JAVASE进阶day08（Map双列集合）

news2025/2/23 1:42:42

HashMap

1.HashMap基本使用

package com.lu.day08.map;

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

public class MapDome {
    public static void main(String[] args) {
        HashMap<String , String> map = new HashMap<>();
        //添加后者修改->如果key不存在则是添加，如果key已经存在就是修改
        map.put("曹雪芹","红楼梦");
        map.put("施耐庵","水浒传");
        map.put("吴承恩","西游记");
        map.put("杜甫","登高");
        System.out.println(map);
        //通过key获取值，如果key不存在则，获取到的值为null
        System.out.println(map.get("吴承恩"));
        System.out.println(map.get("李白"));
        map.remove("吴承恩");
        System.out.println("--------------");
        map.put("杜甫","琵琶行");
        System.out.println(map);

        HashMap<Integer, String> map1 = new HashMap<>();
        map1.put(1,"one");
        map1.put(2,"two");
        map1.put(3,"three");
        map1.put(4,"four");
        System.out.println(map1.containsKey(7));
        System.out.println(map1.containsValue("one"));

        System.out.println(map1.isEmpty());
        System.out.println(map.size());
        //获取键集合
        Set<String> strings = map.keySet();
        for (String key : strings) {
            System.out.println(key+"-->"+map.get(key));
        }
        System.out.println("-----------------");
        //entrySet 返回一个存放键值对set集合
        //entry是一个map接口中的内部类->通过put（k，v）放置的键值对，实际上存到了entry对象中->对象由HashMap中的内部类中的node实现
        //实现其中有两个属性（key,value）
        Set<Map.Entry<String, String>> entries = map.entrySet();
        for (Map.Entry<String, String> entry : entries) {
            System.out.println(entry.getKey()+"-->"+entry.getValue());
        }
        System.out.println("-----------------");
        //函数式的遍历方法forEach
        map.forEach((k,v)->{
            System.out.println(k+"-->"+v);
        });//只能key取value不能反过来
    }


}

TreeMap基本使用

1.TreeMap基本使用

package com.lu.day08.map;

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

public class TreeMapDemo {
    public static void main(String[] args) {//TreeMap只能对key排序
        Comparator<Integer> c = (o1,o2)->o2-o1;
        TreeMap<Integer, String> map = new TreeMap<>(c);
        map.put(1, "a");
        map.put(2, "b");
        map.put(3, "c");
        System.out.println(map);

    }
}

LinkedHashMap

package com.lu.day08.map;

import java.util.LinkedHashMap;
import java.util.LinkedHashSet;

public class LinkedHashMapDemo {
    public static void main(String[] args) {
        //保证存取有序
        LinkedHashMap<Integer, String> map = new LinkedHashMap<>();
        map.put(1, "张三");
        map.put(2, "李四");
        map.put(3, "王五");
        map.put(4, "赵六");
        map.forEach((k, v) -> {
            System.out.println(k + ":" + v);
        });
        LinkedHashSet<Integer> set = new LinkedHashSet<>();
        set.add(1);
        set.add(2);
        set.add(3);
        set.forEach(System.out::println);

    }
}

集合嵌套

package com.lu.day08.map;

import java.util.ArrayList;
import java.util.HashMap;
import java.util.List;
import java.util.Map;

public class Test {
    public static void main(String[] args) {
        HashMap<String, List<String>> map = new HashMap<>();
        ArrayList<String> xiaomi = new ArrayList<>();
        xiaomi.add("小米1");
        xiaomi.add("小米2");
        xiaomi.add("小米3");
        map.put("小米", xiaomi);

        ArrayList<Map<String,Double>> list = new ArrayList<>();
        HashMap<String, Double> clothMap = new HashMap<>();
        clothMap.put("耐克",2300.0);
        clothMap.put("贵人鸟",7800.0);
        clothMap.put("凡客",50.0);
    }
}

Collections

1.集合辅助工具类

package com.lu.day08.collections;

import java.util.ArrayList;
import java.util.Collections;

public class CollectionsDemo {
    public static void main(String[] args) {
        ArrayList<Integer> list = new ArrayList<>();
        list.add(1);
        list.add(5);
        list.add(2);
        list.add(4);
        list.add(3);
        System.out.println(list);
        // 反转
        Collections.reverse(list);
        System.out.println(list);
        // 排序
        Collections.sort(list);
        System.out.println(list);
        // 随机
        Collections.shuffle(list);
        System.out.println(list);

    }
}

可变参数

package com.lu.day08.collections;

/**
 * 可变参数
 */
public class VaryParam {
    public static void main(String[] args) {
        print(1,2,3,4,5,6,7,8,9);
    }
    public static void print(int... a){
        for (int j : a) {
            System.out.println(j);
        }
    }
}

不可变集合

package com.lu.day08.collections;


import java.util.List;
import java.util.Map;
import java.util.Set;

/**
 * jdk9新增不可变集合
 */
public class Test {
    public static void main(String[] args) {
        //创建list不可变集合
        List<Integer> integers = List.of(1, 2, 3);
        integers.forEach(System.out::println);
        //创建set不可变集合
        Set<Integer> integers1 = Set.of(1, 2, 3);
        System.out.println(integers1);
        //创建map不可变集合
        Map<String, Integer> a = Map.of("A", 1, "B", 2, "C", 3);
        System.out.println(a);
    }
}