Java - collection 2-5 (Map)

Map

Overview and usage of Map collection

Overview of the Map collection

Interface Map<K,V> K: type of key; V: type of value
An object mapping keys to values; cannot contain duplicate keys; each key can map to at most one value
Example: student ID and name
001 Xiaobai
002 Xiaohei
003 Xiaohong

Create an object of the Map collection

polymorphic way
The specific implementation class HashMap

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

/*
Map集合概述

- Interface Map<K,V> K:键的类型；V:值的类型
- 将键映射到值的对象；不能包含重复的键；每个键可以映射到最多一个值
- 举例：学生的学号和姓名
001 小白
002 小黑
003 小红

创建Map集合的对象

- 多态的方式
- 具体的实现类HashMap
*/
public class MapDemo {
    
    
	public static void main(String[] args) {
    
    
		//创建集合对象
		Map<String, String> map = new HashMap<String, String>();//HashMap保证了键的唯一性

		//添加方法：V put(K key,V value)将指定的值与该映射中的指定键相关联
		map.put("001", "小白");
		map.put("002", "小黑");
		map.put("003", "小红");
		map.put("003", "小绿");//当键重复时，会把原先的值替代调

		//输出集合对象
		System.out.println(map);

	}
}

Running results:
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Basic functions of the Map collection

method name	illustrate
V put(K key,V value)	add element
V remove(Objedt key)	Delete key-value pair elements based on key
void clear()	remove all key-value elements
boolean containsKey(Object key)	Determine whether the collection contains the specified key
boolean containsValue(Object value)	Determine whether the collection contains the specified value
boolean isEmpty()	Determine whether the collection is empty
int size()	The length of the collection, that is, the number of key-value pairs in the collection

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

public class MapDemo {
    
    
	public static void main(String[] args) {
    
    
		//创建集合对象
		Map<String, String> map = new HashMap<String, String>();//HashMap保证了键的唯一性

		//添加方法：V put(K key,V value)将指定的值与该映射中的指定键相关联
		map.put("小黑", "小白");
		map.put("小绿", "小红");
		map.put("小蓝", "小紫");

		//V remove(Objedt key)根据键删除键值对元素
		System.out.println(map.remove("小蓝"));//返回小紫
		System.out.println(map.remove("小黄"));//返回null

		//void clear()移除所有的键值对元素
		map.clear();//{}

		//boolean containsKey(Object key)判断集合是否包含指定的键
		System.out.println(map.containsKey("小黑"));//true
		System.out.println(map.containsKey("小白"));//false

		//boolean containsValue(Object value)判断集合是否包含指定的值
		System.out.println(map.containsValue("小红"));//true
		System.out.println(map.containsValue("小绿"));//false

		//boolean isEmpty()判断集合是否为空
		System.out.println(map.isEmpty());//false

		//int size()集合的长度，也就是集合中键值对的个数
		System.out.println(map.size());//3

		//输出集合对象
		System.out.println(map);

	}
}

The acquisition function of the Map collection

method name	illustrate
V get(Object key)	get value by key
Set keySet()	Get the set of all keys
Collection values()	Get a collection of all values
Set<Map.Entry<K,V>>entrySet()	Get a collection of all key-value pair objects

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

/*
Map集合的获取功能

V get(Object key)  根据键获取值 
Set<K> keySet()获取所有键的集合
Collection<V> values()获取所有值的集合
*/
public class MapDemo2 {
    
    
	public static void main(String[] args) {
    
    
		//创建集合对象
		Map<String, String> map = new HashMap<String, String>();

		//添加元素
		map.put("小黑", "小白");
		map.put("小绿", "小红");
		map.put("小蓝", "小紫");

		//V get(Object key)  根据键获取值 
		//		System.out.println(map.get("小黑"));//小白
		//		System.out.println(map.get("小小"));//null

		//Set<K> keySet()获取所有键的集合
		//		System.out.println(map.keySet());//[小蓝, 小黑, 小绿]
		//遍历键集合
		Set<String> ks = map.keySet();
		for (String s : ks) {
    
    
			System.out.println(s);
		}

		//Collection<V> values()获取所有值的集合
		//		System.out.println(map.values());//[小紫, 小白, 小红]
		//遍历值集合
		Collection<String> v = map.values();
		for (String s : v) {
    
    
			System.out.println(s);
		}

	}
}

Running result:
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traversal of the Map collection