Java Collections unmodifiableMap() Method

The unmodifiableMap() method in Java is a utility method provided by the java.util.Collections class. It returns an unmodifiable view of the specified map, meaning that any attempts to modify the map through this view will result in an UnsupportedOperationException. This method is useful when you need to provide a read-only view of a map, ensuring that the original map remains unchanged.

Table of Contents

1. Introduction
2. unmodifiableMap() Method Syntax
3. Examples
   • Basic Usage of unmodifiableMap()
   • Using unmodifiableMap() with Custom Classes
4. Real-World Use Case
5. Conclusion

Introduction

The Collections.unmodifiableMap() method allows you to create a read-only view of an existing map. The returned map does not allow any modifications such as adding, removing, or updating entries. This is useful in scenarios where you want to share a map with other parts of your program without allowing them to alter it, ensuring data integrity and immutability.

unmodifiableMap() Method Syntax

The syntax for the unmodifiableMap() method is as follows:

public static <K, V> Map<K, V> unmodifiableMap(Map<? extends K, ? extends V> m)

Parameters:

• m: The map for which an unmodifiable view is to be returned.

Returns:

• An unmodifiable view of the specified map.

Throws:

• NullPointerException if the specified map is null.

Examples

Basic Usage of unmodifiableMap()

The following example demonstrates how to use the unmodifiableMap() method to create a read-only view of a map.

Example

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

public class UnmodifiableMapExample {
    public static void main(String[] args) {
        // Create a map with initial entries
        Map<String, String> map = new HashMap<>();
        map.put("Apple", "Fruit");
        map.put("Carrot", "Vegetable");
        map.put("Banana", "Fruit");

        // Create an unmodifiable view of the map
        Map<String, String> unmodifiableMap = Collections.unmodifiableMap(map);

        // Display the unmodifiable map
        System.out.println("Unmodifiable Map: " + unmodifiableMap);

        // Attempt to modify the unmodifiable map (will throw UnsupportedOperationException)
        try {
            unmodifiableMap.put("Date", "Fruit");
        } catch (UnsupportedOperationException e) {
            System.out.println("Error: Cannot modify an unmodifiable map");
        }

        // Display the original map after attempted modification
        System.out.println("Original Map: " + map);
    }
}

Output:

Unmodifiable Map: {Apple=Fruit, Carrot=Vegetable, Banana=Fruit}
Error: Cannot modify an unmodifiable map
Original Map: {Apple=Fruit, Carrot=Vegetable, Banana=Fruit}

Using unmodifiableMap() with Custom Classes

You can also use the unmodifiableMap() method with maps containing instances of custom classes.

Example

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

class Student {
    String name;

    Student(String name) {
        this.name = name;
    }

    @Override
    public String toString() {
        return name;
    }

    @Override
    public boolean equals(Object obj) {
        if (this == obj) return true;
        if (obj == null || getClass() != obj.getClass()) return false;
        Student student = (Student) obj;
        return name.equals(student.name);
    }

    @Override
    public int hashCode() {
        return name.hashCode();
    }
}

public class CustomUnmodifiableMapExample {
    public static void main(String[] args) {
        // Create a map of students and their scores
        Map<Student, Integer> studentScores = new HashMap<>();
        studentScores.put(new Student("Amit"), 85);
        studentScores.put(new Student("Neha"), 90);
        studentScores.put(new Student("Raj"), 78);

        // Create an unmodifiable view of the student map
        Map<Student, Integer> unmodifiableStudentScores = Collections.unmodifiableMap(studentScores);

        // Display the unmodifiable student map
        System.out.println("Unmodifiable Student Map: " + unmodifiableStudentScores);

        // Attempt to modify the unmodifiable student map (will throw UnsupportedOperationException)
        try {
            unmodifiableStudentScores.put(new Student("Vikram"), 88);
        } catch (UnsupportedOperationException e) {
            System.out.println("Error: Cannot modify an unmodifiable student map");
        }

        // Display the original student map after attempted modification
        System.out.println("Original Student Map: " + studentScores);
    }
}

Output:

Unmodifiable Student Map: {Neha=90, Amit=85, Raj=78}
Error: Cannot modify an unmodifiable student map
Original Student Map: {Neha=90, Amit=85, Raj=78}

Explanation:

1. Unmodifiable View: The unmodifiableMap() method returns a read-only view of the specified map, ensuring that any attempts to modify the map through this view will result in an UnsupportedOperationException.

2. Immutable Nature: The example shows that any modification attempts result in an exception, demonstrating the immutability of the unmodifiable map.

3. Custom Class: The method works with custom class instances, allowing you to create unmodifiable views of maps containing user-defined objects.

Real-World Use Case

Providing Read-Only Access to a Map

In real-world applications, the unmodifiableMap() method can be used to provide read-only access to a map, such as when returning a map from a method that should not be modified by the caller.

Example

Imagine a scenario where you have a class that manages a map of configuration settings, and you want to provide read-only access to the settings.

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

class Configuration {
    private final Map<String, String> settings;

    public Configuration() {
        settings = new HashMap<>();
        settings.put("timeout", "30s");
        settings.put("maxConnections", "100");
        settings.put("enableLogging", "true");
    }

    // Method to get an unmodifiable view of the configuration settings
    public Map<String, String> getSettings() {
        return Collections.unmodifiableMap(settings);
    }
}

public class ConfigurationExample {
    public static void main(String[] args) {
        Configuration config = new Configuration();

        // Get the unmodifiable view of configuration settings
        Map<String, String> settings = config.getSettings();

        // Display the configuration settings
        System.out.println("Configuration Settings: " + settings);

        // Attempt to modify the configuration settings (will throw UnsupportedOperationException)
        try {
            settings.put("newSetting", "value");
        } catch (UnsupportedOperationException e) {
            System.out.println("Error: Cannot modify configuration settings");
        }
    }
}

Output:

Configuration Settings: {enableLogging=true, timeout=30s, maxConnections=100}
Error: Cannot modify configuration settings

Explanation:

1. Read-Only Access: The getSettings() method returns an unmodifiable view of the configuration settings, ensuring that the settings cannot be modified externally.

2. Immutable Map: The example demonstrates the use of an unmodifiable map to protect the integrity of configuration data.

Conclusion

The Collections.unmodifiableMap() method is a powerful utility for creating unmodifiable (read-only) views of maps in Java. By providing a simple way to ensure immutability, it enhances the flexibility and safety of your code by preventing unintended modifications. This method is particularly valuable in scenarios where you need to protect the integrity of maps while providing access to them, improving the robustness and maintainability of your Java applications.