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javauniqueidentifieridentifier

Generating unique auto-incrementing instance IDs

I have been trying to generate unique IDs for class instances starting from 1, like so:

public abstract class foo {
    static int ID = 0;
    final int id;

    public foo() { this.id = ++ID; }
}

I then realised that I have several classes that require this behaviour, and so I tried to make an abstract class from which to inherit, but I found that static fields can't really be inherited.

Is there any way I can reuse this common behaviour in Java?

Edit:
I did not make it clear, but I need the indexes for each subclass to be not only unique, but also consecutive.

For example, if I have bar and baz inheriting from foo:

bar b1 = new bar();
baz b2 = new baz();
bar b3 = new bar();

b2 should have id == 1, and b3 should have id == 2.

Solution

  • Not really! There are a couple of things you might try:

    1. Have a single IDGenerator class with a static method to generate a new ID for a specific class. The .generate() method would take a Class<?> parameter, and would maintain a HashMap mapping classes to most recently given out identifiers. This would put all the logic into one class, but wouldn't really simplify things.
    2. Have a static reference to a new instance of an IDGenerator class inside each class that wants to use this kind of behaviour. In this case, the IDGenerator class wouldn't need a Class<?> parameter because there would be one instance for each class using it. Its .generate() method would be a no-arg method. Again, I'm not sure this makes things much simpler.
    3. Abandon the requirement for consecutive IDs. That way, you could have a class IDGenerator with a static no-arg .generate() method that would just dish out the next unused identifier. Your identifiers would then be unique across your application, and not consecutive within a class.

    What you've got is pretty fool-proof and slimline.

    One more minor note: you say you want your IDs to start from 0, but the way you've written it, they'll start from 1. You want this.id = ID++ rather than this.id = ++ID.