read only collections

<T> Collection<T> java.util.Collections.unmodifiableCollection(Collection<? extends T> c)

Returns an unmodifiable view of the specified collection.

This method allows modules to provide users with "read-only" access to internal collections.

Query operations on the returned collection "read through" to the specified collection. If we will try to modify the returned collection whether direct or via its iterator, result in an UnsupportedOperationException.

The returned collection will be serializable if the specified collection is serializable.

import java.util.ArrayList;

import java.util.Collection;

import java.util.Collections;

import java.util.Iterator;

import java.util.List;

/**

 * Collection read only by using

 * Collections.unmodifiableCollection()

 */

public class ReadOnlyCollection {

     public static void main(String args[]) {

          //making existing ArrayList readonly in Java

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

          readableList.add("element 1");

          readableList.add("element 2");

          //creating read only Collection in Java

          Collection<String> readOnlyCollection =

                       Collections.unmodifiableCollection(readableList);

          Iterator<String> itr = readOnlyCollection.iterator();

          while(itr.hasNext()) {

                 System.out.println(itr.next());

          }

             

          //add will throw Exception because List is read only

          readOnlyCollection.add("element 3");

     }

}

Output:

element 1

element 2

Exception in thread "main" java.lang.UnsupportedOperationException

at java.util.Collections$UnmodifiableCollection.add(Unknown Source) at core.readonly.collections.ReadOnlyCollection.main(ReadOnlyCollection.java:30)