目录

常用方法和遍历方式

 HashTable

Properties 

如何选择集合实现类 

 TreeMap

结构图

package com.map_.Map_;

import java.util.HashMap;

public class Demo {
    public static void main(String[] args) {
        HashMap map = new HashMap();
        map.put("no1","张三");//ok
        map.put("no2","李四");//ok
        map.put("no3","王五");//ok
        map.put("no3","王五");//no
        map.put(1,"呜啊");//key和valueCan be any reference type data
        map.put("no3","马六");//key相同时 Equivalence and substitution
        map.put(null,null);// Map 的key 和value可以为null
        map.put("no4",null);//key只能有一个是null 但value可以有多个为null
        System.out.println(map);
        System.out.println(map.get("no2"));//可以通过key得到value
    }
}

常用方法和遍历方式

package com.map_.Map_;

import java.util.*;

//Map接口方法 六大遍历方法
public class Demo02 {
    public static void main(String[] args) {
        //添加 put
        HashMap map = new HashMap();
        map.put("no1","张三");
        map.put("no2","李四");
        map.put("no3","王五");
        map.put("no4",new People("马六"));
        map.put(null,"刘七");
        System.out.println(map);
        //删除 remove
        map.remove("no3");
        System.out.println(map);
        //获取 get
        System.out.println(map.get("no2"));
        //获取元素个数 size
        System.out.println(map.size());
        //清空 clear
        //map.clear();
        //查找键是否存在
        System.out.println(map.containsKey("no1"));
        //Find if the value exists
        System.out.println(map.containsValue("李四"));

        //遍历方法
        //第一组：先取出所有的key,通过key取出对应的Value
        Set keyset = map.keySet();
        //1.增强for
        System.out.println("========增强for========");
        for (Object key :keyset) {
            System.out.println(key +"-"+map.get(key));
        }
        //2.迭代器
        System.out.println("=========迭代器============");
        Iterator iterator = keyset.iterator();
        while (iterator.hasNext()) {
            Object key =  iterator.next();
            System.out.println(key + "-" + map.get(key));
        }

        //第二组 取出所有的值 用collocation的遍历方法
        Collection values = map.values();
        System.out.println("======The second set of iterators==========");
        Iterator iterator1 = values.iterator();
        while (iterator1.hasNext()) {
            Object obj =  iterator1.next();
            System.out.println(obj);
        }
        System.out.println("=========The second group of enhancementsfor======");
        for (Object o : values) {
            System.out.println(o);
        }
        //第三组 通过EntrySet 来获取 k-v
        Set entrySet = map.entrySet();
        //增强for
        System.out.println("=======使用entrySet 的增强for=======");
        for (Object entry :entrySet) {
            //将entry 转成 Map.Entry
           Map.Entry m = (Map.Entry)entry;
            System.out.println(m.getKey()+"-"+m.getValue());
        }
        //迭代器
        System.out.println("========使用entrySet的迭代器");
        Iterator iterator2 = entrySet.iterator();
        while (iterator2.hasNext()) {
            Object o =  iterator2.next();
            System.out.println(o);

        }


    }
}
class People{
    private String name;

    public People(String name) {
        this.name = name;
    }

    public String getName() {
        return name;
    }

    public void setName(String name) {
        this.name = name;
    }

    @Override
    public String toString() {
        return "People{" +
                "name='" + name + '\'' +
                '}';
    }


}

小结

 HashTable

底层有数组 Hashtable$Entry[]初始化大小为11

临界值 threshold 8 = 11*0.75

底层使用addEntry（hash k,value index）添加键值对.

扩容机制：when the critical value is reached,Shift the old array one bit to the left+1（两倍+1）

Properties 

继承了hashTable

通过k-v存放数据,key和value不能为空.

 由于继承HashTable类,使用方法与HashTable相差不大.

如何选择集合实现类 

 TreeMap

TreeSet和TreeMap的区别,TreeSet中的e放的是key, presentIt is a static object,

TreeMap,e放的是key,prent位置放的是value.

排序方法 

和TreeSetThere is also no ordering in the case of no-argument constructors 

The default is ascending alphabetical order  ？

比较方法和TreeSet一样,You can write your own comparison method.比如字符串长度.length（）.

package com.map_.Map_;

import java.util.Comparator;
import java.util.TreeMap;

public class TreeMap_ {
    public static void main(String[] args) {
        //按照传入的Key（String） 的大小进行排序
        TreeMap treeMap = new TreeMap(new Comparator() {
            @Override
            public int compare(Object o1, Object o2) {
                return ((String)o1).compareTo((String)o2);
            }
        });
        treeMap.put("jack","杰克");
        treeMap.put("tom","汤姆");
        treeMap.put("kristina","克瑞斯提诺");
        treeMap.put("smith","史密斯");
        System.out.println(treeMap);
    }
}