><span class="token punctuation">.</span>name<span class="token punctuation">.</span><span class="token function">equals</span><span class="token punctuation">(</span>p<span class="token punctuation">.</span>name<span class="token punctuation">)</span> <span class="token operator">&amp;&amp;</span> <span class="token keyword">this</span><span class="token punctuation">.</span>age <span class="token operator">==</span> p<span class="token punctuation">.</span>age<span class="token punctuation">;</span>  
        <span class="token punctuation">}</span> <span class="token keyword">else</span> <span class="token punctuation">{<!-- --></span>  
            <span class="token keyword">return</span> <span class="token boolean">false</span><span class="token punctuation">;</span>  
        <span class="token punctuation">}</span>  
    <span class="token punctuation">}</span>  
  
    @Override  
    <span class="token keyword">public</span> String <span class="token function">toString</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token punctuation">{<!-- --></span>  
  
        <span class="token keyword">return</span> <span class="token string">"Person@name:"</span> <span class="token operator">+</span> <span class="token keyword">this</span><span class="token punctuation">.</span>name <span class="token operator">+</span> <span class="token string">" age:"</span> <span class="token operator">+</span> <span class="token keyword">this</span><span class="token punctuation">.</span>age<span class="token punctuation">;</span>  
    <span class="token punctuation">}</span>  
  
<span class="token punctuation">}</span>  
<span class="token punctuation">}</span>

复制代码

TreeMap

TreeMap的排序，TreeMap可以对集合中的键进行排序。如何实现键的排序？

方式一：元素自身具备比较性

和TreeSet一样原理，需要让存储在键位置的对象实现Comparable接口，重写compareTo方法，也就是让元素自身具备比较性，这种方式叫做　　元素的自然排序也叫做默认排序。

方式二：容器具备比较性

当元素自身不具备比较性，或者自身具备的比较性不是所需要的。那么此时可以让容器自身具备。需要定义一个类实现接口Comparator，重　　写compare方法，并将该接口的子类实例对象作为参数传递给TreeMap集合的构造方法。

注意：当Comparable比较方式和Comparator比较方式同时存在时，以Comparator的比较方式为主；

注意：在重写compareTo或者compare方法时，必须要明确比较的主要条件相等时要比较次要条件。（假设姓名和年龄一致的人为相同的人，　　如果想要对人按照年龄的大小来排序，如果年龄相同的人，需要如何处理？不能直接return 0，以为可能姓名不同（年龄相同姓名不同的人　　是不同的人）。此时就需要进行次要条件判断（需要判断姓名），只有姓名和年龄同时相等的才可以返回0.）

通过return 0来判断唯一性。

public class Demo4 { 
    public static void main(String[] args) { 
        TreeMap<String, Integer> tree = new TreeMap<String, Integer>(); 
        tree.put("张三", 19); 
        tree.put("李四", 20); 
        tree.put("王五", 21); 
        tree.put("赵六", 22); 
        tree.put("周七", 23); 
        tree.put("张三", 24); 
        System.out.println(tree); 
        System.out.println("张三".compareTo("李四"));//-2094 
    } 
} 

 
 
 
 1
2
3
4
5
6
7
8
9
10
11
12
13

自定义元素排序

复制代码
    public class Demo3 {  
        public static void main(String[] args) {  
            TreeMap<Person, String> hm = new TreeMap<Person, String>(  
                    new MyComparator());  
            hm.put(new Person("jack", 20), "1001");  
            hm.put(new Person("rose", 18), "1002");  
            hm.put(new Person("lucy", 19), "1003");  
            hm.put(new Person("hmm", 17), "1004");  
            hm.put(new Person("ll", 25), "1005");  
            System.out.println(hm);  
            System.out.println(hm.put(new Person("rose", 18), "1006"));  
        Set<span class="token operator">&lt;</span>Entry<span class="token operator">&lt;</span>Person<span class="token punctuation">,</span> String<span class="token operator">&gt;&gt;</span> entrySet <span class="token operator">=</span> hm<span class="token punctuation">.</span><span class="token function">entrySet</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>  
        Iterator<span class="token operator">&lt;</span>Entry<span class="token operator">&lt;</span>Person<span class="token punctuation">,</span> String<span class="token operator">&gt;&gt;</span> it <span class="token operator">=</span> entrySet<span class="token punctuation">.</span><span class="token function">iterator</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>  
        <span class="token keyword">while</span> <span class="token punctuation">(</span>it<span class="token punctuation">.</span><span class="token function">hasNext</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">)</span> <span class="token punctuation">{<!-- --></span>  
            Entry<span class="token operator">&lt;</span>Person<span class="token punctuation">,</span> String<span class="token operator">&gt;</span> next <span class="token operator">=</span> it<span class="token punctuation">.</span><span class="token function">next</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>  
            Person key <span class="token operator">=</span> next<span class="token punctuation">.</span><span class="token function">getKey</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>  
            String value <span class="token operator">=</span> next<span class="token punctuation">.</span><span class="token function">getValue</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>  
            System<span class="token punctuation">.</span>out<span class="token punctuation">.</span><span class="token function">println</span><span class="token punctuation">(</span>key <span class="token operator">+</span> <span class="token string">" = "</span> <span class="token operator">+</span> value<span class="token punctuation">)</span><span class="token punctuation">;</span>  
        <span class="token punctuation">}</span>  
    <span class="token punctuation">}</span>  
<span class="token punctuation">}</span>  
  
<span class="token keyword">class</span> <span class="token class-name">MyComparator</span> <span class="token keyword">implements</span> <span class="token class-name">Comparator</span><span class="token operator">&lt;</span>Person<span class="token operator">&gt;</span> <span class="token punctuation">{<!-- --></span>  
  
    @Override  
    <span class="token keyword">public</span> int <span class="token function">compare</span><span class="token punctuation">(</span>Person p1<span class="token punctuation">,</span> Person p2<span class="token punctuation">)</span> <span class="token punctuation">{<!-- --></span>  
        <span class="token keyword">if</span> <span class="token punctuation">(</span>p1<span class="token punctuation">.</span><span class="token function">getAge</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token operator">&gt;</span> p2<span class="token punctuation">.</span><span class="token function">getAge</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">)</span> <span class="token punctuation">{<!-- --></span>  
            <span class="token keyword">return</span> <span class="token operator">-</span><span class="token number">1</span><span class="token punctuation">;</span>  
        <span class="token punctuation">}</span> <span class="token keyword">else</span> <span class="token keyword">if</span> <span class="token punctuation">(</span>p1<span class="token punctuation">.</span><span class="token function">getAge</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token operator">&lt;</span> p2<span class="token punctuation">.</span><span class="token function">getAge</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">)</span> <span class="token punctuation">{<!-- --></span>  
            <span class="token keyword">return</span> <span class="token number">1</span><span class="token punctuation">;</span>  
        <span class="token punctuation">}</span>  
        <span class="token keyword">return</span> p1<span class="token punctuation">.</span><span class="token function">getName</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">.</span><span class="token function">compareTo</span><span class="token punctuation">(</span>p2<span class="token punctuation">.</span><span class="token function">getName</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">)</span><span class="token punctuation">;</span>  
    <span class="token punctuation">}</span>  
  
<span class="token punctuation">}</span>  
  
<span class="token keyword">class</span> <span class="token class-name">Person</span> <span class="token keyword">implements</span> <span class="token class-name">Comparable</span><span class="token operator">&lt;</span>Person<span class="token operator">&gt;</span> <span class="token punctuation">{<!-- --></span>  
    <span class="token keyword">private</span> String name<span class="token punctuation">;</span>  
    <span class="token keyword">private</span> int age<span class="token punctuation">;</span>  
  
    <span class="token function">Person</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token punctuation">{<!-- --></span>  
  
    <span class="token punctuation">}</span>  
  
    <span class="token keyword">public</span> <span class="token function">Person</span><span class="token punctuation">(</span>String name<span class="token punctuation">,</span> int age<span class="token punctuation">)</span> <span class="token punctuation">{<!-- --></span>  
  
        <span class="token keyword">this</span><span class="token punctuation">.</span>name <span class="token operator">=</span> name<span class="token punctuation">;</span>  
        <span class="token keyword">this</span><span class="token punctuation">.</span>age <span class="token operator">=</span> age<span class="token punctuation">;</span>  
    <span class="token punctuation">}</span>  
  
    <span class="token keyword">public</span> String <span class="token function">getName</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token punctuation">{<!-- --></span>  
        <span class="token keyword">return</span> name<span class="token punctuation">;</span>  
    <span class="token punctuation">}</span>  
  
    <span class="token keyword">public</span> <span class="token keyword">void</span> <span class="token function">setName</span><span class="token punctuation">(</span>String name<span class="token punctuation">)</span> <span class="token punctuation">{<!-- --></span>  
        <span class="token keyword">this</span><span class="token punctuation">.</span>name <span class="token operator">=</span> name<span class="token punctuation">;</span>  
    <span class="token punctuation">}</span>  
  
    <span class="token keyword">public</span> int <span class="token function">getAge</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token punctuation">{<!-- --></span>  
        <span class="token keyword">return</span> age<span class="token punctuation">;</span>  
    <span class="token punctuation">}</span>  
  
    <span class="token keyword">public</span> <span class="token keyword">void</span> <span class="token function">setAge</span><span class="token punctuation">(</span>int age<span class="token punctuation">)</span> <span class="token punctuation">{<!-- --></span>  
        <span class="token keyword">this</span><span class="token punctuation">.</span>age <span class="token operator">=</span> age<span class="token punctuation">;</span>  
    <span class="token punctuation">}</span>  
  
    @Override  
    <span class="token keyword">public</span> int <span class="token function">hashCode</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token punctuation">{<!-- --></span>  
  
        <span class="token keyword">return</span> <span class="token keyword">this</span><span class="token punctuation">.</span>name<span class="token punctuation">.</span><span class="token function">hashCode</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token operator">+</span> age <span class="token operator">*</span> <span class="token number">37</span><span class="token punctuation">;</span>  
    <span class="token punctuation">}</span>  
  
    @Override  
    <span class="token keyword">public</span> boolean <span class="token function">equals</span><span class="token punctuation">(</span>Object obj<span class="token punctuation">)</span> <span class="token punctuation">{<!-- --></span>  
        <span class="token keyword">if</span> <span class="token punctuation">(</span>obj <span class="token keyword">instanceof</span> <span class="token class-name">Person</span><span class="token punctuation">)</span> <span class="token punctuation">{<!-- --></span>  
            Person p <span class="token operator">=</span> <span class="token punctuation">(</span>Person<span class="token punctuation">)</span> obj<span class="token punctuation">;</span>  
            <span class="token keyword">return</span> <span class="token keyword">this</span><span class="token punctuation">.</span>name<span class="token punctuation">.</span><span class="token function">equals</span><span class="token punctuation">(</span>p<span class="token punctuation">.</span>name<span class="token punctuation">)</span> <span class="token operator">&amp;&amp;</span> <span class="token keyword">this</span><span class="token punctuation">.</span>age <span class="token operator">==</span> p<span class="token punctuation">.</span>age<span class="token punctuation">;</span>  
        <span class="token punctuation">}</span> <span class="token keyword">else</span> <span class="token punctuation">{<!-- --></span>  
            <span class="token keyword">return</span> <span class="token boolean">false</span><span class="token punctuation">;</span>  
        <span class="token punctuation">}</span>  
    <span class="token punctuation">}</span>  
  
    @Override  
    <span class="token keyword">public</span> String <span class="token function">toString</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token punctuation">{<!-- --></span>  
  
        <span class="token keyword">return</span> <span class="token string">"Person@name:"</span> <span class="token operator">+</span> <span class="token keyword">this</span><span class="token punctuation">.</span>name <span class="token operator">+</span> <span class="token string">" age:"</span> <span class="token operator">+</span> <span class="token keyword">this</span><span class="token punctuation">.</span>age<span class="token punctuation">;</span>  
    <span class="token punctuation">}</span>  
  
    @Override  
    <span class="token keyword">public</span> int <span class="token function">compareTo</span><span class="token punctuation">(</span>Person p<span class="token punctuation">)</span> <span class="token punctuation">{<!-- --></span>  
  
        <span class="token keyword">if</span> <span class="token punctuation">(</span><span class="token keyword">this</span><span class="token punctuation">.</span>age <span class="token operator">&gt;</span> p<span class="token punctuation">.</span>age<span class="token punctuation">)</span> <span class="token punctuation">{<!-- --></span>  
            <span class="token keyword">return</span> <span class="token number">1</span><span class="token punctuation">;</span>  
        <span class="token punctuation">}</span> <span class="token keyword">else</span> <span class="token keyword">if</span> <span class="token punctuation">(</span><span class="token keyword">this</span><span class="token punctuation">.</span>age <span class="token operator">&lt;</span> p<span class="token punctuation">.</span>age<span class="token punctuation">)</span> <span class="token punctuation">{<!-- --></span>  
            <span class="token keyword">return</span> <span class="token operator">-</span><span class="token number">1</span><span class="token punctuation">;</span>  
        <span class="token punctuation">}</span>  
        <span class="token keyword">return</span> <span class="token keyword">this</span><span class="token punctuation">.</span>name<span class="token punctuation">.</span><span class="token function">compareTo</span><span class="token punctuation">(</span>p<span class="token punctuation">.</span>name<span class="token punctuation">)</span><span class="token punctuation">;</span>  
    <span class="token punctuation">}</span>  
  
<span class="token punctuation">}</span>

复制代码

注意：Set的元素不可重复，Map的键不可重复，如果存入重复元素如何处理

Set元素重复元素不能存入add方法返回false

Map的重复健将覆盖旧键，将旧值返回。

学习微站公众平台

关注

0
点赞
踩
2

收藏

觉得还不错? 一键收藏
0
评论
Map集合的五种遍历方式及TreeMap方法

Map集合：链接: Map集合的五种遍历方式及Treemap方法 Set集合：链接: Java中遍历Set集合的三种方法 TreeSet集合：链接: Java深入了解TreeSet，和迭代器遍历方法 LIst集合：链接: Java中List集合的三种遍历方式(全网最详) 集合区别：链接: java中list，set，map集合的区别，及面试要点//循环遍历map的方法public class MapF { public static void main(String[] args) { Map&l
复制链接

扫一扫