Hash is a useful tool in everyday programming work. Here is a brief summary.

HashMap & HashSet

HashSet

import java.util.HashSet;
import java.util.Iterator;

public class hashSetTest {
    private static final HashSet<String> set = new HashSet<String>() {
        {
            add("A");
            add("B");
            add("C");
            add("D");
            add("E");
        }
    };
    public static void main(String[] argc) {
        if(set.isEmpty()) {
            System.out.println("is set empty? " + set.isEmpty());
        }
        if(set.contains("A")) {
            System.out.println("is set contains A? " + set.contains("A"));
        }

        Iterator i = set.iterator();
        while(i.hasNext()) {
            System.out.println("has values: " + i.next());
        }

        set.remove("A");
        if(set.contains("A")) {
            System.out.println("is set contains A? " + set.contains("A"));
        }

        System.out.println("set size = " + set.size());
        set.clear();
        System.out.println("set size = " + set.size());
    }

}

Run result

is set contains A? true
has values: D
has values: E
has values: A
has values: B
has values: C
set size = 4
set size = 0

Process finished with exit code 0

HashMap

import java.util.HashMap;

public class hashTest {
    private static final HashMap<String, Integer> map = new HashMap<String, Integer> () {
        {
            put("A", 1);
            put("B", 2);
            put("C", 3);
            put("D", 4);
        }

    };
    public static void main(String[] argc) {
        if(map.containsKey("A")) {
            System.out.println("is map Contains key A? " + map.containsKey("A"));
        }
        if(map.containsValue(1)) {
            System.out.println("is map Contains value 1? " + map.containsKey("A"));
        }
        if(map.isEmpty()){
            System.out.println("is map empty? " + map.isEmpty());
        }
        System.out.println("map size = " + map.size());
        for(String key : map.keySet()) {
            System.out.println(key + "->" + map.get(key));
        }

        map.clear();
        if(map.isEmpty()){
            System.out.println("is map empty? " + map.isEmpty());
        }
    }
}

Run result

is map Contains key A? true
is map Contains value 1? true
map size = 4
D->4
A->1
B->2
C->3
is map empty? true

Process finished with exit code 0

Well, that's all.