Use the keySet() method of the HashMap class in Java to obtain all keys in the map

Use the keySet() method of the HashMap class in Java to obtain all keys in the map

HashMap is one of the commonly used collection classes in Java. It provides a mapping relationship that can be used through key-value pairs. way to store and access data. In actual development, we often need to obtain all keys in HashMap for corresponding processing. The keySet() method provided by HashMap is used to obtain all keys in the map.

The keySet() method is a method that the HashMap class inherits from the parent class AbstractMap. Its return value is a Set collection, which contains all the keys in the HashMap. Below we use an example to demonstrate how to use the keySet() method to obtain all keys in a HashMap.

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

public class HashMapDemo {

    public static void main(String[] args) {
        // 创建一个HashMap对象
        HashMap<String, Integer> hashMap = new HashMap<>();

        // 向HashMap中添加键值对
        hashMap.put("张三", 18);
        hashMap.put("李四", 20);
        hashMap.put("王五", 22);

        // 使用keySet()方法获取所有的键
        Set<String> keys = hashMap.keySet();

        // 遍历所有的键并输出
        for (String key : keys) {
            System.out.println(key);
        }
    }
}

In the above code, we first created a HashMap object and added three key-value pairs to it through the put() method. Then, we use the keySet() method to get all the keys and save them in a Set collection. Finally, we output each key to the console by looping through the Set collection.

Execute the above code, the output result is:

张三
李四
王五

As you can see, we successfully obtained all the keys in the HashMap through the keySet() method, and output them in the order of addition.

It should be noted that since the keys in the HashMap are unique, the returned Set collection will not contain duplicate keys. In addition, since HashMap is unordered, the order of the keys obtained may not be consistent with the order in which they were added.

In summary, using the keySet() method of HashMap can easily obtain all the keys in the HashMap, which can help us better process and operate the data in the HashMap. I hope this article will help you understand the keySet() method of HashMap.

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