Create Immutable HashMap in Java

In this tutorial, we will learn how to make immutable HashMap with an example.

Learn and master Java Collections Framework at Learn Java Collections Framework

We will see two examples to make immutable HashMap:

• First, we will see how to create an immutable HashMap using Collections.unmodifiableMap() method with an example.
• Second, we will see how to create an immutable HashMap using Java 9 provided Map.ofEntries() static factory method.

1. Create Immutable HashMap with Collections.unmodifiableMap() Method 

Prior to Java 9, we create an immutable HashMap using Collections.unmodifiableMap() method like:

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}

2. Create immutable HashMap with Java 9 Map.ofEntries() factory method 

The below example shows how to create an immutable HashMap using Java 9 provided Map.ofEntries() static factory method.

import java.util.HashMap;
import java.util.Map;


public class ImmutableHashMap {

    public static void main(String[] args) {

        Map < String, String > fruits = new HashMap < String, String > ();
        fruits.put("1", "Banana");
        fruits.put("2", "Mango");
        fruits.put("3", "Apple");

        // java 9 with factory methods
        Map < String, String > map = Map.ofEntries(
            Map.entry("1", "Banana"),
            Map.entry("2", "Mango"),
            Map.entry("3", "Apple"));

        System.out.println(map);
    }
}

Output:

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

Conclusion

In this tutorial, we have seen how to make immutable HashMap using Collections.unmodifiableMap() method and Java 9 provided Map.ofEntries() static factory method with an example.

Collections Examples

• Java LinkedHashMap Example
  
• Java HashSet Example
  
• Java LinkedList Example
  
• Java ArrayList Example
  
• How To Remove Duplicate Elements From ArrayList In Java?
  
• Different Ways to Iterate over List, Set, and Map in Java
• Java Comparator Interface Example
• Java Comparable Interface Example
• Java IdentityHashMap Example
• Java WeakHashMap Example
• Java EnumMap Example
• Java CopyOnWriteArraySet Example
• Java EnumSet Class Example
• Guide to Java 8 forEach Method
• Different Ways to Iterate over a List in Java [Snippet]
• Different Ways to Iterate over a Set in Java [Snippet]
• Different Ways to Iterate over a Map in Java [Snippet]
• Iterate over TreeSet in Java Example
• Iterate over LinkedHashSet in Java Example
• Remove First and Last Elements of LinkedList in Java
• Iterate over LinkedList using an Iterator in Java
• Search an Element in an ArrayList in Java
• Iterate over ArrayList using Iterator in Java
• Remove Element from HashSet in Java
• Iterating over a HashSet using Iterator

References

• https://www.javaguides.net/2019/12/java-9-map.ofentries-method-create.html

• https://www.javaguides.net/p/java-9-tutorial.html