In this article, we will learn to create an unmodifiable Map in Java 9. First, we will learn about Map and Unmodifiable Map and after that we will be knowing the different ways through which we can create an unmodifiable map (Map.of method and unmodifiableMap() Method).

Map

In Java, a Map is an interface in the Collections framework used to store data in the form of key-value pairs. Each key in the map is unique and maps to some value.

Some of the implementations of the Map interface are:

What is an Unmodifiable Map?

An unmodifiable Map is one whose keys and values can't be added, removed, or updated once an unmodifiable instance of a map has been created. Calling any mutator method on the map causes an UnsupportedOperationException.

Different ways to create an unmodifiable Map

The following are two different ways to create an unmodifiable map in Java 9:

Using Map.of() Method
Using unmodifiableMap() Method

Using Map.of() Method

The static factory methods: Map.of() method provides an easy way to create unmodifiable maps in Java 9.

Syntax

Initialization of an unmodifiable map using the Map.of() method. The below Map.of() method accepts the null values only:

Map<Integer> immutableEmptyMap = Map.of();

In addition to this method, the Map interface provides various polymorphic variants that accepts different parameters. Using these you can create an unmodifiable map with a single key-value pair, two key-value pairs, three key-value pairs... up to 10 key-value pairs.

If we want to generate an immutable map for a single key-value pair where k1 is the key and v1 is the value, we can use:

static <K,V> Map<K,V> of(K k1, V v1)

If we want to generate an immutable map of 2 elements where the input key-value pairs are k1, v1, and k2, v2, respectively, we can use:

static <K,V> Map<K,V> of(K k1, V v1, K k2, V v2)

An instance of a map created by using the Map.of() methods has the following characteristics.

An immutable map doesn't allow null keys and values. If any attempt to create with null keys or values, then it throws a NullPointerException.
The duplicate keys are rejected at the time of creation itself. Passing duplicate keys to a static factory method causes an IllegalArgumentException.
The immutable maps are serializable if all keys and values are serializable.
The order of iteration of mappings is unspecified and subject to change.

Example

Below is an example of creating an unmodifiable map in Java 9:

import java.util.Map;
public class UnmodifiableMapTest {
   public static void main(String[] args) {
      Map<String, String> empMap = Map.of("101", "Raja", "102", "Adithya",
      "103", "Jai", "104", "Chaitanya");
      System.out.println("empMap - " + empMap);
   }
}

Output

empMap - {104=Chaitanya, 103=Jai, 102=Adithya, 101=Raja}

Using unmodifiableMap() Method

The standard way to create an unmodifiable map was by wrapping a mutable map using Collections.unmodifiableMap() in Java 9.

Syntax

Initialization of an unmodifiable map using the unmodifiableMap() method:

Map<Integer> immutableEmptyMap  = Collections.unmodifiableMap(muttableMap);

An instance of a map created by using the unmodifiableMap() method has the following characteristics:

Collections.unmodifiableMap() wraps an existing map and prevents modification through the wrapper.
Unlike the Map.of() method, the unmodifiableMap() method allows null elements if present in the original Map.
The Key-value pairs in the unmodifiable Map maintain the same order as the original map.

Example

Below is an example of creating an unmodifiable map using the unmodifiableMap() method in Java 9:

import java.util.*;
public class UnmodifiableMapMethodTest {
    public static void main(String[] args) {
        Map<String, String> empMap = new HashMap<>();
        empMap.put("101", "Raja");
        empMap.put("102", "Adithya");
        empMap.put("103", "Jai");
        empMap.put("104", "Chaitanya");
        Map<String, String> immutablemap = Collections.unmodifiableMap(empMap);
        System.out.println("empMap - " + immutablemap);
    }
}

Output

empMap - {101=Raja, 102=Adithya, 103=Jai, 104=Chaitanya}

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