Java - immutable ArrayList in Java

In this short article, we would like to how in Java create immutable List - read-only list.

This technique can be used with all classes that implements List interface. Main advantage of this approach is: the list is only covered by class - list is not copied that makes solution very fast.

Quick solution:

Collections.unmodifiableList(list);

Practical example

import java.util.Arrays;
import java.util.List;
import java.util.ArrayList;
import java.util.Collections;

public class Example {

    public static void main(String[] args) {

        List<String> list = new ArrayList<>(Arrays.asList("one", "two", "three"));
        List<String> immutableList = Collections.unmodifiableList(list);

        immutableList.add("four");
    }
}

Output:

Exception in thread "main" java.lang.UnsupportedOperationException
	at java.base/java.util.Collections$UnmodifiableCollection.add(Collections.java:1060)
	at Example.main(Example.java:13)

Note: in read-only list we are not able to do any modification.