Collection has an unmodifiableCollection method that returns a collection object that cannot be modified. It means that you cannot add or remove any element from the collection.
import java.util.*;public class UnmodifiableCollection{ public static void main(String args[]) { UnmodifiableCollection unmodifiableCollectionIns = new UnmodifiableCollection(); unmodifiableCollectionIns.proceed(); } private void proceed() { List modifiableList = new ArrayList(); modifiableList.add("Java"); modifiableList.add("C++"); modifiableList.add(".Net"); modifiableList.add("Python"); System.out.println("Modifiable list: " + modifiableList); Collection unmodifiableCollection = Collections.unmodifiableCollection(modifiableList); System.out.println("Unmodifiable list: " + unmodifiableCollection); System.out.println("Attempting to add new item to unmodifiableCollection..."); //This will thrown and exception since we are attempting to add to an unmodifiable collection unmodifiableCollection.add("Perl"); }}/*
Expected output:
[root@mypc]# java UnmodifiableCollectionModifiable list: [Java, C++, .Net, Python]Unmodifiable list: [Java, C++, .Net, Python]Attempting to add new item to unmodifiableCollection...Exception in thread "main" java.lang.UnsupportedOperationException at java.util.Collections$UnmodifiableCollection.add(Unknown Source) at UnmodifiableCollection.proceed(UnmodifiableCollection.java:31) at UnmodifiableCollection.main(UnmodifiableCollection.java:14)*/
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