Convert a mutable class to immutable class

[Pankaj Kumarkk]

Hi I have a class which is mutable. This class is available in a 3rd party jar and thus I do not own the source code.

I have to use this class in multithreaded scenario and I thus wanted to have a immutable version of it so that I can use it freely in my threads.

The existing class looks like

class Order
{
private int orderNum;

private ArrayList items;

private Date orderDate;

getters method

setters method

}

Can somebody tell me how do I create a immutable version of this class.

[Matthew Brown]

You could write your own wrapper class. Each instance of your class contains an instance of Order. You write getter methods that delegate to Order, but don't write any setters. You'll need a constructor to set all Tue values on creation.

Any use?

[Jesper de Jong]

Matthew's suggestion is good, but ofcourse that will not allow you to use your own wrapper class in place of the original class - if the API of the 3rd party JAR expects Order objects to be passed to its methods, then you can only pass it Order objects (or objects of subclasses of Order).

There's no way to make an existing class, that you can't modify, immutable so that other existing code will suddenly treat it as if it is immutable.

[Maneesh Godbole]

Pankaj Kumarkk wrote:Can somebody tell me how do I create a immutable version of this class.

Check this out http://www.javaranch.com/journal/2003/04/immutable.htm Of course what Jesper said, still stands.

[Rob Spoor]

Matthew Brown wrote:You could write your own wrapper class. Each instance of your class contains an instance of Order. You write getter methods that delegate to Order, but don't write any setters. You'll need a constructor to set all Tue values on creation.

That's not enough. All getters must return defensive copies of the data it returns. For instance, it can't return order.getOrderDate() since Date is mutable. It must therefore return a clone of that date instead. The same needs to be done for all mutable values; the ArrayList can be wrapped using Collections.unmodifiableList.