Map接口

　　实现Map接口的类用来存储键—值 对。

Map接口的实现类有HashMap和TreeMap等。

Map类中存储的键—值通过键来标识，所以键值不能重复

Object put (Object key, Object value）//返回值是旧的value值， 如果传入的key已经存在的话，新的value会替换旧的value

Object get (Object key) ;//拿到相对应的value对象

Object remove (Object key) ;//移除相对应的value对象

boolean containsKey (Object key) ;//是否包含这个key

boolean containsValue（Object value）//是否包含这个值

int size （）；//一共多少对对象

boolean isEmpty（）；是不是空的

void putAll(Map t);把另外一个Map全部的东西都加进来

void clear（）；//清除

 

Map集合遍历的方式1

package com.liushuaishuai;
/*
Map集合的遍历（方式1）
    1获取所有键的集合用keySet
    2遍历键的集合，获取每一个键，用增强for
    3根据键去找值，用get（Object key）方法实现
 */

import java.util.HashMap;
import java.util.Map;
import java.util.Set;

public class MapDemo01 {
    public static void main(String[] args) {
        //创建集合对象
        Map<String,String> map = new HashMap<>();
        
        //添加元素
        map.put("杨过","小龙女");
        map.put("郭靖","黄蓉");
        map.put("张无忌","周芷若");
        //获取所有的键组成的集合，用keySet（）方法实现
        Set<String> keyset = map.keySet();
        
//        遍历上述得到的键的集合，使用增强for
        for(String s:keyset) {
            //根据键找值，使用get（Object key）方法实现
            String s1 = map.get(s);
            System.out.println(s+","+s1);
        }

    }
}

 Map集合遍历方式2

 

 

 

 

例子如下：

import java.util.List;
import java.util.LinkedList;
import java.util.Collections;
import java.util.*;
public class Test {
	public static void main(String[] args) {
		Map m1 = new HashMap();Map m2 = new TreeMap();
		m1.put("one",new Integer(1));
		m1.put("two",new Integer(2));
		m1.put("three",new Integer(3));
		m2.put("A",new Integer(1));
		m2.put("B",new Integer(2));
		System.out.println(m1.size());
		System.out.println(m1.containsKey("one"));
		System.out.println(m2.containsValue(new Integer(1)));
		if(m1.containsKey("two")) {
			int i = ((Integer)m1.get("two")).intValue();
			System.out.println(i);
		}
		Map m3 = new HashMap(m1);
		m3.putAll(m2);
		System.out.println(m3);
		
		
		
		
		
		
		
		
		
		
		
		
		/*List l1 = new LinkedList();
		l1.add(new Name("Karl","M"));
		l1.add(new Name("Steven","Lee"));
		l1.add(new Name("John","O"));
		l1.add(new Name("Tom","M"));
		System.out.println(l1);
		Collections.sort(l1);
		System.out.println(l1);*/
		
		
		
		
		
		
		
		
		
		/*
		Set s = new HashSet();
		s.add("hello");
		s.add("world");
		s.add(new Name("f1","11"));
		s.add(new Integer(100));
		*/
		
		/*
		
		s.add("hello");
		s.add("hello");
		*/
		//Set 
		/*
		Set s1 = new HashSet();
		Set s2 = new HashSet();
		s1.add("a");s1.add("b");s1.add("c");
		s2.add("d");s2.add("a");s2.add("b");
		Set sn = new HashSet(s1);
		sn.retainAll(s2);
		Set su = new HashSet(s1);
		su.addAll(s2);
		
		System.out.println(sn);
		
		System.out.println(su);
		*/
		
		
		
		
		
		
		
		
		/*
		Collection c = new HashSet();
		c.add("hello");
		c.add(new Name("f1","11"));
		c.add(new Name("f2","12"));
		c.add(new Name("f3","13"));
		c.add(new Integer(100));
		c.remove("hello");
		c.remove(new Integer(100));
		
		Iterator i = c.iterator();
		while(i.hasNext()) {
			Name n = (Name)i.next();
			System.out.print(n.getfirstName()+" ");
		}*/
		/*System.out.println(c.remove(new Name("f1","11")));
		System.out.println(c);*/
	}
}
class Name implements Comparable {
	private String firstName,secondName;
	public Name(String firstName,String secondName) {
		this.firstName = firstName;
		this.secondName = secondName;
	}
	public String getfirstName() {return firstName;}
	public String getsecondName() {return secondName;}
	public String toString() {
		return firstName+" "+secondName;
	}
	
	public boolean equals(Object obj) {
		if(obj instanceof Name) {
			Name name = (Name) obj;
			return (firstName.equals(name.firstName))&&(secondName.equals(name.secondName));
		}
		return super.equals(obj);
	}
	
	public int hashCode() {
		return firstName.hashCode();
	}
	
	public int compareTo(Object o) {
		Name n = (Name) o;
		int lastCmp = 
				secondName.compareTo(n.secondName);
		return
					(lastCmp!=0 ? lastCmp:firstName.compareTo(n.firstName));
	}
}

 用hashmap来统计字符串中字母出现的次数

package com.maptest01;

import java.util.HashMap;
import java.util.Scanner;
import java.util.Set;

public class staticchar {
    public static void main(String[] args) {
        Scanner sc = new Scanner(System.in);
        System.out.println("请输入一个字符串");
        String s = sc.nextLine();
        HashMap<Character, Integer> hm1 = new HashMap<Character, Integer>();


        for (int i = 0; i < s.length(); i++) {
            char key = s.charAt(i);
            Integer value = hm1.get(key);
            if (value == null) {
                hm1.put(key, 1);

            } else {
                value++;
                hm1.put(key, value);
            }
        }
        Set<Character> keyset = hm1.keySet();
        for (Character x : keyset) {
            Character key = x;
            Integer value = hm1.get(key);
            System.out.print(key + "(" + value + ")");
        }
    }
}