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 initialisation. Effectively, I'd like a final variable that I can initialise outside of the constructor.

Currently, I'm improvising this with a setter that throws an Exception as follows:

public class Example {

    private long id = 0;

    // Constructors and other variables and methods deleted for clarity

    public long getId() {
        return id;
    }

    public void setId(long id) throws Exception {
        if ( this.id == 0 ) {
            this.id = id;
        } else {
            throw new Exception("Can't change id once set");
        }
    }
}

Is this a good way of going about what I'm trying to do? I feel like I should be able to set something as immutable after it's initialised, or that there is a pattern I can use to make this more elegant.

java reference final
Edit
Report