Java HashMap example

By mkyong | Updated: April 8, 2019

Viewed: 1,498 (+14 pv/w)

Tags: hashmap java

HashMap is an object that stores both key=value as a pair. This HashMap permits null values and the null key, unsynchronized and no guarantees to the order of the map.

["key","value"] = ["java","mkyong.com"]

1. HashMap Basic

1.1 Add an Item

	Map map = new HashMap<String, String>();
	map.put("PostgreSQL", "Free Open Source Enterprise Database");

1.2 Get an Item

	map.get("PostgreSQL"); // output : Free Open Source Enterprise Database

1.3 Update an Item

	map.put("PostgreSQL", "Still the best!");
	map.get("PostgreSQL"); // output : Still the best!

	// @Since 1.8
	map.replace("PostgreSQL", "Still the best! 2");
	map.get("PostgreSQL"); // output : Still the best! 2

1.4 Remove an Item

	map.remove("PostgreSQL");
	map.get("PostgreSQL"); // output : null

1.5 Remove everything

	map.clear();

1.6 Get Size

	map.size();

2. Loop HashMap

There are 3 ways to loop or iterate a HashMap

2.1 If possible, always uses Java 8 forEach, simple and nice.

	Map<String, String> map = new HashMap<>();
    map.forEach((key, value) -> System.out.println("[Key] : " + key + " [Value] : " + value));

2.2 Normal for loop.

	Map<String, String> map = new HashMap<>();
	
	for (Map.Entry<String, String> entry : map.entrySet()) {
		System.out.println("[Key] : " + entry.getKey() + " [Value] : " + entry.getValue());
	}

2.3 Iterator, classic.

	Map<String, String> map = new HashMap<>();
       
	Iterator iter = map.entrySet().iterator();
	while (iter.hasNext()) {
		Map.Entry entry = (Map.Entry) iter.next();
		System.out.println("[Key] : " + entry.getKey() + " [Value] : " + entry.getValue());
	}

3. Collections.synchronizedMap

3.1 This HashMap is unsynchronized, if multiple threads access a HashMap concurrently, it will mess up the values. To use HashMap in a multiple thread’s environment, try Collections.synchronizedMap(new HashMap<>()) to create a synchronized map.

package com.mkyong;

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

public class HashMapSynchronized {

    public static void main(String[] args) {

		// this map is synchronized
        Map<String, Integer> map = Collections.synchronizedMap(new HashMap<>());
		
        map.put("web", 1024);
        map.put("backend", 2048);

        map.forEach((key, value) -> System.out.println("[Key] : " + key + " [Value] : " + value));

    }

}

4. HashMap

A full example, just for reference.

package com.mkyong;

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

public class HashMapExample {

    public static void main(String[] args) {

        Map<String, String> map = new HashMap<>();
        map.put("PostgreSQL", "Free Open Source Enterprise Database");
        map.put("DB2", "Enterprise Database , It's expensive");
        map.put("Oracle", "Enterprise Database , It's expensive");
        map.put("MySQL", "Free Open SourceDatabase (no more, try MariaDB)");

        // Get
        System.out.println(map.get("PostgreSQL")); // Free Open Source Enterprise Database

        // Update
        map.put("PostgreSQL", "Still the best!");
        System.out.println(map.get("PostgreSQL")); // Still the best!

        // @Since 1.8
        map.replace("PostgreSQL", "Still the best! 2");
        System.out.println(map.get("PostgreSQL")); // Still the best! 2

        // Remove
        map.remove("PostgreSQL");
        System.out.println(map.get("PostgreSQL")); // null

        // Size
        System.out.println(map.size()); // 3

        // loop
        System.out.println("Iterator loop...");
        Iterator iter = map.entrySet().iterator();
        while (iter.hasNext()) {
            Map.Entry entry = (Map.Entry) iter.next();
            System.out.println("[Key] : " + entry.getKey() + " [Value] : " + entry.getValue());
        }

        System.out.println("for loop...");
        for (Map.Entry<String, String> entry : map.entrySet()) {
            System.out.println("[Key] : " + entry.getKey() + " [Value] : " + entry.getValue());
        }

        // Java 8
        System.out.println("forEach loop...");
        map.forEach((key, value) -> System.out.println("[Key] : " + key + " [Value] : " + value));

        // clear everything
        map.clear();

		// nothing
        map.forEach((key, value) -> System.out.println("[Key] : " + key + " [Value] : " + value));

    }

}

Output

Free Open Source Enterprise Database
Still the best!
Still the best! 2
null
3
Iterator loop...
[Key] : DB2 [Value] : Enterprise Database , It's expensive
[Key] : MySQL [Value] : Free Open SourceDatabase (no more, try MariaDB)
[Key] : Oracle [Value] : Enterprise Database , It's expensive
for loop...
[Key] : DB2 [Value] : Enterprise Database , It's expensive
[Key] : MySQL [Value] : Free Open SourceDatabase (no more, try MariaDB)
[Key] : Oracle [Value] : Enterprise Database , It's expensive
forEach loop...
[Key] : DB2 [Value] : Enterprise Database , It's expensive
[Key] : MySQL [Value] : Free Open SourceDatabase (no more, try MariaDB)
[Key] : Oracle [Value] : Enterprise Database , It's expensive

References

• Java HashMap JavaDocs