Java uses the putAll() function of the HashMap class to add a Map to another Map

Map is a commonly used data structure in Java, used to represent a collection of key-value pairs. In Java's collection framework, HashMap is a commonly used implementation class. It provides the putAll() function, which is used to add one Map to another Map to facilitate data merging and copying. This article will introduce how to use the putAll() function and provide corresponding code examples.

First, let’s understand the basic usage of HashMap. HashMap is implemented based on a hash table, which can store key-value pairs and quickly find the corresponding value through the key. The following is an example of a simple HashMap:

import java.util.HashMap;

public class HashMapExample {
    public static void main(String[] args) {
        // 創(chuàng)建一個HashMap實例
        HashMap<String, Integer> map = new HashMap<>();

        // 添加鍵值對
        map.put("張三", 18);
        map.put("李四", 20);
        map.put("王五", 22);

        // 獲取值
        int age = map.get("李四");
        System.out.println("李四的年齡是：" + age);

        // 判斷是否包含某個鍵
        boolean contains = map.containsKey("張三");
        System.out.println("是否包含張三：" + contains);

        // 刪除鍵值對
        map.remove("王五");
        System.out.println("刪除王五后的HashMap：" + map);
    }
}

The above example creates a HashMap object and adds three key-value pairs. We obtained the value corresponding to "李思" through the get() method. At the same time, the containsKey() method is used to determine whether the key "Zhang San" is included. Finally, we deleted the key-value pair "王五" through the remove() method.

Next, we will introduce the use of putAll() method. The putAll() method is a function used to add a Map to another Map. Its definition is as follows:

void putAll(Map<? extends K, ? extends V> m)

putAll() method accepts a parameter m, which is a Map object. It adds all key-value pairs in m to the current Map. If the current Map already contains a key-value pair in m, then the value of the key-value pair will be replaced with the corresponding value in m.

The following is a sample code using the putAll() method:

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

public class PutAllExample {
    public static void main(String[] args) {
        // 創(chuàng)建兩個HashMap實例
        HashMap<String, Integer> map1 = new HashMap<>();
        HashMap<String, Integer> map2 = new HashMap<>();

        // 添加鍵值對到map1
        map1.put("張三", 18);
        map1.put("李四", 20);

        // 添加鍵值對到map2
        map2.put("王五", 22);
        map2.put("趙六", 25);

        // 使用putAll()方法將map2添加到map1
        map1.putAll(map2);

        // 輸出合并后的map1
        System.out.println("合并后的HashMap：" + map1);
    }
}

The above sample code creates two HashMap instances map1 and map2, and adds different key-value pairs. Next, we use the putAll() method to add the key-value pairs of map2 to map1. Finally, we verify the merged results by printing map1.

Run the sample code, the output is as follows:

The merged HashMap: {Zhang San=18, Li Si=20, Zhao Liu=25, Wang Wu=22}

As can be seen from the results, map1 contains all key-value pairs in map2. If map1 originally contains a key-value pair, the value of the key-value pair will be replaced with the corresponding value in map2.

To summarize, the HashMap class in Java provides the putAll() method to implement the function of adding a Map to another Map. Such a function is very convenient for merging and copying data. Through the putAll() method, we can easily merge the key-value pairs in one Map into another Map without adding them one by one. Programmers can flexibly use the putAll() method according to actual needs to improve the efficiency and readability of the code.

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