How to iterate over Map or HashMap in java

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

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

Initialize ArrayList with values in Java

In this article, we will learn to initialize ArrayList with values in Java. ArrayList is an implementation class of List interface in Java. It is used to store elements. It is based on a dynamic array concept that grows accordingly. We can Initialize ArrayList with values in several ways. Let’s see some of them with […]

Core Java interview Java Collections

17 March

PriorityQueue in Java 8

In this post, we will see about Java 8 PriorityQueue. When the objects are supposed to be processed on the basis of their priority, in that scenario we use PriorityQueue. It’s a special type of queue (also, unbound queues) where the elements can be ordered either as per their natural ordering or based on a […]

Java Collections

06 October

2d Arraylist java example

In this post, we will see how to create 2d Arraylist in java. Best way to create 2d Arraylist is to create list of list in java. [crayon-600b98a1819df369036266/] Let’s create a program to implement 2d Arraylist java. [crayon-600b98a1819e3348539558/] Output: 2nd element in list3 : List3_Str2 3nd element in list1 : List1_Str3 1st element in list2 […]

HashMap Java Collections HashMap

04 May

How HashMap works in java

Most common interview questions are <code>How HashMap works in java</code>, “How get and put method of HashMap work internally”. Here I am trying to explain internal functionality with an easy example. [crayon-600b98a19310e323030817-i/] is one of the most used Collections in java.Rather than going through theory, we will start with example first, so that you will […]

Java Collections

21 October

How to Sort HashSet in Java

In this post, we will see how to sort HashSet in java. HashSet is a collection which does not store elements in any order. You might come across a situation where you need to sort HashSet. There can be many ways to sort HashSet, we will see two methods here. Using TreeSet You can use […]

Conversion Java Collections

20 October

Java Array to Set

In this post, we will learn java array to set conversion. There are many ways to convert array to set. 1. Using Java 8’s Stream If you are using Java 8, I would recommend using this method. [crayon-600b98a1824e5242038942/] Output [John, Martin, Mary] 2. Using HashSet constructor() We can directly call HashSet‘s constructor for java set […]

How to iterate over Map or HashMap in java

Java HashMap tutorial: