- HashMap is Hash table based implementation of the Map interface.
- HashMap provides constant-time performance for the basic operations like get & put.
- HashMap based implementation is not thread safe.
- In multi-threading environment, HashMap should be synchronized externally.
- If multiple threads access a hashmap concurrently then HashMap must synchronized externally.
1. Methods to remove element or objects from HashMap collection in java
No. | Method Name | Description |
---|---|---|
1 | V remove(Object key) | Removes the mapping for the specified key from this map if present. |
2 | boolean remove(Object key, Object value) | Removes the entry for the specified key only if it is currently mapped to the specified value. |
We will demonstrate the removal of keys using entrySet iterator.
2. Remove elements/string objects from HashMap collection (java/example)
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; public class DemoRemoveHashMap { 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("Demo HashMap collection remove methods : "); demoRemoveMethod(mapSportsPersonality); } private static void demoRemoveMethod(Map<String, String> mapSportsPersonality) { // {Tennis=Federer, Cricket=Bradman, Golf=Woods, Basketball=Jordan, Boxer=Ali} System.out.println("1. Orignal hashmap:\n" + mapSportsPersonality); // Remove Cricket from map mapSportsPersonality.remove("Cricket"); // {Tennis=Federer, Golf=Woods, Basketball=Jordan, Boxer=Ali} System.out.println("2. Removed Cricket from map : \n" + mapSportsPersonality); mapSportsPersonality.remove("Boxer","Ali"); // {Tennis=Federer, Golf=Woods, Basketball=Jordan} System.out.println("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("4. Removed Basketball,Golf during iteration:\n " + mapSportsPersonality); } }
3. Remove elements/string objects from HashMap collection (java/example)
Demo HashMap collection remove methods: 1. Orignal hashmap: {Tennis=Federer, Cricket=Bradman, Golf=Woods, Basketball=Jordan, Boxer=Ali} 2. Removed Cricket from map : {Tennis=Federer, Golf=Woods, Basketball=Jordan, Boxer=Ali} 3. Removed entry from map with Key=Boxer and Value=Ali: {Tennis=Federer, Golf=Woods, Basketball=Jordan} 4. Removed Basketball,Golf during iteration: {Tennis=Federer}