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

You can simply add a boolean flag, and in your setId(), set/check the boolean. If I understood the question right, we don't need any complex structure/pattern here. How about this:

public class Example {

private long id = 0;
private boolean touched = false;

// Constructors and other variables and methods deleted for clarity

public long getId() {
    return id;
}

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

}

in this way, if you setId(0l); it thinks that the ID is set too. You can change if it is not right for your business logic requirement.

not edited it in an IDE, sorry for the typo/format problem, if there was...