How to iterate over Map or HashMap in java

Table of Contents

Java HashMap tutorial:
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In this post, we will see how can we iterate a map in java. There are four ways of iterating over a map, HashMap or TreeMap.

Java HashMap tutorial:

Using keyset() and for each loop(Java 5)
Using keyset() and java Iterator
Using EntrySet() and for each loop(Java 5)
Using EntrySet() and java Iterator

if you remove elements while iterating , then 1st and 3rd option will throw java.util.ConcurrentModificationException.

If you understand internal working of HashMap, then it may be easier for you to iterate an HashMap

Lets take an example:

1. IterateMapMain.java

package org.arpit.java2blog;

import java.util.HashMap;

import java.util.Iterator;

import java.util.Map.Entry;

public class IterateMapMain {

public static void main(String args[])

{

// HashMap with Country as key and capital as value

HashMap<String,String> countryCapitalMap=new HashMap<String,String>();

countryCapitalMap.put("India","Delhi");

countryCapitalMap.put("Japan","Tokyo");

countryCapitalMap.put("France","Paris");

countryCapitalMap.put("Russia","Moscow");

// Iterating Using keySet() and for each loop

System.out.println("Iterating Using keySet() and for each loop");

for (String countryKey:countryCapitalMap.keySet()) {

System.out.println("Country:"+ countryKey +" and Capital:"+countryCapitalMap.get(countryKey));

}

System.out.println("-----------------------------");

// Iterating Using keySet() and java iterator

System.out.println("Iterating Using keySet() and java Iterator");

Iterator countryKeySetIterator=countryCapitalMap.keySet().iterator();

while(countryKeySetIterator.hasNext()){

String countryKey=countryKeySetIterator.next();

System.out.println("Country:"+ countryKey +" and Capital:"+countryCapitalMap.get(countryKey));

}

System.out.println("-----------------------------");

// Iterating Using entrySet() and for each loop

System.out.println("Iterating Using entrySet() and for each loop");

for (Entry<String,String> entry:countryCapitalMap.entrySet()) {

System.out.println("Country:"+ entry.getKey() +" and Capital:"+entry.getValue());

}

System.out.println("-----------------------------");

// Iterating Using entrySet() and java iterator

System.out.println("Iterating Using entrySet() and and java Iterator");

Iterator<Entry<String,String>> entryIterator=countryCapitalMap.entrySet().iterator();

while(entryIterator.hasNext())

{

Entry<String,String> entry=entryIterator.next();

System.out.println("Country:"+ entry.getKey() +" and Capital:"+entry.getValue());

}

System.out.println("-----------------------------");

}

Run it and you will get following output:

Iterating Using keySet() and for each loop

Country:France and Capital:Paris

Country:Russia and Capital:Moscow

Country:Japan and Capital:Tokyo

Country:India and Capital:Delhi

-----------------------------

Iterating Using keySet() and java Iterator

Country:France and Capital:Paris

Country:Russia and Capital:Moscow

Country:Japan and Capital:Tokyo

Country:India and Capital:Delhi

-----------------------------

Iterating Using entrySet() and for each loop

Country:France and Capital:Paris

Country:Russia and Capital:Moscow

Country:Japan and Capital:Tokyo

Country:India and Capital:Delhi

-----------------------------

Iterating Using entrySet() and and java Iterator

Country:France and Capital:Paris

Country:Russia and Capital:Moscow

Country:Japan and Capital:Tokyo

Country:India and Capital:Delhi

-----------------------------

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How to iterate over Map or HashMap in java