[数据结构与算法]Map接口集合

1、总体结构

2、特点

       2.1、Map接口存储的形式是<K,V>
       2.2、LinkedHashMap的底层是哈希表加链表，能保证存取顺序
       2.3、HashTable和HashMap的区别是：
                     <1>、HashTable是线程安全的，KV值不能为null
                     <2>、HashMap是线程不安全的，KV值可以有null

3、常用的四个方法：

   put(),    get(),    remove(),       containsKey()

  public static void main(String[] args) {
        demo01();
    }

    //测试map集合的四个常用方法
    public static void demo01() {
        HashMap<String, Integer> hashMap = new HashMap<>();
        //1、put()
        hashMap.put("小李",23);
        hashMap.put("小张",28);
        hashMap.put("小王",14);
        System.out.println(hashMap);

        //2、get()
        Integer integer = hashMap.get("小李");
        System.out.println("获取小李的年龄为："+integer);

        //3、containsKey()
        boolean b = hashMap.containsKey("小张");
        System.out.println("是否包含小张:"+b);

        //4、remove()
        Integer remove = hashMap.remove("小王");
        System.out.println("被移除的小王的年龄是："+remove);

4、Map集合的遍历方式

      <1>通过keySet()，先拿到key，再通过key拿到value
      <2>entrySet()，把<K,V>看作一个整体拿出来

4.1通过KeySet()遍历Map集合

    public class demoForMap_keySet {
    public static void main(String[] args) {
        demo01();
    }
    public static void demo01() {
        HashMap<String, Integer> hashMap = new HashMap<>();
        hashMap.put("小李",23);
        hashMap.put("小张",28);
        hashMap.put("小王",14);
        //通过keySet遍历Map集合
        Set<String> strings = hashMap.keySet();
        Iterator<String> iterator = strings.iterator();
        while (iterator.hasNext()){
            String key = iterator.next();
            Integer value = hashMap.get(key);
            System.out.println(key+".."+value);
        }
    }
}

4.2通过EntrySet()遍历Map集合

public class demoForMap_EntrySet {
    public static void main(String[] args) {
        demo01();
    }
    public static void demo01() {
        HashMap<String, Integer> hashMap = new HashMap<>();
        hashMap.put("小李",23);
        hashMap.put("小张",28);
        hashMap.put("小王",14);
        //通过EntrySet遍历Map集合
        Set<Map.Entry<String, Integer>> entrySet = hashMap.entrySet();
        Iterator<Map.Entry<String, Integer>> iterator = entrySet.iterator();
        while (iterator.hasNext()){
            Map.Entry<String, Integer> entry = iterator.next();
            String key = entry.getKey();
            Integer value = entry.getValue();
            System.out.println(key+".."+value);
        }
    }
}

4.3、存储第三方元素作为key,为保证Key的唯一性，第三方对象要重写hashCode()和equels()

public class DemoFor_Map_EntrySet {
    public static void main(String[] args) {
        demo01();
    }
    public static void demo01() {
        HashMap<person, String> hashMap = new HashMap<>();
        hashMap.put(new person("张三",23),"上海");
        hashMap.put(new person("李四",24),"北京");
        hashMap.put(new person("王五",25),"成都");
        //通过EntrySet遍历
        Set<Map.Entry<person, String>> entries = hashMap.entrySet();
        Iterator<Map.Entry<person, String>> iterator = entries.iterator();
        while (iterator.hasNext()){
            Map.Entry<person, String> next = iterator.next();
            System.out.println(next.getKey()+next.getValue());
        }
    }
}

public class person {
    @Override
    public boolean equals(Object o) {
        if (this == o) return true;
        if (o == null || getClass() != o.getClass()) return false;
        person person = (person) o;
        return Objects.equals(name, person.name) &&
                Objects.equals(age, person.age);
    }
    @Override
    public int hashCode() {
        return Objects.hash(name, age);
    }
}

5、注意点

  Map中<K，V>的K唯一，当两个K相同，而V不同时，后来的V会覆盖之气的V