Learn How to Iterate HashMap in Java

Updated on October 21, 2024

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5 min read

ARTICLE OUTLINE

Introduction to HashMap Methods of Iterate Over a HashMap Conclusion FAQs

In Java, a HashMap is part of the Java Collection framework and provides a convenient way to store key-value pairs. It allows for fast retrieval, insertion, and deletion of elements. However, when accessing the data stored in a HashMap, you can iterate through its entries. This article will explore various methods to iterate over a HashMap.

Introduction to HashMap

This is a powerful data structure that is used for storing key-value pairs. This makes it easier to retrieve and manipulate the data stored. It also forms one of the Java Collections frameworks and utilizes a Hash table to achieve average time complexity on operations such as insertion, deletion, and access of order O(1). It is allowed to have one null key and any amount of null values of the HashMap, which opens great opportunities for their usage.

However, they don’t preserve the order of the elements; if the order is important, it could be a drawback. They manage collisions- circumstances under which different keys give the same hash value – using chaining or open addressing methods. Some applications incorporating HashMaps include caching, counting, and storing tables and associative arrays, hence qualifying as a general-purpose map.

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Methods of Iterate Over a HashMap

There are several ways to iterate through a HashMap.

Iterate through a HashMap EntrySet using iterators
Iterate through a HashMap KeySet using Iterator
Iterate HashMap using for-each Loop
Iterating through a HashMap using Lambda Expression
Loop through a HashMap using Stream API

Method 1: Using a for loop to iterate through a HashMap

In the HashMap using a for loop in Java, you typically use the entrySet() method, which provides a set of key-value pairs. You can access each entry as a Map by using an enhanced for loop. Entry object. It allows you to retrieve and populate both keys with key-value pairs. You can loop through the entries to print each key and its associated value. This method is straightforward and leverages Java’s collection framework to iterate through the HashMap in a clean and readable manner, making it easy to handle the data stored within the map.

The following program iterates the HashMap using the loop.

Program

import java.util.HashMap ;

import java.util.Map ;

class Main{

public static void main(String args[]){

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

ob.put("A", "Amit") ;

ob.put("J","Java") ;

ob.put("P", "Python") ;

ob.put("H", "Hibernate") ;

for(Map.Entry<String,String> set: ob.entrySet()) {

System.out.println(set.getKey()+" ="+set.getValue()) ;

}

}

}

Output

P =Python

A =Amit

H =Hibernate

J =Java

Method 2: Using a forEach to iterate through a HashMap

In the second method, the forEach function is used to iterate the key-value pairs.

import java.util.HashMap ;

import java.util.Map ;

class Main{

public static void main(String args[]){

Map<Character,String> ob = new HashMap<Character,String>() ;

ob.put('J', "java") ;

ob.put('H', "Hibernate") ;

ob.put('P', "Python") ;

ob.put('A', "Angular") ;

ob.forEach((key,value)-> System.out.println(key+"="+value));

}

}

Output

P=Python

A=Angular

H=Hibernate

J=java

Method 3: Using an iterator to iterate through a HashMap

The iterator is being used to iterate each mapped pair in HashMap as shown below in the java program.

import java.util.HashMap;

import java.util.Iterator;

import java.util.Map;

import java.util.Map.Entry;

public class Main {

public static void main(String[] arguments)

{

Map<Integer, String> intType

= new HashMap<Integer, String>();

intType.put(1, "First");

intType.put(2, "Second");

intType.put(3, "Third");

intType.put(4, "Fourth");

Iterator<Entry<Integer, String> > ob

= intType.entrySet().iterator();

while (ob.hasNext()) {

Map.Entry<Integer, String> new_Map

= (Map.Entry<Integer, String>)

ob.next();

System.out.println(new_Map.getKey() + " = "

+ new_Map.getValue());

}

}

}

Output

1 = First

2 = Second

3 = Third

4 = Fourth

Method 4: Iterating through a HashMap using Lambda Expressions

Iterating through a HashMap using lambda expressions in Java is a concise and modern way to access and process the key-value pairs. We can use the forEach method that is available in the Map interface.

Program

import java.util.HashMap;

import java.util.Map;

class HashMapInteration{

public static void main(String[]args){

Map<String,Integer> map = new HashMap<>() ;

map.put("Apple",1) ;

map.put('Banana',2) ;

map.put('Cherry',3) ;

map.forEach((key,value)->{

System.out.println("Key"+key+"value"+value) ;

});

}

}

Output

1 = First

2 = Second

3 = Third

4 = Fourth

Method 5: Loop through a HashMap using Stream API

Using the Stream API in Java, we can also iterate through a HashMap, which provides a functional approach to processing collections. This method allows you to leverage various stream operations like filtering, mapping, and collecting.

Program

import java.util.HashMap;

import java.util.Map;

public class HashMapStreamIteration {

public static void main(String[] args) {

Map<String, Integer> map = new HashMap<>();

map.put("Apple", 1);

map.put("Banana", 2);

map.put("Cherry", 3);

map.entrySet().stream()

.forEach(entry -> {

String key = entry.getKey();

Integer value = entry.getValue();

System.out.println("Key: " + key + ", Value: " + value);

});

}

}

Output

Key: Apple, Value: 1

Key: Cherry, Value: 3

Key: Banana, Value: 2

Also Read: Java Interview Questions and Answers

Conclusion

Iterating through a HashMap in Java can be tailored to your needs using various methods. For optimal performance when accessing both keys and values, entrySet() is recommended. If you only need keys or values, consider keySet() or values(), respectively. For scenarios requiring modification during iteration, an Iterator is the safest choice. Lastly, Java 8 streams offer a functional and succinct way to handle entries. Select the iteration technique that best fits your use case for clarity and efficiency. Want to learn more about Java? Consider pursuing the Certificate Program in Application Development offered by Hero Vired.

FAQs

Is the Iteration order guaranteed in a HashMap?

No, HashMap does not guarantee any specific order of its entries. If you need ordered iteration, consider using LinkedHashMap, which maintains insertion order, or TreeMap, which sorts by keys.

What should I do if I need to maintain an insertion order?

They use a LinkedHashMap instead of a HasMap to preserve the order of insertion during iteration.

Can I use a HashMap in a multi-threaded environment?

HashMap does not support thread safety. If a client requires it to be used in a multi-threaded environment, then the synchronized key is used, or we use ConcurrentHashMap.

What happens If I insert a duplicate key into a HashMap?

If you insert a duplicate key, the new value will overwrite the existing value associated with that key.

Is it possible to have a HashMap with null keys and values?

Yes, a HashMap allows one null key and multiple null values.

Updated on October 21, 2024

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