Java for each hashmap

How to Iterate Over a HashMap in Java

If you have a HashMap , what is the best way to iterate over all the elements in the Map ?

Java has a couple of solutions we can use to iterate over just the map keys, the values, or each key-value entry. In the approaches below, we will only consider iterating over map entries that will allow you to access both the map keys and map values during the iteration.

Perhaps the most straightforward approach to iterating over a HashMap is to use a for-each loop to iterate over each entry. Using the HashMap.entrySet() will return a set view of the mappings or entries in the HashMap.

import java.util.HashMap; import java.util.Map; public class Example < public static void main(String[] args) < MapmyMap = new HashMap<>() >; iterateUsingForEach(myMap); > public static void iterateUsingForEach(Map map) < for (Map.Entryentry : map.entrySet()) < String key = entry.getKey(); String value = entry.getValue(); System.out.println("Key=" + key + ", Value=" + value); >> > Click to Copy 

Using a While Loop and Iterator

This next approach uses the same for-each syntax as before, but this time we can make use of an Iterator . Since the HashMap.entrySet() method returns a Set , which extends the Collection interface, we can use the Iterator instance returned by Map.entrySet().iterator .

public void iterateUsingForEachIterator(Map map) < Iterator iterator = map.entrySet().iterator(); while (iterator.hasNext()) < Map.Entryentry = iterator.next(); String key = entry.getKey(); String value = entry.getValue(); System.out.println("Key=" + key + ", Value=" + value); > > Click to Copy 

In this approach, we make use of lambda expressions, which have been available in Java since version 8. A lambda expression operates on input parameters and returns a value. The lambda expression solution to this problem does not need each key-value entry to be converted into an entry set.

public void iterateUsingLambda(Map map) < map.forEach((key, value) ->< System.out.println("Key=" + key + ", Value=" + value); >); > Click to Copy 

In conclusion, we have looked at three approaches to iterating over a HashMap. Each approach is useful for retrieving both the map keys and the map values. We also looked at using lambdas, which use fewer lines of code to achieve the same iteration as non-lambda approaches.

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8 Best ways to Iterate through HashMap in Java

As a Java developer, everyone should know how to Iterate through HashMap, as it will be part of his routine programming. Unlike other Collections, we cannot iterate through HashMap directly we need to get the keySet or entrySet to iterate.

In this article, we will learn about all the different ways of iterating a HashMap in Java.

8 Best ways to Iterate through HashMap in Java

Method 1. Iterate through a HashMap EntrySet using Iterator

Map interface didn’t extend a Collection interface and hence it will not have its own iterator. entrySet() returns a Set and a Set interface which extends the Collection interface and now on top of it, we can use the Iterator.

Now we can get the key-value pair easily using the getKey() and getValue() method.

package com.javainterviewpoint.iterate; import java.util.HashMap; import java.util.Iterator; import java.util.Map; public class IterateHashMap1 < public static void main(String[] args) < HashMaphm = new HashMap<>(); hm.put("Cricket", "Sachin"); hm.put("Football", "Zidane"); hm.put("Tennis", "Federer"); Iterator> entrySet = hm.entrySet().iterator(); while (entrySet.hasNext()) < Map.Entryentry = entrySet.next(); System.out.println("Key : "+entry.getKey()+" Value : "+entry.getValue()); > > >

Method 2. Iterate through HashMap KeySet using Iterator

The keySet() method returns the Set of all the Keys in the HashMap. Since it is a Set again we can use the Iterator to iterate it.

package com.javainterviewpoint.iterate; import java.util.HashMap; import java.util.Iterator; public class IterateHashMap2 < public static void main(String[] args) < HashMaphm = new HashMap<>(); hm.put("Cricket", "Sachin"); hm.put("Football", "Zidane"); hm.put("Tennis", "Federer"); Iterator keySetIterator = hm.keySet().iterator(); while (keySetIterator.hasNext()) < String key = keySetIterator.next(); System.out.println("Key : "+key+" Value : "+hm.get(key)); >> >

Method 3. Iterate HashMap using For-each Loop

The For-Each loop is available for all the classes which implement the Iterable interface. entrySet() method returns Set interface, Set interface extends the Collection interface which in turn extends the Iterable Interface.

The for-each loop (or) enhanced for loop in Java will invoke the iterator() method internally.

package com.javainterviewpoint.iterate; import java.util.HashMap; import java.util.Map; import java.util.Set; public class IterateHashMap3 < public static void main(String[] args) < HashMaphm = new HashMap<>(); hm.put("Cricket", "Sachin"); hm.put("Football", "Zidane"); hm.put("Tennis", "Federer"); Set> entrySet = hm.entrySet(); for(Map.Entry entry : entrySet) < System.out.println("Key : "+entry.getKey()+" Value : "+entry.getValue()); >> >

Method 4. Iterate a HashMap using For-each Loop Java for each hashmap

package com.javainterviewpoint.iterate; import java.util.HashMap; import java.util.Set; public class IterateHashMap4 < public static void main(String[] args) < HashMaphm = new HashMap<>(); hm.put("Cricket", "Sachin"); hm.put("Football", "Zidane"); hm.put("Tennis", "Federer"); Set keySet = hm.keySet(); for(String key : keySet) < System.out.println("Key : "+key+" Value : "+hm.get(key)); >> >

Method 5. Iterating through a HashMap using Lambda Expressions

The forEach() method of the HashMap takes up the BiConsumer functional interface as the argument and hence we can pass it a lambda expression that takes two inputs as argument key and value

package com.javainterviewpoint.iterate; import java.util.HashMap; public class IterateHashMap5 < public static void main(String[] args) < HashMaphm = new HashMap<>(); hm.put("Cricket", "Sachin"); hm.put("Football", "Zidane"); hm.put("Tennis", "Federer"); hm.forEach((k,v) -> ); > >

We can also use map.entrySet().forEach() which belongs to the Iterable interface, it takes up Consumer Functional Interface as the argument.

package com.javainterviewpoint.iterate; import java.util.HashMap; public class IterateHashMap6 < public static void main(String[] args) < HashMaphm = new HashMap<>(); hm.put("Cricket", "Sachin"); hm.put("Football", "Zidane"); hm.put("Tennis", "Federer"); hm.entrySet().forEach((entry) -> ); > >

Method 6. Loop through a HashMap using Stream API

The Stream represents a sequence of elements from a source, the source can be a Collection or array which can provide data to a Stream.

In the below code we will get EntrySet from the entrySet() method, from the entrySet we will get the Stream through the stream() method which we will be iterating using forEach(). The entry reference will be further passed to a lambda

package com.javainterviewpoint.iterate; import java.util.HashMap; public class IterateHashMap7 < public static void main(String[] args) < HashMaphm = new HashMap<>(); hm.put("Cricket", "Sachin"); hm.put("Football", "Zidane"); hm.put("Tennis", "Federer"); hm.entrySet().stream().forEach (entry-> System.out.println("Key : "+entry.getKey()+" Value : "+entry.getValue())); > >

Method 7. Using Stream of() method

We will be converting a HashMap into a Stream using the Stream.of() method, on top which we can use the forEach() method to iterate.

package com.javainterviewpoint.iterate; import java.util.HashMap; import java.util.stream.Stream; public class IterateHashMap8 < public static void main(String[] args) < HashMaphm = new HashMap<>(); hm.put("Cricket", "Sachin"); hm.put("Football", "Zidane"); hm.put("Tennis", "Federer"); Stream.of(hm.entrySet()) .forEach((entry) ->); > >

Method 8. Iterate a HashMap using Iterator.forEachRemaining() method

The forEachRemaining() method is newly added to Iterator interface in Java 8. As we have seen it earlier we can get the iterator of a Map through a Set [entrySet()]

package com.javainterviewpoint.iterate; import java.util.HashMap; public class IterateHashMap9 < public static void main(String[] args) < HashMaphm = new HashMap<>(); hm.put("Cricket", "Sachin"); hm.put("Football", "Zidane"); hm.put("Tennis", "Federer"); hm.entrySet().iterator().forEachRemaining(entry-> System.out.println("Key : "+entry.getKey()+" Value : "+entry.getValue())); > >

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