【Java】ArrayList、HashMap、HashSet

ArrayList

1. Add to
2. insert
3. access
4. modify
5. delete
6. Traverse

package com.itheima;

import java.util.ArrayList;
import java.util.Iterator;

public class Main {
    
    
    public static void main(String[] args) {
    
    
        ArrayList<String> c1 = new ArrayList<>();
        c1.add("Red");
        c1.add("Green");
        c1.add("Black");
        c1.add("White");
        c1.add("Pink");//添加
        System.out.println(c1);

        //一、遍历
        for(int i=0;i<c1.size();i++){
    
    
            System.out.println(c1.get(i)); // 访问
        }

        //二、强循环遍历
        for(String str:c1)
            System.out.println(str);

        //迭代器遍历
        Iterator<String> iterator = c1.iterator();
        while (iterator.hasNext())
            System.out.println(iterator.next());
    }
}

Copy an array list to another array list: Collections.copy(List1, List2);

Array list swaps two elements: Collections.swap(c1, 0, 2);

Connect two array lists: c2.addAll(c1);

Clone an array list to another array list: ArrayList newc1 = (ArrayList)c1.clone();

Clear the array list: c1.removeAll(c1);

Extract part of the array list: List sub_List = list_Strings.subList(0, 3);

HashMap

1. Add to
2. access
3. modify
4. delete
5. Traverse

package com.itheima;

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

public class Main {
    
    
    public static void main(String[] args) {
    
    
        HashMap<Integer, String> map = new HashMap<>();
        map.put(1,"Red");
        map.put(2,"Green");
        map.put(3,"Black");
        map.put(4,"White");
        map.put(5,"Blue"); // 添加

        for(Integer i:map.keySet()){
    
     //遍历键集合
            System.out.print("键："+i + ",");
            System.out.println("值：" + map.get(i)); //通过键访问值
        }

        for(String str:map.values()){
    
     //遍历值集合
            System.out.println("值：" + str);
        }

        for(Map.Entry x : map.entrySet()){
    
    
            System.out.println("键："+x.getKey() +" 值：" + x.getValue() );
        }
    }
}

Copy all maps from the specified map to another map: hash_map2.putAll(hash_map1);

Delete all mappings in the mapping: hash_map.clear();

Get a shallow copy of the HashMap instance: new_hash_map = (HashMap)hash_map.clone();

HashSet

1. Add to
2. delete
3. Traverse
4. contain

package com.itheima;


import java.util.HashSet;
import java.util.Iterator;

public class Main {
    
    
    public static void main(String[] args) {
    
    
        HashSet<String> set = new HashSet<>();
        set.add("Red");
        set.add("Green");
        set.add("Black");
        set.add("White");
        set.add("Pink");
        set.add("Yellow"); // 添加
        System.out.println(set);
        for (String str : set){
    
    
            System.out.println(str);
        }
        Iterator<String> iterator = set.iterator();
        while (iterator.hasNext())
            System.out.println(iterator.next());
        System.out.println(set.contains("Pink")); //判断集合是否存在该元素

    }
}