Java basics - Map collection traversal method

(1) Method 1: find value by key

First obtain the Set collection of all keys of the Map collection.
Traverse the Set collection of keys, and then extract the corresponding value through the key.

APIs involved:

method name	illustrate
Set<K> keySet()	Get the set of all keys
V get(Object key)	get value by key

(2) Method 2: Key-value pairs

First convert the Map collection into a Set collection, and each element in the Set collection is a key-value pair implementation type.
Iterates through the Set collection, then extracts the keys and extracts the values.

APIs involved:

method name	illustrate
Set<Map.Entry<K,V>> entrySet()	Get a collection of all key-value pair objects
K getKey()	get key
V getValue()	get value

(3) Method 3: lambda expression

Thanks to the new technology Lambda expression started by JDK8, it provides a simpler and more direct way to traverse the collection.

APIs involved:

method name	illustrate
default void forEach(BiConsumer<? super K, ? super V>action)	Combining lambda to traverse the Map collection

process:

maps.forEach((k,v) -> {

System.out.println(k + ":" + v);

});

(4) The codes of the three traversal methods are as follows:

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

/**
 * Map集合遍历方式
 */
public class MapDemo {
    public static void main(String[] args) {
        Map<String,Integer> maps = new HashMap<>();
        //无序，不重复，无索引
        maps.put("大大",23);
        maps.put("小小",21);
        maps.put("清清",20);
        maps.put("心心",24);
        System.out.println(maps);

        //1.键找值
        //1.1先拿到集合的全部键
        Set<String> keys = maps.keySet();
        //1.2遍历每个键，根据键提取值
        for (String key : keys) {
            int value = maps.get(key);
            System.out.println(key + ":" + value);
        }

        System.out.println(maps);//{大大=23, 小小=21, 心心=24, 清清=20}
        //2.键值对
        /**
         maps = {大大=23, 小小=21, 心心=24, 清清=20}
         使用foreach遍历Map集合，发现Map集合的键值对元素直接是没有类型的，所以不能使用foreach遍历集合
         可以通过调用Map的方法，entrySet把Map集合转换成Set集合形式 maps.entrySet()
         Set<Map.Entry<String,Integer>>entries = [(大大=23), (小小=21), (心心=24), (清清=20)]
         此时就可以使用foreach遍历
         */
        //2.1把Map集合转换成Set集合 快捷键：maps.entrySet() + ctrl+alt+v
        Set<Map.Entry<String, Integer>> entries = maps.entrySet();
        //2.2开始遍历
        for(Map.Entry<String, Integer> entry : entries){
            String key = entry.getKey();
            int value = entry.getValue();
            System.out.println(key + ":" + value);
        }

        System.out.println(maps);//{大大=23, 小小=21, 心心=24, 清清=20}
        //3.Lambda表达式
//        maps.forEach(new BiConsumer<String, Integer>() {
//            @Override
//            public void accept(String key, Integer value) {
//                System.out.println(key + ":" + value);
//            }
//        });
        
        //简化代码
        maps.forEach ((k, v) -> {System.out.println(k + ":" + v);});
    }
}

Java basics - Map collection traversal method

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