How to create a variable that can be set only once but isn't final in Java

Question

I want a class that I can create instances of with one variable unset (the id), then initialise this variable later, and have it immutable after initial

Answer 1

I think the singleton pattern might be something you should look into. Google around a bit to check if this pattern meets your design goals.

Below is some sudo code on how to make a singleton in Java using enum. I think this is based off Joshua Bloch's design outlined in Effective Java, either way it's a book worth picking up if you don't have it yet.

public enum JavaObject {
    INSTANCE;

    public void doSomething(){
        System.out.println("Hello World!");
    }
}

Usage:

JavaObject.INSTANCE.doSomething();