Question

I have this code:

abstract class Entity
{
// blah-blah-blah
}

abstract class BaseCollection
{
    public void add(Entity entity);
}

And I derive from the Entity and BaseCollection classes:

class User extends Entity
{
}

class UserCollection extends BaseCollection
{
   public void add(User user) { // blah-blah-blah }
}

Is this an example of Liskov Substitution Principle violation? If it is, how can I solve the issue?

Answer 1

As User is a subtype of Entity it is perfectly reasonable to add such objects to BaseCollection (via UserCollection) — each user is an Entity

Passing UserCollection where BaseCollection is expected, will not work on the other hand: you are expecrted to be able to add an Entity, but you need a User — or in other words: when you get an element out of the UserCollection, you might get an Entity after this, where you expect a User.