Java 9 Map.ofEntries() Method - Create Immutable Map Example

With Java 9, new factory methods are added to List, Set and Map interfaces to create immutable instances. These factory methods are convenience factory methods to create a collection in less verbose and in a concise way.

In this post, I show you how to create an immutable Map using Java 9 provided Map.ofEntries() static factory method.

Create Immutable Map

Map factory methods are the same as List or Set overloaded factory methods. The only difference is that the signatures of the methods take alternating keys and values as arguments. e.g.

static <K,V> Map<K,V>   of()
static <K,V> Map<K,V>   of(K k1, V v1)
static <K,V> Map<K,V>   of(K k1, V v1, K k2, V v2)
...
...
static <K,V> Map<K,V>   ofEntries(Map.Entry<? extends K,? extends V>... entries)

Create Immutable Map Before Java 9 Example

Prior to Java 9, the creation of an immutable Map was some kind of verbose task. For Example:

public class ImmutableMapExample {

    public static void main(String[] args) {

        // Creating a HashMap
        Map < String, Integer > numberMapping = new HashMap < > ();

        // Adding key-value pairs to a HashMap
        numberMapping.put("One", 1);
        numberMapping.put("Two", 2);
        numberMapping.put("Three", 3);

        Collections.unmodifiableMap(numberMapping);
        System.out.println(fruits);
    }
}

Output:

{One=1, Two=2, Three=3}

Create Immutable Map Example - Java 9 Map.ofEntries() Method

Let’s take an example of creating an immutable Map in java 9.

package net.javaguides.corejava.java9;

import java.util.Map;

public class ImmutableMapExample {

    public static void main(String[] args) {
        Map < String, String > fruits = Map.ofEntries(
            Map.entry("1", "Banana"),
            Map.entry("2", "Apple"),
            Map.entry("3", "Mango"),
            Map.entry("4", "Orange"));

        System.out.println(fruits);
    }

}

Output:

{1=Banana, 2=Apple, 3=Mango, 4=Orange}

Related Java 9 Posts

• Java 9 Private Methods in Interface with Examples - Learn how to use private methods in interface with examples.

• Java 9 List.of() Method - Create Immutable List Example - In this post, I show you how to create an immutable list using Java 9 provided List.of() static factory method.

• Java 9 Set.of() Method - Create Immutable Set Example - In this post, I show you how to create an immutable Set using Java 9 provided Set.of() static factory method.

• Java 9 Map.ofEntries() Method - Create Immutable Map Example - In this post, I show you how to create an immutable Map using Java 9 provided Map.ofEntries() static factory method.

• Java 9 - Stream API Improvements with Examples - In this article, we will learn what are the Stream API improvements made in Java 9 with an example.

• Java 9 - Optional Class Improvements with Examples - Learn Optional class improvements with examples.