Automatically initializing an int messageID in a constructor

Question

How can I initialize a final int in the constructor in Java to be with 1 bigger than the previous instance and can I do that at all? I mean i have a final int messageID; which must be unique for every instance, how can I do that?

possible duplicate of [Final variable manipulation in Java](http://stackoverflow.com/questions/1249917/final-variable-manipulation-in-java) — Alexandru Cimpanu, Jun 18 '14 at 11:06
you need something like this http://stackoverflow.com/a/3478862/821057 — Zaheer Ahmed, Jun 18 '14 at 11:06
possible duplicate of [how many instances for a class exists at runtime in java](http://stackoverflow.com/questions/3478795/how-many-instances-for-a-class-exists-at-runtime-in-java) — Zaheer Ahmed, Jun 18 '14 at 11:06
Possible duplicate of http://stackoverflow.com/questions/12336030/automatic-id-generation — Raedwald, Jun 18 '14 at 11:44

score 5 · Answer 1 · answered Jun 18 '14 at 11:05

5

Keep a

private static final AtomicInteger NEXT_MESSAGE_ID = new AtomicInteger();

Then in your constructor do

this.messageId = NEXT_MESSAGE_ID.getAndIncrement();

answered Jun 18 '14 at 11:05

Ray

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Thank you! But is there a different way to do that? – I.Anchev Jun 18 '14 at 11:08
1

Why do you want a different way? – Ray Jun 18 '14 at 11:09
1

My university professor does not allow us to use things we have not learnt yet.. It's a bit silly, I know... – I.Anchev Jun 18 '14 at 11:13
Then you have to go with the other proposed answer, although it is not threadsafe. – Ray Jun 18 '14 at 11:15

Jørgen R · Answer 2 · 2014-06-18T11:25:09.353

0

A soulution could be to use a static variable to hold the largest message ID, then increment it each time a new object is created.

class Foo {
    static int maxMessageID = 0;
    final int messageID;

    public Foo() {
        this.messageID = ++maxMessageID;
    }
}

EDIT

To make this thread safe, put the incrementing of the variable inside a method using synchronized.

class Foo {
    static int maxMessageID = 0;
    final int messageID;

    public Foo() {
        this.messageID = this.getID();
    }

   private synchronized int getID() {
       return ++maxMessageID;
   }
}

edited Jun 18 '14 at 11:25

answered Jun 18 '14 at 11:08

Jørgen R

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@assylias That is true. I just wanted to show how it can be done using simple techniques. – Jørgen R Jun 18 '14 at 11:17

Automatically initializing an int messageID in a constructor

2 Answers2