Add no duplicates to List of supperclass (check for equality with subclasses)

Question

I have a List<Item> items = new ArrayList<>(); I add two different Items to, both subclasses of Item : PortableItem and SceneryItem .

public class Item implements Comparable<item> {
    public String id;
    public String desc;

    ...

    public int compareTo(Item o) {
        return getId().compareTo(o.getId());
    }  
}

Now I would like to detect Items with duplicate ID s before adding a new one to my List.

    PortableItem a = new PortableItem("a");
    SceneryItem b  = new SceneryItem("a");
    items.add(a);
    items.contains(b); 

Returns false. How can I change that behavior?

Answer 1

You could add an equals method that compared on id, be default an Object equals another when they are == - ie the same instance. This isn't what you want.

public class Item implements Comparable<Item> {

    public String id;
    public String desc;

    public String getId() {
        return id;
    }

    @Override
    public int compareTo(Item o) {
        return getId().compareTo(o.getId());
    }

    @Override
    public int hashCode() {
        int hash = 7;
        hash = 17 * hash + Objects.hashCode(this.id);
        return hash;
    }

    @Override
    public boolean equals(Object obj) {
        if (obj == this) {
            return true;
        }
        if (!(obj instanceof Item)) {
            return false;
        }
        final Item other = (Item) obj;
        if (!Objects.equals(this.id, other.id)) {
            return false;
        }
        return true;
    }
}

This is really good practice anyway given your compareTo method and the requirement that is is consistent with equals - now a.equals(b) will return true if they have the same id .

As you now have an equals method you have to have a hashCode() method, again the consistent with equals requirement.

Note that if you override the equals method in a superclass this obviously won't work unless you use a call to return super.equals at the end.

Now, using a List the contains method is guaranteed O(n) - this is very slow . I would recommend using a Set where contains is guaranteed O(1). If you need to maintain order use a LinkedHashSet or even better a TreeSet which will use your compareTo method to order items automagically.

You can always turn the thing into a List afterwards with one O(n) call anyway...

Answer 2

You need to override the equals() and hashCode() methods in the base class to compare by ID.
compareTo() is only used for sorting.