Question

I’ve read that you cannot declare static variables/methods inside a generic class and I really have no idea how to solve my problem or work around it so I ask for your guidance.

What I want is a generic “index” that all of my core classes will extend.
I’m creating a game-engine and an example is that I will have different gamestates who all extends State who in turn extends Nexus<State>. The reason I want the static head and tail is so that I can keep a linked list of all gamestates since they’re all added to that list upon creation.

Another example is that I will have different gameobjects who all extends GameObject who in turn extends Nexus<GameObject>.

This is the index called Nexus:

public abstract class Nexus<T> 
{

    private static T head = null;
    private static T tail = null;

    private T next = null;
    private static int num = 0;

    protected Nexus() { this.Add( (T)this ); }

    public T Add( T obj )
    {

        ((Nexus)obj).next = null;
        if( num++ == 0 ) head = tail = obj;
        else             tail = ( tail.next = obj );

        return obj;

    }

}

If anyone got another solution or a workaround I’m all ears!

Answer 1

Try this approach: Define a protected abstract method that subclasses implement to return a static object for their class.

There may be some logic issues etc, but the basics of the answer are here (ie this compiles):

EDITED: Now delegating to HeadAndTail

/** <T> A subclass of Nexus */
abstract class Nexus<T extends Nexus<T>> { // This syntax lets you confine T to a subclass of Nexus
    private T next;

    protected Nexus() {
        this.add((T) this);
    }

    public T add(T obj) {
        // Delegate to HeadAndTail
        return getHeadAndTail().add(obj);
    }

    /** @return a static for the class */
    protected abstract HeadAndTail<T> getHeadAndTail();
}

/** Bundled into one Object for simplicity of API */
class HeadAndTail<T extends Nexus<T>> {
    T head = null;
    T tail = null;
    int num = 0;

    public T add(T obj) {
        obj.next = null;
        if (num++ == 0)
            head = tail = obj;
        else
            tail = tail.next = obj;

        return obj;
    }
}

class ConcreteNexus extends Nexus<ConcreteNexus> {
    // This is the static object all instances will return from the method
    private static HeadAndTail<ConcreteNexus> headAndTail = new HeadAndTail<ConcreteNexus>();

    protected HeadAndTail<ConcreteNexus> getHeadAndTail() {
        return headAndTail; // return the static
    }
}