Put,Remove,Sort,iterate,Get methods of HashMap (example)

What is HashMap in java?

• HashMap is Hash table based implementation of the Map interface.
• HashMap provides constant-time performance for the basic operations (get and put).
• HashMap based implementation is not thread safe.
• HashMap must synchronized externally, If multiple threads access a hashmap concurrently.

List of Important methods of HashMap in java

• Remove

• put

• Sort

• Iterate

• Get

• Contains And

• Clear

Program – HashMap functions in java

package org.learn.collection.map.hmap;

import java.util.HashMap;
import java.util.Iterator;
import java.util.Map;
import java.util.Map.Entry;
import java.util.Set;
import java.util.TreeMap;

public class DemoHashMap {

	public static void main(String[] args) {
		Map<String, String> mapSportsPersonality = new HashMap<>();
		mapSportsPersonality.put("Tennis", "Federer");
		mapSportsPersonality.put("Cricket", "Bradman");
		mapSportsPersonality.put("Basketball", "Jordan");
		mapSportsPersonality.put("Golf", "Woods");
		mapSportsPersonality.put("Boxer", "Ali");

		System.out.println("1. Demo of remove methods: ");
		demoRemoveMethod(mapSportsPersonality);

		System.out.println("\n2. Demo of put methods: ");
		demoPutMethod(mapSportsPersonality);

		System.out.println("\n3. Sort elements of HashMap: ");
		demoSortMethod(mapSportsPersonality);

		System.out.println("\n4. Iterate through HashMap: ");
		demoIterateMethod(mapSportsPersonality);

		System.out.println("\n6. Demo of get method of HashMap: ");
		System.out.println("Element with key Golf: " + mapSportsPersonality.get("Golf"));
		System.out.println("Element with key Tennis: " + mapSportsPersonality.get("Tennis"));

		System.out.println("\n7. Demo of contains method of HashMap: ");
		System.out.println("Check HashMap contains Boxer: " + mapSportsPersonality.containsKey("Boxer"));
		System.out.println("Check HashMap contains soccer: " + mapSportsPersonality.containsKey("soccer"));

		System.out.println("\n8.Demo of clear method of HashMap: ");
		mapSportsPersonality.clear();
		System.out.println("Clear HashMap: " + mapSportsPersonality);
	}

	private static void demoRemoveMethod(Map<String, String> mapSportsPersonality) {
		// {Tennis=Federer, Cricket=Bradman, Golf=Woods, Basketball=Jordan,
		// Boxer=Ali}
		System.out.println("1.1. Orignal hashmap:\n" + mapSportsPersonality);

		// Remove Cricket from map
		mapSportsPersonality.remove("Cricket");
		// {Tennis=Federer, Golf=Woods, Basketball=Jordan, Boxer=Ali}
		System.out.println("1.2. Removed Cricket from map : \n" + mapSportsPersonality);

		mapSportsPersonality.remove("Boxer","Ali");
		// {Tennis=Federer, Golf=Woods, Basketball=Jordan}
		System.out.println("1.3. Removed entry from map with Key=Boxer and Value=Ali :\n " + mapSportsPersonality);

		Set<Entry<String, String>> entrySet = mapSportsPersonality.entrySet();
		Iterator<Entry<String, String>> iterator = entrySet.iterator();
		while (iterator.hasNext()) {
			Map.Entry<String, String> entry = iterator.next();
			if (entry.getKey().equals("Basketball")) {
				iterator.remove();
			} else if (entry.getKey().equals("Golf")) {
				iterator.remove();
			}
		}
		// {Tennis=Federer}
		System.out.println("1.4. Removed Basketball,Golf during iteration:\n " + mapSportsPersonality);
	}

	private static void demoPutMethod(Map<String, String> mapSportsPersonality) {
		System.out.println("2.1. Orignal HashMap:\n" + mapSportsPersonality);

		mapSportsPersonality.put("Tennis", "Federer");
		mapSportsPersonality.put("Cricket", "Bradman");
		System.out.println("2.2. Added Tennis &  Cricket using put: \n" + mapSportsPersonality);

		Map<String, String> newSportsMap = new HashMap<>();
		newSportsMap.put("Golf", "Woods");
		newSportsMap.put("Boxer", "Ali");

		mapSportsPersonality.putAll(newSportsMap);
		System.out.println("2.3. Added another map using putAll to map:\n" + mapSportsPersonality);

		mapSportsPersonality.putIfAbsent("Baseball", "Trout");

		// key Baseball already added to map, so "Kershaw" would not be added
		mapSportsPersonality.putIfAbsent("Baseball", "Kershaw");
		System.out.println("2.4. Added Baseball using putIfAbsent to map:\n" + mapSportsPersonality);
	}

	private static void demoSortMethod(Map<String, String> mapSportsPersonality) {
		// {Tennis=Federer, Cricket=Bradman, Golf=Woods, Baseball=Trout, Boxer=Ali}
		System.out.println("Orignal HashMap:" + mapSportsPersonality);

		System.out.println("\n3.1. Sort HashMap by ascending key: ");
		TreeMap<String, String> mapSorted = new TreeMap<>(mapSportsPersonality);
		mapSorted.forEach((key, value) -> {
			System.out.println(key + ", " + value);
		});

		System.out.println("\n3.2. Sort HashMap by descending key: ");
		mapSorted.descendingMap().forEach((key, value) -> {
			System.out.println(key + ", " + value);
		});
	}

	private static void demoIterateMethod(Map<String, String> mapSportsPersonality) {

		System.out.println("4.1. Iterate using forEach:");
		// Output using Java 8
		mapSportsPersonality.forEach((key, value) -> {
			System.out.println("Game:" + key + ", Player:" + value);
		});

		System.out.println("\n4.2. Iterate using iterator:");
		// Output use Iterator:
		Set<Entry<String, String>> entrySet = mapSportsPersonality.entrySet();
		Iterator<Entry<String, String>> iterator = entrySet.iterator();
		while (iterator.hasNext()) {
			Map.Entry<String, String> entry = iterator.next();
			System.out.println("Game:" + entry.getKey() + ", Player:" + entry.getValue());
		}
	}
}

Output – Various method of HashMap in java

1. Demo of remove methods: 
1.1. Orignal hashmap:
{Tennis=Federer, Cricket=Bradman, Golf=Woods, Basketball=Jordan, Boxer=Ali}
1.2. Removed Cricket from map : 
{Tennis=Federer, Golf=Woods, Basketball=Jordan, Boxer=Ali}
1.3. Removed entry from map with Key=Boxer and Value=Ali :
 {Tennis=Federer, Golf=Woods, Basketball=Jordan}
1.4. Removed Basketball,Golf during iteration:
 {Tennis=Federer}

2. Demo of put methods: 
2.1. Orignal HashMap:
{Tennis=Federer}
2.2. Added Tennis &  Cricket using put: 
{Tennis=Federer, Cricket=Bradman}
2.3. Added another map using putAll to map:
{Tennis=Federer, Cricket=Bradman, Golf=Woods, Boxer=Ali}
2.4. Added Baseball using putIfAbsent to map:
{Tennis=Federer, Cricket=Bradman, Golf=Woods, Baseball=Trout, Boxer=Ali}

3. Sort elements of HashMap: 
Orignal HashMap:{Tennis=Federer, Cricket=Bradman, Golf=Woods, Baseball=Trout, Boxer=Ali}

3.1. Sort HashMap by ascending key: 
Baseball, Trout
Boxer, Ali
Cricket, Bradman
Golf, Woods
Tennis, Federer

3.2. Sort HashMap by descending key: 
Tennis, Federer
Golf, Woods
Cricket, Bradman
Boxer, Ali
Baseball, Trout

4. Iterate through HashMap: 
4.1. Iterate using forEach:
Game:Tennis, Player:Federer
Game:Cricket, Player:Bradman
Game:Golf, Player:Woods
Game:Baseball, Player:Trout
Game:Boxer, Player:Ali

4.2. Iterate using iterator:
Game:Tennis, Player:Federer
Game:Cricket, Player:Bradman
Game:Golf, Player:Woods
Game:Baseball, Player:Trout
Game:Boxer, Player:Ali

6. Demo of get method of HashMap: 
Element with key Golf: Woods
Element with key Tennis: Federer

7. Demo of contains method of HashMap: 
Check HashMap contains Boxer: true
Check HashMap contains soccer: false

8.Demo of clear method of HashMap: 
Clear HashMap: {}