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.

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References

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