Abstract class is using it's own abstract method?

Question

I'm looking over some code in a game and I came across something that I haven't seen before and I don't really know whats going on.

public abstract class Entity
{

    public Entity(World world)
    {
        // irrelevent code
        entityInit();
    }

    protected abstract void entityInit();
}

What's going on here? What happens when it calls on entityInit()?

Answer 1

An abstract class is never instantiated. Only its concrete subclasses can be instantiated. So, when the concrete subclass (let's call it Foo) constructor is called, it calls super(world). The Entity constructor then calls entityInit(), which has been overridden by Foo. It thus calls the Foo entityInit concrete method.

Note that this is bad practice, because the entityInit method will be called on a not yet fully constructed object. The subclass thus has to make sure this method doesn't access any field it might declare, because they will all be unititialized.