Read only List, Map and Set in Java
A 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 a 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 versions 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.
