1. What is HashMap collection in java?
- HashMap is Hash table based implementation of the Map interface.
- HashMap provides constant-time performance for the basic operations like get & put.
- HashMap is not thread safe.
- HashMap must synchronized externally, If multiple threads access the hashmap concurrently.
- We will iterate or loop through HashMap collection using following methods:
- Iterate HashMap<String, String> using forEach method of java 8
- Iterate HashMap<String, String> using entrySet Iterator.
2. Class hierarchy of HashMap collection:
3. Iterate /loop HashMap collection having string objects (Java8/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 DemoIterationHashMap { 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: Iterate HashMap collection in java " ); demoIterateMethod(mapSportsPersonality); } private static void demoIterateMethod(Map<String, String> mapSportsPersonality) { System.out.println( "1. Iterate HashMap using forEach method:" ); // Output using Java 8 mapSportsPersonality.forEach((key, value) -> { System.out.println( "Game:" + key + ", Player:" + value); }); System.out.println( "\n2. Iterate HashMap collection using entrySet 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()); } } } |
4. Output – Iterate /loop HashMap collection having string objects in Java8
Demo: Iterate HashMap collection in java 1. Iterate HashMap using forEach method: Game:Tennis, Player:Federer Game:Cricket, Player:Bradman Game:Golf, Player:Woods Game:Basketball, Player:Jordan Game:Boxer, Player:Ali 2. Iterate HashMap collection using entrySet iterator: Game:Tennis, Player:Federer Game:Cricket, Player:Bradman Game:Golf, Player:Woods Game:Basketball, Player:Jordan Game:Boxer, Player:Ali |