Raja PLECI: How to create read only List, Map and Set in Java â€“ unmodifiable example

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

Example to create read only List Set and Map in 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.

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