How to Create Read Only List, Set, Map in Java 8, Java 9 and java 10

1. Introduction

We'll learn what are the ways to create a read only collection List, Set, or Map in java 8 and older versions. In other ways, It is called as Imuutale or unmodifiable collection.

Usually, Whatever the objects that we create using a new keyword will create mutable collections objects as below.

List<String> list = new ArrayList<String>();

Next, On list instance, you can call add() or remove() methods of List interface.

list.add("one");
list.add("two");

list.remove("two");

list.add("three");

System.out.println("List values : "+list);

Output:

List values : [one, three]

You can observe that previously added value "two" is removed from the list and added a new value "three" to it.

Our main core concept of this article is to make the existing list as read only after adding the needed values.

2. Collections.unmodifiableCollection() Method

The Collections utility class is added with several methods to make the collection as Read Only. All of these are static methods so directly can be accessed with the Collections class.

Collections.unmodifiableCollection() method returns an unmodifiable view of the specified collection. This method works for both list and set examples.

Syntax:

public static <T> Collection<T> unmodifiableCollection(Collection<? extends T> c)

Collections.unmodifiableCollection() Example:

package com.javaprogramto.readonly;

import java.util.ArrayList;

import java.util.Collection;

import java.util.Collections;

import java.util.List;


public class ReadOnlyCollectionExample {

    public static void main(String[] args) {

        List<String> list = new ArrayList<String>();

        list.add("one");

        list.add("two");

        list.add("three");


        list.remove("two");


        System.out.println("List values : " + list);


        Collection<String> readOnlyList = Collections.unmodifiableCollection(list);


        System.out.println("Read only collection : " + readOnlyList);

    }
}

Output:

List values : [one, three]
Read only collection : [one, three]

Now try to add the new value to the readOnlyList. Then, it will produce the runtime exception saying "UnsupportedOperationException".

readOnlyList.add("new value");

Error:

Exception in thread "main" java.lang.UnsupportedOperationException
at java.util.Collections$UnmodifiableCollection.add(Collections.java:1055)
at ReadOnlyCollectionExample.main(ReadOnlyCollectionExample.java:25)

3. Collections.unmodifiableList() - To make List as read only

Returns an unmodifiable view of the specified list. Input argument must be a List interface implementation.

package com.javaprogramto.readonly;

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


public class ReadOnlyCollectionunmodifiableListExample {

    public static void main(String[] args) {

        List<String> list = new ArrayList<String>();

        list.add("one");

        list.add("two");

        list.add("three");

        list.remove("two");

        System.out.println("List values : " + list);


        List<String> readOnlyList = Collections.unmodifiableList(list);


        System.out.println("Read only collection : " + readOnlyList);


        if (readOnlyList.getClass().getName().contains("Unmodifiable")) {

            System.out.println("readOnlyList is a Read only collections");

        } else {

            System.out.println("This is not Read only collections");

        }

    }
}

Output:

List values : [one, three]
Read only collection : [one, three]
readOnlyList is a Read only collections

Note: Making List as read only can be done in another way using Arrays.asList() method but Arrays class a utility method to only for List and not for Set, Map implementations.

4. Collections.unmodifiableSet() - To make Set as read only

Collections.unmodifiableSet() returns a unmodifiable view for the given set. Input arguemnt must be Set interface implementation and either HashSet or LikedHashSet.

package com.javaprogramto.readonly;

import java.util.Collections;
import java.util.LinkedHashSet;
import java.util.Set;


public class ReadOnlyCollectionunmodifiableSetExample {

    public static void main(String[] args) {

        Set<String> set = new LinkedHashSet<String>();

        set.add("one");
        set.add("two");
        set.add("three");
       
        set.remove("two");


        System.out.println("Set values : " + set);


        Set<String> readOnlySet = Collections.unmodifiableSet(set);


        System.out.println("Read only collection : " + readOnlySet);


        if (readOnlySet.getClass().getName().contains("Unmodifiable")) {

            System.out.println("readOnlySet is a Read only collections");

        } else {

            System.out.println("readOnlySet not Read only collections");
        }
       
    }
}

Output:

Set values : [one, three]
Read only collection : [one, three]
readOnlySet is a Read only collections

5. Collections.unmodifiableMap() - To make Map as read only

Collections.unmodifiableMap() returns a unmodifiable view for the given Map. Input arguments must be Set interface implementation and either HashMap or LikedHashMap or TreeMap.

package com.javaprogramto.readonly;

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


public class ReadOnlyCollectionunmodifiableMapExample {

    public static void main(String[] args) {

        Map<Integer, String> map = new HashMap<Integer, String>();

        map.put(100, "one");
        map.put(200, "two");
        map.put(300, "three");

        map.remove(100);

        System.out.println("Set map : " + map);


        Map<Integer, String> readOnlyMap = Collections.unmodifiableMap(map);


        System.out.println("Read only map : " + readOnlyMap);


        if (readOnlyMap.getClass().getName().contains("Unmodifiable")) {

            System.out.println("readOnlyMap is a Read only collections");

        } else {

            System.out.println("readOnlyMap not Read only collections");

        }
    }
}

Output:

Set map : {200=two, 300=three}
Read only map : {200=two, 300=three}
readOnlyMap is a Read only collections

6. Java 9 Read Only List

In java 9, It is very easy to do converting an Array into a List as read-only

List interface is added with static method List.of() which takes the varargs.

String[] stringArray = {"1", "2"};

List<String> stringList = List.of(stringArray);

7. Java 10 Read Only List

In java 10 api, List is added with copyOf() and Collectors added with toUnmodifiableList() method.

List.copyOf(list);

List<String> readOnlyListJava10 = list.stream().collect(Collectors.toUnmodifiableList());

Set<String> readOnlySetJava10 = list.stream().collect(Collectors.toUnmodifiableSet());

Map<Integer, String> readOnlyMapJava10 = list.stream().collect(Collectors.toUnmodifiableMap());

8. Conclusion

In this article, You've seen how to make read only Collection classes in Java 10, java 9, and older versions.

As usual, all examples are over GitHub.

Static Method
Collections API
Collections. unmodifiableCollection()
List.copyOf()
List.of()
Collectors.toUnmodifiableList()
GeekForGeeks