Should I extend ArrayList to add attributes that isn't null?

Question

I would like to add a collection of objects to an arrayList ,only if the particular attribute is not null.

I am thinking of extending the ArrayList and implementing the check inside the child class.

One alternate way is to check for the the attribute before putting it in a Arraylist, but that would mean , i will have to scatter the if checks every where if i need to add the objects to the arraylist based on the logic.

I would like to know your thoughts on it ... on a second thought is it a overkill ?

Answer 1

Decorator pattern

I would actually recommend wrapping ArrayList using well-documented Decorator pattern. You simply wrap your ArrayList with another List implementation that delegates most of the methods but adds validation logic:

public class ValidatingListDecorator extends AbstractList<MyBusinessObject>
{

    private final List<MyBusinessObject> target;

    public ValidatingListDecorator(List<MyBusinessObject> target) {
        this.target = target;
    }

    @Override
    public MyBusinessObject set(int index, MyBusinessObject element)
    {
        validate(element);
        return target.set(index, element);
    }

    @Override
    public boolean add(MyBusinessObject o)
    {
        validate(o);
        return target.add(o);
    }

    //few more to implement
    
}

Advantages:

• You can still access raw list without validation if you want (but you can restrict this)
• Easier to stack different validations, turn them on and off selectively.
• Promotes composition over inheritance as noted by @helios
• Improves testability
• Does not tie you to a specific List implementation, you can add validation to LinkedList or Hibernate-backed persistent lists. You can even think about generic Collection decorator to validate any collection.

Implementation notes

Despite the implementation remember there are quite a lot of methods you have to remember about while overriding: add(), addAll(), set(), subList() (?), etc.

Also your object must be immutable, otherwise the user can add/set valid object and modify it afterwards to violate the contract.

Good OO design

Finaly I wrote:

validate(element)

but consider:

element.validate()

which is a better design.

Stacking validations

As noted before if you want to stack validations, validating each proprty/apsect in a single, separate class, consider the following idiom:

public abstract class ValidatingListDecorator extends AbstractList<MyBusinessObject>
{

    private final List<MyBusinessObject> target;

    public ValidatingListDecorator(List<MyBusinessObject> target) {
        this.target = target;
    }

    @Override
    public MyBusinessObject set(int index, MyBusinessObject element)
    {
        validate(element);
        return target.set(index, element);
    }

    protected abstract void validate(MyBusinessObject element);

}

...and few implementations:

class FooValidatingDecorator extends ValidatingListDecorator {

    public FooValidatingDecorator(List<MyBusinessObject> target)
    {
        super(target);
    }

    @Override
    protected void validate(MyBusinessObject element)
    {
        //throw if "foo" not met
    }
}

class BarValidatingDecorator extends ValidatingListDecorator {

    public BarValidatingDecorator(List<MyBusinessObject> target)
    {
        super(target);
    }

    @Override
    protected void validate(MyBusinessObject element)
    {
        //throw if "bar" not met
    }
}

Want to only validate foo?

List<MyBusinessObject> list = new FooValidatingDecorator(rawArrayList);

Want to validate both foo and bar?

List<MyBusinessObject> list = 
  new BarValidatingDecorator(new FooValidatingDecorator(rawArrayList));