Make ArrayList Read Only

Question

In Java, how can you make an ArrayList read-only (so that no one can add elements, edit or delete elements) after initialization?

Answer 1

Pass the ArrayList to Collections.unmodifiableList () . It returns an unmodifiable representation of the specified list. Use this returned list and never use the original ArrayList.

Answer 2

Pass the list object to Collections.unmodifiableList (). The following is an example of an example.

  import java.util.*; public class CollDemo { public static void main(String[] argv) throws Exception { List stuff = Arrays.asList(new String[] { "a", "b" }); List list = new ArrayList(stuff); list = Collections.unmodifiableList(list); Set set = new HashSet(stuff); set = Collections.unmodifiableSet(set); Map map = new HashMap(); map = Collections.unmodifiableMap(map); System.out.println("Collection is read-only now."); } } 

Answer 3

Are you sure you want to use ArrayList in this case?

It might be better to populate the ArrayList with all your information first, and then convert the ArrayList to a finite array when the Java program initializes

Answer 4

Check the Google Collections , and for your needs look at the ImmutableList. But all there is to see.

Answer 5

Pass the collection object to its equivalent non-modifiable function in the Collections class, the following code shows the use of unmodifiableList

 import java.util.ArrayList; import java.util.Collections; import java.util.List; public class Temp { public static void main(String[] args) { List<Integer> objList = new ArrayList<Integer>(); objList.add(4); objList.add(5); objList.add(6); objList.add(7); objList = Collections.unmodifiableList(objList); System.out.println("List contents " + objList); try { objList.add(9); } catch(UnsupportedOperationException e) { e.printStackTrace(); System.out.println("Exception occured"); } System.out.println("List contents " + objList); } } 

just like you can create other collections that are not modifiable,

• Collections.unmodifiableMap (map);
• Collections.unmodifiableSet (set);