Programming Reflection: How to create read only List, Map and Set in Java

Read only List, Map and Set in Java

Read only List means a List where you can not perform modification operations like add, remove or set. You can only read from the List by using get method or by using Iterator of List, This kind of List is good for certain requirement where parameters are final and can not be changed. In Java you can use Collections.unModifiableList() method to create read only List , Collections.unmodifiableSet() for creating read-only Set like read only HashSet and similarly creating a read only Map in Java, as shown in below example. Any modification in read only List will result in java.lang.UnSupportedOperationException in Java. This read-only List example is based on Java 5 generics but also applicable to other Java version like JDK 1.4 or JDK 1.3, just remove Generics code i.e. angle bracket which is not supported prior to Java 5. One common mistake programmer makes is that assuming fixed size List and read only List as same. As shown in our 3 example of converting Array to Array List , we can use Arrays.asList() method to create and initialized List at same line. List implementation returned by this method is fixed size and it doesn’t allow adding or removal of element but it is not read only because you can update objects by calling set(index) method. How to make a collection read only is also a popular Java collection interview question, which makes this Java collection tutorial even more important.

Read only List, Set and Map Example - Java

Here is sample Java program which demonstrate method of creating read only List, Set and Map in Java. You can make any List, Set or Map implementation as read only by following code example. Just remember that Collections.unModifiableList() , Collections.unModifiableSet() and Collections.unModifiableMap() method in Java. On read only List add, remove and set operation is not permitted, on read only Set you can not add or remove elements and in read only Map you can not put new entries or update existing entries. You can use same methods to convert any List implementation like ArrayList, LinkedList or Vector to read only ArrayList , LinkedList and Vector in Java.

package example;

import java.util.ArrayList;
import java.util.Collections;
import java.util.HashMap;
import java.util.HashSet;
import java.util.List;
import java.util.Map;
import java.util.Set;

/**
* Java program to create read only List, Set and Map in Java. You can first create

* a List or Set and than make it unmodifiable or read-only by

* using Collections.unmodifiableList() or Collections.unmodifiableSet() method.
*
* @author Javin Paul
*/
public class ReadOnlyListSetMap {

public static void main(String args[]) {

// creating List in Java
List<String> contents = new ArrayList<String>();

// initializing List in Java
contents.add("Example");
contents.add("Tutorial");
contents.add("Program");

// Currently This List is not read only, you can add or remove elements from List
contents.add("Tips"); //should not be allowed if List is read only.

System.err.println("normal List in Java : " + contents);

//creating readonly List from contents
contents = Collections.unmodifiableList(contents);

//java.lang.UnsupportedOperationException -- no modification in read only list

//not allowed as it is read-only List in Java
contents.add("Can I add object into read only List - No");

contents.remove("Example"); //remove not allowed in read only list

//java.lang.UnSupportedOperation - List update not allowed
contents.set(0, "Can I override or set object in read-only Set - No");

//Creating read only Set in Java
//similar to read-only List you can also create a Set which is read only
//i.e. addition , removal and modification operation is not permitted on list

//Creating a Set based on contents of List
Set<String> readOnlySet = new HashSet<String>(contents);

System.out.println("original Set in Java : " + readOnlySet);

//Set is not yet read-only you can still add elements into Set
readOnlySet.add("Override");

System.out.println("Set before making read only : " + readOnlySet);

//making Set readonly in Java - no add remove or set operation permitted
readOnlySet = Collections.unmodifiableSet(readOnlySet);

//trying to add element in read only Set - java.lang.UnSupportedOperationException
readOnlySet.add("You can not add element in read Only Set");

//trying to remove element from read only set
readOnlySet.remove("Example"); //you can not remove elements from read only Set

// Creating read only Map in Java
// Similar to List and Set you can also create read only or unmodifiable Map in Java
// add , remove and override is not allowed on read only Map in Java

Map<String, String> contries = new HashMap<String, String>();
contries.put("India", "New Delhi");

//Map is not read only yet, you can still add entries into
contries.put("UK", "London");

System.out.println("Map in Java before making read only: " + contries);

//Making Map read only in Java
Map readOnlyMap = Collections.unmodifiableMap(contries);

//you can not put a new entry in read only Map in Java
readOnlyMap.put("USA", "Washington"); //java.lang.UnSupportedOperation

//you can not remove keys from read only Map in Java
readOnlyMap.remove("UK"); //java.lang.UnSupportedOperation

}

}

That’s all on how to create read only List, Set and Map in Java. Its very easy to make any List implementation read only by wrapping it with Collections.unModifiableList(). Use same technique to convert any other Collection into read only in Java.

Other Java collection tutorial from Javarevisited

Difference between ArrayList and Vector in Java

Difference between ArrayList and LinkedList in Java

10 example of Hashtable in Java

4 ways to traverse Map in Java

How to convert ArrayList to HashSet in Java

How to sort ArrayList in ascending and descending order in Java

Please share with your friends if like this article