What is Hashmap in Java? Explained with Examples

HashMap is a collection class in Java. You can use it to store key and value pairs. Its performance depends on the initial capacity and the load factor. HashMap has various methods that allow you to use its hash table data structure. 

In the following article, we’ll explore what HashMap is and learn about its various constructors through examples. 

What is HashMap?

HashMap is a collection class based on Map. We use it to store Key & value pairs. You denote a HashMap Java as HashMap<K, V>, where K stands for Key and V stands for Value. 

HashMap is similar to the Hashtable class. The difference between the two is that HashMap is unsynchronized while Hashtable isn’t. Also, unlike Hashtable, HashMap allows null values and the null key. 

Since HashMap isn’t an ordered collection, it doesn’t return the keys and values in the order you insert them. Also, HashMap doesn’t sort the stored keys and values. If you want to use the HashMap class and its methods, you’ll have to import java.util.HashMap (or its superclass).

HashMap gives you a simple implementation of Java’s Map interface. You can access the key-value pairs you store through HashMap by using an index of another type (such as an integer). Keep in mind that if you use a duplicate key, it replaces the element of the relevant key. Also, you can only use one null key object; however, there can be any number of null values. 

HashMap implements Cloneable, Serializable, Map<K, V> interfaces. It extends the AbstractMap<K, V> class, and its direct subclasses are PrinterStateReasons and LinkedHashMap. 

Features of HashMap 

Following are the primary features in HashMap: 

• It is a component of the java.util.package.
• You can use duplicate values, but HashMap doesn’t allow duplicate keys. It means one key can’t have multiple values, but multiple keys can have a single value. 
• You can use null keys only once, but you can use various null values. 
• HashMap provides no guarantees regarding the map’s order. So, it doesn’t guarantee if the order will remain constant. 
• HashMap is very similar to Hashtable, with the only difference being that HashMap is unsynchronized. 
• HashMap implements a Serializable and Cloneable interface. 

HashMap Constructors

There are four constructors in HashMap:

1. HashMap()

HashMap() is the default constructor that creates an instance with a load factor of 0.75 and an initial capacity of 16.

Example: 

// Showing how HashMap() constructor works 

import java.io.*;

import java.util.*;

class AddElementsToHashMap {

    public static void main(String args[])

    {

        // You don’t have to mention the

        // Generic type twice

        HashMap<Integer, String> hm1 = new HashMap<>();

        // Using Generics to initialize HashMap

        HashMap<Integer, String> hm2

            = new HashMap<Integer, String>();

        // Use the put method to add any element

        hm1.put(1, “A”);

        hm1.put(2, “B”);

        hm1.put(3, “C”);

        hm2.put(4, “D”);

        hm2.put(5, “E”);

        hm2.put(6, “F”);

        System.out.println(“Results of hm1 are : “

                           + hm1);

        System.out.println(“Results of HashMap hm2 are : “

                           + hm2);

    }

}

Output: 

Results of hm1 are : {1=A, 2=B, 3=C}

Results of hm2 are : {4=D, 5=E, 6=F}

2. HashMap(int initialCapacity)

HashMap(int initialCapacity) would create an instance with load 0.75 and a specific initial capacity. 

Example:

// Showing how HashMap(int initialCapacity) constructor works 

import java.io.*;

import java.util.*;

class AddElementsToHashMap {

    public static void main(String args[])

    {

        // You don’t have to mention the

        // Generic type twice

        HashMap<Integer, String> hm1 = new HashMap<>(10);

        // Using Generics to initialize HashMap

        HashMap<Integer, String> hm2

            = new HashMap<Integer, String>(2);

        // Use the put method to add any element

        hm1.put(1, “A”);

        hm1.put(2, “B”);

        hm1.put(3, “C”);

        hm2.put(4, “D”);

        hm2.put(5, “E”);

        hm2.put(6, “F”);

        System.out.println(“Results of hm1 are : “

                           + hm1);

        System.out.println(“Results of HashMap hm2 are : “

                           + hm2);

    }

}

Output: 

Results of hm1 are : {1=A, 2=B, 3=C}

Results of hm2 are : {4=D, 5=E, 6=F}

3. HashMap(int initialCapacity, float loadFactor) 

HashMap(int initialCapacity, float loadFactor) generates an instance with a specific load factor and initial capacity.

Example:

// Showing how  HashMap(int initialCapacity, float loadFactor) Constructor works

import java.io.*;

import java.util.*;

class AddElementsToHashMap {

    public static void main(String args[])

    {

        // You don’t have to mention the

        // Generic type twice

        HashMap<Integer, String> hm1

            = new HashMap<>(5, 0.75f);

        // Using Generics to initialize the HashMap

        HashMap<Integer, String> hm2

            = new HashMap<Integer, String>(3, 0.5f);

        // Use the put method to add elements

        hm1.put(1, “A”);

        hm1.put(2, “B”);

        hm1.put(3, “C”);

        hm2.put(4, “D”);

        hm2.put(5, “E”);

        hm2.put(6, “F”);

        System.out.println(“Results of hm1 are : “

                           + hm1);

        System.out.println(“Results of HashMap hm2 are : “

                           + hm2);

    }

}

Output: 

Results of hm1 are : {1=A, 2=B, 3=C}

Results of hm2 are : {4=D, 5=E, 6=F}

4. HashMap(Map map)

HashMap(Map map) would create an instance with the same mappings as the map you specify. 

Example:

// Showing how HashMap(Map map) constructor works 

import java.io.*;

import java.util.*;

class AddElementsToHashMap {

    public static void main(String args[])

    {

        // You don’t have to mention the

        // Generic type twice

        Map<Integer, String> hm1 = new HashMap<>();

         // Using Generics to initialize HashMap

        HashMap<Integer, String> hm2

            = new HashMap<Integer, String>(hml);

        // Use the put method to add any element

        hm1.put(1, “A”);

        hm1.put(2, “B”);

        hm1.put(3, “C”);

        hm2.put(4, “D”);

        hm2.put(5, “E”);

        hm2.put(6, “F”);

        System.out.println(“Results of hm1 are : “

                           + hm1);

        System.out.println(“Results of HashMap hm2 are : “

                           + hm2);

    }

}

Output: 

Results of hm1 are : {1=A, 2=B, 3=C}

Results of hm2 are : {4=D, 5=E, 6=F}

Learn more about Java

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Conclusion

In this article, we learned what HashMap is, its different constructors, and how you can implement them through examples. HashMap has a ton of applications in Java – you can use HashMap to add items, remove them, and manage them as required. 

What are your thoughts on this guide on HashMap? Share them in the comment section below. 

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