The usage of HashSet in java and the difference from HashMap

HashSet implements the Set interface. It is an unordered collection that does not allow repeated elements. It is not thread-safe. It is used to store objects and uses the add() method to add elements. The example is as follows:

import java.util.HashSet;

public class test1 {
    public static void main(String[] args) {
        //初始化HashSet的对象
        HashSet<String> testHashSet = new HashSet<>();
        //使用add()添加元素
        testHashSet.add("1");
        testHashSet.add("2");
        testHashSet.add("3");
        testHashSet.add("4");
        //重复元素不会被添加
        testHashSet.add("1");
        testHashSet.add("2");
        //输出
        System.out.println(testHashSet);
        System.out.println(testHashSet.size());
    }
}

output:

[1, 2, 3, 4]
4

Differences from HashMap:

HashMap implements the Map interface, does not allow duplicate keys, but allows duplicate values, stores key-value pairs, and uses the put() method to add elements.