Java static variable for auto-increment (userID) ObjectOutputStream

Question

I have this assignment at school. It includes making a "Person" class, and put each object in a list. Each Person object should have a unique ID. This is achieved by defining a static int in the class, like this:

public class MyClass implements Serializable
{
  private static int nextmember;
  private int userid;

  public MyClass()
  {
    userid = nextmember++;
  }
}

This works well enough, first object gets userid 1, next object userid 2 etc. My challenge is how to deal with this when also saving to file? I use ObjectOutputStream and ObjectInputStream.

So if I create 3 objects, 1, 2 and 3, close my program (everything is saved to file), re-open the program (all 3 objects are present), and create a fourth object, the fourth object is given userid 1. How can I preserve the nextmember value through closing / re-opening the program?

Tried googling for it, but all I could find on the subject is that "it doesn't make sense to serialize static variables", so maybe I should find a different approach to userid management?

Thanks,
Erik

Solution 1

I am not saying it's a good solution, but the easiest way to solve this is by also persisting the value of your static "nextmember" field to the file.

e.g.

ObjectOutputStream objectOutputStream = ...;
objectOutputStream.writeInt(MyClass.nextmember);
// write your users

ObjectInputStream objectInputStream = ...;
MyClass.nextmember = objectInputStream.readInt()
// read your users

Solution 2

There is really no sense to persist the sequence with the object.

Let's play with this thought, we will save the nextmember with an object. Imagine the following series:

• Create instance1 of MyClass, id=1, seq=1
• Create instance2, id=2, seq=2
• Serialize instance1, id=1, seq=2
• Create instance3, id=3, seq=3
• Deserialize instance1, set seq=2
• Create instance4, id=3, seq=3

So, you got 2 instances, (instance3 and instance4) with the same id.

If you really want to seralize the state, you could have a separate class (and instance) for the sequence. But it still should be a static property of the class.

public class Seq implements Serializable {
    private int next = 1;
    public synchronized int getNext() { 
        return next++; 
    }
}

public class MyClass implements Serializable {
    private static Seq seq;
    public MyClass() {
        userid = seq.getNext();
    }
}

You should (de)serialize the sequence separately, and set it to the MyClass with separate getters/setters.

Notice that the sequence number may be accessed from more than 1 threads, so the access to it should be synchronized.

Comments

[Xander]

I guess it should be "public synchronized int getNext()" without the static keyword, just saying.

[Erik Haider Forsén]

Thanks for the input. I see I'll have to read upon serialization, but for this assignment I think I'll stick to @Xander solution. Definitely something I should look more into though.

[Erik Haider Forsén]

Thanks! Helped a lot. I'm not the one to judge how good your solution is (yet), but this solution is at least something that's in line with what we've learned so far.