How to Create a Read-Only ArrayList in Java

How can I create a read-only ArrayList in Java that prevents any modifications after it is initialized?

List<String> readOnlyList = Collections.unmodifiableList(new ArrayList<>(Arrays.asList("Element1", "Element2", "Element3")));

In Java, to create a read-only ArrayList, you can utilize the `Collections.unmodifiableList()` method that ensures the list cannot be modified after initialization. This approach is beneficial when you want to provide a stable API without risks of unintended modifications.

import java.util.*;

public class ReadOnlyArrayListExample {
    public static void main(String[] args) {
        List<String> originalList = new ArrayList<>(Arrays.asList("Element1", "Element2", "Element3"));
        List<String> readOnlyList = Collections.unmodifiableList(originalList);
        
        System.out.println(readOnlyList); // Output: [Element1, Element2, Element3]
        // The following line will throw UnsupportedOperationException:
        // readOnlyList.add("Element4");
    }
}

Causes

• Modifying an ArrayList can lead to data inconsistencies in your application.
• You may want to expose data without allowing modifications.

Solutions

• Use `Collections.unmodifiableList()` to create a read-only version of an ArrayList.
• Create an immutable list using `List.of()` in Java 9 and later, which inherently does not allow any modifications.

Mistake: Using a modifiable ArrayList instead of creating a wrapped unmodifiable list.

Solution: Wrap the ArrayList with `Collections.unmodifiableList()` before exposing it.

Mistake: Not handling UnsupportedOperationException when modifying a read-only list.

Solution: Always be cautious of the exception and avoid operations that change the list.