How can i make a class Immutable if it has a instance variable of java.lang.Object?

Question

final public class ImmutableWithObject {

    final Object obj;
    final List myList;

    ImmutableWithObject(Object obj1, List list)
    {
        this.obj = obj1;
        this.myList = ((List) ((ArrayList) list).clone());
    }

    public Object getObj() {
        return this.obj;
    }

    public List getMyList() {
        return (List) ((ArrayList<String>) this.myList).clone();
    }

    public static void main(String[] args) {

        ImmutableWithObject io = new ImmutableWithObject(new Date(), new ArrayList());

        ((Date) io.getObj()).setDate(22);

        System.out.println((Date) io.getObj());
    }
}

o/p : Mon Aug 22 00:50:04 IST 2011

which is incorrect.

Answer 1

Immutable means that once the object has been constructed, its state does not change.

• Make class final ( which you have already done )
• Make the instance variables as private and final
• Dont provide methods that change the state
• When passing instance variables, send copies instead of original.

From EJ Item 15 <-- Lot more information in there

Classes should be immutable unless there's a very good reason to make them mutable. If a class cannot be made immutable, limit its mutability as much as possible.

Answer 2

You cannot make it immutable since this object cannot create copies of the contents of the list or the Object. Assuming that you mean to have getters for accessing those properties, the properties themselves were created elsewhere and can be changed in code external to this class that has a reference to it.

The only exception to this is if the contents of Object and List are themselves immutable. Then you can create an immutable copy of the list and you would be done.