Different ways to iterate Map in Java

Learn different ways to iterate Map in Java using for-each loop keySet, entrySet, for loop, while loop.

1. Ways to iterate Map in Java

There are different ways of traversal of Map HashMap , ConcurrentHashMap object in java for getting key/value pair . In day-to-day life, we almost use Map (either get or put operation) every day in a java programming language. But the way of iteration is different, it depends upon the requirement and thing in mind at that point in time. Most of the time while iteration, we don’t know the key and its value. We have to iterate over and best on that we usually process.

2. Traversing Using JDK 5 for-each with EntrySet

This is simplest of all and elegant of all the iteration, developer used this way very frequently. This is one of the way of traversing of map using for-each loop with EntrySet.

// Using JDK 5 for-each with EntrySet
System.out.println("Using JDK 5 for-each with EntrySet...");
for (Map.Entry<String, String> entry : countryMap.entrySet()) {
   System.out.printf("Key : %s and Value: %s \n ", entry.getKey(), entry.getValue());
}

3. Iteration Using for-each loop with keySet()

In this section we learn by using for-each loop with keySt, this is one of the ways to iterate Map in Java.

// Using for-each loop with keySet()
System.out.println("Using for-each loop with keySet()........");
for (String key : countryMap.keySet()) {
   System.out.printf(" key : %s and value: %s \n", key, countryMap.get(key));
}

4. Using while loop with entrySet()

In this section while loop has been used with entrySet() to iterate Map in Java.

// Using  while loop with entrySet()
System.out.println("Using  while loop with entrySet()..............");
Iterator> itr = countryMap.entrySet().iterator();
while(itr.hasNext()) {
  Map.Entry entry = itr.next();
  System.out.printf(" key : %s and value: %s \n",  entry.getKey(), entry.getValue());
}

5. Traversing Using while loop with keySet()

// Using while loop with keySet()
System.out.println("Using while loop with keySet()............................");
Iterator itr2 = countryMap.keySet().iterator();
while (itr2.hasNext()) {
  String key = itr2.next();
  System.out.printf(" key : %s and value: %s \n",  key, countryMap.get(key));
}

6. Complete Working Example to iterate Over Map in different ways

package com.mysoftkey.collection;

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

public class MapIteratorExample {

  public static void main(String[] args) {
		
	 // creating a HashMap object and add few key/value in it.
	 Map countryMap = new HashMap();
	 countryMap.put("IN", "India");
	 countryMap.put("USA", "United States of America");
	 countryMap.put("SG", "Singapore");
	 countryMap.put("CA", "Canada");

	// Using JDK 5 for-each with EntrySet
	System.out.println("Using JDK 5 for-each with EntrySet...");
        for (Map.Entry entry : countryMap.entrySet()) {
            System.out.printf("Key : %s and Value: %s \n", entry.getKey(), entry.getValue());
        }
		
        // Using for-each loop with keySet()
        System.out.println("Using for-each loop with keySet()........");
        for (String key : countryMap.keySet()) {
           System.out.printf(" key : %s and value: %s \n ", key, countryMap.get(key));
        }
		
        // Using  while loop with entrySet()
        System.out.println("Using  while loop with entrySet()..............");
        Iterator itr2 = countryMap.keySet().iterator();
	  while (itr2.hasNext()) {
		String key = itr2.next();
		System.out.printf(" key: %s and value: %s  \n",  key, countryMap.get(key));
	  }
        
	}

}

In the above example you can create LinkedHashMap, ConcurrentHashMap or Collections.synchronizedMap(Map), but iteration remain same, only object of the implementation gets changed.

7. Output of Iterate element in Map

Using JDK 5 for-each with EntrySet...
Key: USA and Value: United States of America 
Key: SG and Value: Singapore 
Key: IN and Value: India 
Key: CA and Value: Canada 
Using for-each loop with keySet()........
 key: USA and value: United States of America 
 key: SG and value: Singapore 
 key: IN and value: India 
 key: CA and value: Canada 
Using  while loop with entrySet()..............
 key: USA and value: United States of America 
 key: SG and value: Singapore 
 key: IN and value: India 
 key: CA and value: Canada 
Using while loop with keySet()............................
 key: USA and value: United States of America 
 key: SG and value: Singapore 
 key: IN and value: India 
 key: CA and value: Canada 

8. Best way to iterate over Map/HashMap

In the above jdk5 for-each with EntrySet is the best way to iterate . this is simple and elegant way of iteration over HashMap or any Map

// Using JDK 5 for-each with EntrySet
System.out.println("Using JDK 5 for-each with EntrySet...");
for (Map.Entry<String, String> entry : countryMap.entrySet()) {
   System.out.printf("Key: %s and Value: %s \n", entry.getKey(), entry.getValue());
}

9. Reference

Visit Oracle Java official Site for more details.

I hope enjoyed different ways to iterate java.util.Map in Java using for-each keySet, for each entrySet, whi visit Core Java tutorial for more details.
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