Create Immutable HashMap in Java

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

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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

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