Different Ways to Iterate over a Map in Java

Published April 25, 2022

To begin with, it is not possible to iterate a Map directly using iterators, because Maps in Java are not collections. The Hava Maps are used to implement the Map interface.

The following are five ways to iterate over HashMap:

a) Using For-Each Loop Method

Using the MapMap.entrySet() method, the SetMap collection-view entry (K, V) of the mappings found in this MapMap is returned. The getKey and getValue methods of Map EntryK allow us to iterate over key-value pairs. This method is the most common and should be used when looping both keys and values.

Example

import java.util.HashMap;

import java.util.Map;

public class Iterator1 {

public static void main(String[] args) {

Map<String,String> method1 = new HashMap<String,String>();

method1.put("First", "Java");

method1.put("Second", "Python");

method1.put("Third", "Kotlin");

method1.put("Fourth", "Matlab");

for (Map.Entry<String,String> entry : method1.entrySet())

System.out.println("Rank = " + entry.getKey() +

", language = " + entry.getValue());

}

b) Using the keyset () and values () method

Using Map keySet (), you can look at the keys in the MapMap; using Map values (), you can see the values in the MapMap. Using for-each loops, you can loop over keys and values from the MapMap

import java.util.HashMap;

import java.util.Map;

public class Iterator2 {

public static void main(String[] args) {

Map<String,String> method2 = new HashMap<String,String>();

method2.put("First", "Java");

method2.put("Second", "Kotlin");

method2.put("Third", "Python");

method2.put("Fourth", "Matlab");

// using keySet() for iteration over keys

for (String name : method2.keySet())

System.out.println("Rank: " + name);

// using values() for iteration over values

for (String url : method2.values())

System.out.println("language: " + url);

}

c) Using iterators Method

In some ways, this is similar to the first method. In the first method, we use a for-each loop. Entry (K, V): However, we use iterators here. With an entry [K, V], we can call the iterator to remove () method to remove entries from the MapMap during iteration.

Example

import java.util.HashMap;

import java.util.Iterator;

import java.util.Map;

public class Iterator3 {

public static void main(String[] args) {

Map<String,String> method3 = new HashMap<String,String>();

method3.put("First", "Java");

method3.put("Second", "Python");

method3.put("Third", "Kotlin");

method3.put("Fourth", "matlab");

// using iterators

Iterator<Map.Entry<String, String>> itr = method3.entrySet().iterator();

while(itr.hasNext())

{

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

System.out.println("Rank = " + entry.getKey() +

", Language = " + entry.getValue());

}

d) Using forEach method

This method uses MapMap for.forEach(action) method and lambda expressions to traverse a map. The benefit of using this technique is that it is fast and clean.

Example

import java.util.HashMap;

import java.util.Map;

public class Iterate4 {

public static void main(String[] args) {

Map<String,String> method4 = new HashMap<String,String>();

method4.put("First", "Java");

method4.put("Second", "Python");

method4.put("Third", "Kotlin");

method4.put("Fourth", "Matlab");

method4.forEach((k,v) -> System.out.println("Rank = "

+ k + ", Language = " + v));

}

e) Iterating over keys and searching for values

Using the MapMap.keySet() method, we first loop over keys (then we search for a value for each key using the MapMap.get(key) method). In practice, this method is not used since it is very slow and inefficient since retrieving values by a key could take a great deal of time.

Example

import java.util.HashMap;

import java.util.Map;

public class Iterate5 {

public static void main(String[] args) {

Map<String,String> method5 = new HashMap<String,String>();

method5.put("First", "Java");

method5.put("Second", "Python");

method5.put("Third", "Kotlin");

method5.put("Fourth", "Matlab");

for (String name : method5.keySet())

{

String url = method5.get(name);

System.out.println("Rank = " + name + ", Language = " + url);

}

Conclusion

All Java maps implement the Map interface so that the following techniques can be applied to any map implementation

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