Java- give every instance of object an unique number

Question

I'm having trobule writing a simple program in java.

I have a class called ticket, where I have:

public class Ticket {
public String movieTitle = null;
public static Integer movieNumber = 0;
public final Integer currentMovieNumber;

public Ticket(String movieTitle) {
    currentMovieNumber = movieNumber++;
    this.movieTitle = movieTitle;
}

}

Now when I call the ticket out in another class and run the program to get the numbers of the id's, they're all the same. If I call out 3 tickets, every id/movieNumber is 3, when I call out 2 tickets, the id will be 2. Can someone please help me with this ? I thought using static and final would help me but I guess there's is something I am missing.

Answer 1

It sounds like you're looking at movieNumber from your other class, which isn't appropriate. I would write it like this:

import java.util.concurrent.atomic.AtomicInteger;

public class Ticket {
    private static final AtomicInteger ticketCounter = new AtomicInteger();
    private final int ticketId;
    private final String movieTitle; // Or a reference to a Movie...

    public Ticket(String movieTitle) {
        this.movieTitle = movieTitle;
        this.ticketId = ticketCounter.incrementAndGet();
    }

    public int getTicketId() {
        return ticketId;
    }

    public String getMovieTitle() {
        return movieTitle;
    }
}

Now that the fields are private, other classes can't look at the wrong value - instead, they can only get at the ID for a particular ticket, and the title. They have no business looking at the counter.

The downside of this is that you can't easily reset the counter, or resume it when you next run the program, etc. To achieve that, you might want a TicketFactory which has an instance field for the counter, and an instance method of createTicket which creates a ticket by assigning it the next ID etc. Ticket itself then wouldn't know about the counter.

Answer 2

I think it is because you are using wrapper classes, i.e. Integer instead of int, which means that inside the class field you are only storing the reference. That means that in your constructor you are actually linking each class' currentMovieNumber to the static movieNumber so that when you increment the movieNumber, since all the instances' numbers point to it, every Ticket will have the same incremented number.

Using int instead of Integer should solve this.