How to iterate or loop a HashMap?

This is one of the favourite question of  Java interviewer's  and  already asked so many time in different forms with freshers as well as with experienced developers in interviews.Transversing a HashMap is little different from other collection framework classes.In this post, we are going to discuss 6 different ways to iterate or loop through HashMap.

We have taken a simple HashMap and looping that.

package com.javainstance;

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


/**
 * @author javainstance
 *
 */
public class IterateHashmap {


public static void main(String[] args) {

//HashMap Creation
HashMap<Integer, String> hashmap = new HashMap<>();

hashmap.put(1, "Jai");
hashmap.put(2, "Abhay");
hashmap.put(3, "Prakash");
hashmap.put(4, "Bhanu");
hashmap.put(5, "Henery");

//Using Iterator and EntrySet

Set<Map.Entry<Integer, String>> entrySet = hashmap.entrySet();
Iterator<Entry<Integer, String>> iterator = entrySet.iterator();
while (iterator.hasNext()) {
Entry<Integer, String> entry = iterator.next();
System.out.println("Key " + entry.getKey() + "  " + "Value " + entry.getValue());

}


//Using keySet

Set<Integer> keys = hashmap.keySet();
for (Integer key : keys) {
System.out.println("Key " + key + "  " + "Value " + hashmap.get(key));

}

//Using Iterator and keySet

Set<Integer> keyvalues = hashmap.keySet();
Iterator<Integer> iteratr = keyvalues.iterator();

while (iteratr.hasNext()) {

int key = iteratr.next();

System.out.println("Key " + key + "  " + "Value " + hashmap.get(key));

}


//Using EntrySet and for loop

Set<Map.Entry<Integer, String>> entrySt = hashmap.entrySet();

for (Entry<Integer, String> ent : entrySt) {

System.out.println("Key " + ent.getKey() + "  " + "Value " + ent.getValue());

}


// In JAVA 8 using forEach

hashmap.forEach((Integer, String) -> {
System.out.println("Key " + Integer + " " + "Value " + String);
}

);



// In JAVA 8 using forEach on entrySet

hashmap.entrySet().forEach(entry -> {
System.out.println("Key : " + entry.getKey() + " Value : " + entry.getValue());
});

}

}

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