Java: Is a reflection called the same every time on every machine?

This is an odd question. I have a method that populates a list with a collection of classes using java reflection.

    import org.reflections.Reflections;
    ...
    Reflections reflections = new Reflections("network.packet");
    Class<?>[] packets = new Class<?>[256];
    for (Class<?> c : reflections.getSubTypesOf(Packet.class))
    {
        try {
            byte opcode = ((Packet)c.newInstance()).getOpcode();
            packets[opcode] = c;
        } catch (InstantiationException e) {
            // TODO Auto-generated catch block
            e.printStackTrace();
        } catch (IllegalAccessException e) {
            // TODO Auto-generated catch block
            e.printStackTrace();
        }
    }

GetOpcode returns a byte registering a byte to a packet. I'm wondering if i can make my code more API friendly, by offering a automatic solution to opcode mapping. Currently I have in the Classes that extend Packet force a call the super constructor with a byte registering an opcode.

setOpcode(byte num)

that sets an opcode for this class.

My problem lyes in whether or not I can trust org.reflection to run the same way every-time on every machine to calculate the codes rather then having them hard coded, and making users of the API not step on already existing codes.

So, does anyone know?