Java集合-12

Map的接口特点

key重复时会进行替换

package com.edu.map;

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

@SuppressWarnings({"all"})
public class Demo01 {
    public static void main(String[] args) {
        Map map = new HashMap<>();
        map.put("no1","韩顺平");
        map.put("no2","张无忌");
        //1、k-v最后都是 HashMap$Node node = newNode(hash,key,value,null)
        //2、k-v为了方便遍历,还会创建 EntrySet集合,该集合存放的元素类型就是Entry,装的是Node<k v>地址指向
        // 而一个Entry对象就有key value EntrySet<Entry<K,V>> 也就是Set<Map.Entry<K,V>> entrySet
        //3、entrySet中，定义的类型是Map.Entry,但是实际上存放的还是HashMap$Node
        //  因为 static class Node<K,v> implements Map.Entry<K,V>
        //4、当把HashMap$Node 对象存放到 EntrySet 就方便我们的遍历，Map.Entry提供了两个重要的方
        // getkey() 和 getValue()
        //keySet和entrySet只是一个指向，并没有真实存储对象
        Set set = map.entrySet();
        System.out.println(set.getClass());//HashMap$EntrySet

        for(Object obj:set){
            System.out.println(obj.getClass());//HashMap$Node 接口实现 向上转型了
            //1、向下转型 因为是Object接受的
            Map.Entry entry = (Map.Entry)obj;
            System.out.println(entry.getKey()+":"+entry.getValue());
        }

    }
}

Map接口的常用方法?

Map的遍历方法

package com.edu.map;

import java.util.*;

public class Demo02 {
    @SuppressWarnings({"all"})
    public static void main(String[] args) {
        Map map = new HashMap();
        map.put("邓超","孙俪");
        map.put("王宝强","马蓉");
        map.put("宋杰","马蓉");
        map.put("六子","null");
        map.put("鹿晗","关晓彤");

        //第一组:先取出key,再通过key获取对应的value
        Set keyset = map.keySet();
        //(1)增强for
        for(Object obj:keyset){
            System.out.println(obj+":"+map.get(obj));
        }
        //(2)迭代器
        System.out.println("-----------------------------");
        Iterator iterator = keyset.iterator();
        while (iterator.hasNext()){
            Object next = iterator.next();
            System.out.println(next+"-"+map.get(next));
        }
        //第二组:取出所有的values
        Collection values = map.values();
        //(1) 增强for
        for (Object value:values){
            System.out.println(value);
        }
        //(2)迭代器

        //第三组:通过EntrySet来获取k-v
        Set entrySet = map.entrySet();
        //(1)增强for
        for (Object o:entrySet){
            Map.Entry obj = (Map.Entry)o;
            System.out.println(obj.getKey()+"--"+obj.getValue());
        }
        //(2)迭代器
        System.out.println("-----------------------------");
        Iterator iterator1 = entrySet.iterator();
        while (iterator1.hasNext()){
            Object next = iterator1.next();
            Map.Entry entry = (Map.Entry) next;
            System.out.println(entry.getKey()+"--"+entry.getValue());
        }
    }


}

根据value反向获取key

package com.edu.map;

import java.security.Key;
import java.util.*;


public class Demo03{
    @SuppressWarnings({"all"})
    public static void main(String[] args) {
        Map map = new HashMap();
        map.put("邓超","孙俪");
        map.put("王宝强","马蓉");
        map.put("宋杰","马蓉");
        map.put("六子","null");
        map.put("鹿晗","关晓彤");
        System.out.println(getKey01(map,"马蓉"));

    }
    //获取一个key 最后一个匹配的
    public static String getKey(Map<String,String> map,String str){
        String resultKey = null;
        for (String key:map.keySet()){
            if (map.get(key).equals(str)){
                resultKey = key;
            }
        }
        return resultKey;
    }

    //获取一个列表
    public static List<String> getKey01(Map<String,String> map,String str){
        List<String> list = new ArrayList<>();
        for (String key:map.keySet()){
            if (map.get(key).equals(str)){
                list.add(key);
            }
        }
        return list;
    }

}

Map排序

package com.edu.map;

import javax.swing.*;
import java.util.*;

@SuppressWarnings({"all"})
public class Demo04 {
    public static void main(String[] args) {
        Map<String,Integer> map = new HashMap<>();
        map.put("Apple",7299);
        map.put("Banana",1211);
        map.put("Cs",12122);
        map.put("Deep",121);
        map.put("Enda",2311);
        System.out.println(map);
        keySort(map);
        System.out.println();
        System.out.println(map);
        System.out.println("------------------------");
        valueSort(map);
        System.out.println("------------------------");
        valueSort02(map);

    }
    //按照key进行排序
    public static void keySort(Map<String,Integer> map){
        Set keySet = map.keySet();
        Object[] arr = keySet.toArray();
        Arrays.sort(arr);
        for (Object i:arr){
            System.out.print(i+"-"+map.get(i)+"\t");
        }
    }
    //按value进行排序
    //重写list.sort方法
    public static void valueSort(Map<String,Integer> map){
       Set entrySet = map.entrySet();
       List<Map.Entry<String,Integer>> list = new ArrayList<>(entrySet);
       list.sort(new Comparator<Map.Entry<String, Integer>>() {
           @Override
           public int compare(Map.Entry<String, Integer> o1, Map.Entry<String, Integer> o2) {
                return o1.getValue()-o2.getValue();
           }
       });
       for (int i =0;i<list.size();i++){
           System.out.println(list.get(i).getKey()+":"+list.get(i).getValue());
       }
    }
    // //重写Collections.sort方法
    public static void valueSort02(Map<String,Integer> map){
        Set entrySet = map.entrySet();
        List<Map.Entry<String,Integer>> list = new ArrayList<>(entrySet);
        Collections.sort(list,new Comparator<Object>(){
            @Override
            public int compare(Object O1,Object  O2){
               Map.Entry<String,Integer> o3 = (Map.Entry)O1;
               Map.Entry<String,Integer> O4 = (Map.Entry)O2;
               return o3.getValue()- O4.getValue();
            }
        });
        for (Map.Entry<String,Integer> mapping:list){
            System.out.println(mapping.getKey()+":"+mapping.getValue());
        }

    }
}

?

?