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HashMap re-set question.

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HashMap re-set question.

Oct 10th, 2008

Hi all,

I have an HashMap with key/value pair. The program processes each element of the hashMap, and should mark the HashMap element as "processed". i.e. I have the follwoing key/value pair.

 Help with Code Tags 
 Java Syntax (Toggle Plain Text) 
1/"090003453bc454"
2/"090003453bc455"
3/"090003453bc456"
and so on...
1/"090003453bc454"
2/"090003453bc455"
3/"090003453bc456"
and so on...

So, when the program process each element, it should mark the key to a negative one. So, each process element should look like the following.

 Help with Code Tags 
 Java Syntax (Toggle Plain Text) 
-1/"090003453bc454"
-1/"090003453bc455"
-1/"090003453bc456"
and so on...
-1/"090003453bc454"
-1/"090003453bc455"
-1/"090003453bc456"
and so on...

So, my question is, how can I set the key to (-1) after processing the elements in the HashMap?

Please advise...

Thanks,

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Ezzaral

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Re: HashMap re-set question.

Oct 10th, 2008

I can't see any reason that you would want to. It would render the map useless.

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Re: HashMap re-set question.

Oct 10th, 2008

In second thaught, now I think, you are right. I would like to update the value and NOT the KEY

so the value of my HashMap would look like:

 Help with Code Tags 
 Java Syntax (Toggle Plain Text) 
1/"Processed"
2/"Processed"
3/"Processed"
and so on...
1/"Processed"
2/"Processed"
3/"Processed"
and so on...

Thanks

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Re: HashMap re-set question.

Oct 10th, 2008

It depends on what is being processed and what information you need to retain. Also, if your keys are just a sequenced index, you don't even need a Map implementation. A List would work fine. You can either mark the object processed or roll them off to another List of processed items.

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Re: HashMap re-set question.

Oct 10th, 2008

Thanks for prompt reply,

I have ran a sample test and HashMap.put(..) does what I was looking for.

Basically, this is what my senario is:

I have a threaded application which spawns multiple threads, say 10. and each thread will operate on that HashMap list. So, soon as a thread chooses an element of the HashMap to operate on, it should mark the HashMap element as "Processed" so the other threads will not process the same element again. It's like simulating pasimestic locking mechanism.

Thanks again.

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Re: HashMap re-set question.

Oct 10th, 2008

Typically a Queue is used for that, often a BlockingQueue. Workers take items off the queue as they become available to process. Is there a reason that you need to leave them in a Map?

Last edited by Ezzaral; Oct 10th, 2008 at 4:46 pm.

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Re: HashMap re-set question.

Oct 10th, 2008

Actually, no. There's no particular reason. This was my initial thaughts. To elaborate alittle more, here I give alittle more specific details.

I am operating on a very huge amount of data (somewhere arround 200,000 elements). I can use sequentally, but I want to reduce the amount of time the program will take.

The skeleton of my program looks something like this: I think this will give a clearer picture of what I am trying to acheive.

 Help with Code Tags 
 java Syntax (Toggle Plain Text) 
public myClass extends Thread {
    public static void main(String[] arg) {
        myClass m = new myClass ();
        HashMap map = m.loadHashMap ();  // this will only load the HashMap wil key/value as mentioned above
        for (int i =0; i < 10; i++) {
            m.start();
        } 
    }
 
    public void run () {
        for (int j=0; j < map.size(); j++) {
            if (!map.get(new Integer(j)).toString().equalsIgnoreCase("Processed")) {
                map.put(new Integer(j), new String("Processed"));
                processElement(map.get(new Integer(j)).toString()); // this will the actual process
            }
        }
    }
}
public myClass extends Thread {
    public static void main(String[] arg) {
        myClass m = new myClass ();
        HashMap map = m.loadHashMap ();  // this will only load the HashMap wil key/value as mentioned above
        for (int i =0; i < 10; i++) {
            m.start();
        } 
    }
    
    public void run () {
        for (int j=0; j < map.size(); j++) {
            if (!map.get(new Integer(j)).toString().equalsIgnoreCase("Processed")) {
                map.put(new Integer(j), new String("Processed"));
                processElement(map.get(new Integer(j)).toString()); // this will the actual process
            }
        }
    }
}

So, what I am asking, basically, will this designe work? in other words, will each thread operate exactly on on element of the Hashmap, which is not marked yet as "Processed"?

I am open to ideas.

Thanks in advance...

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Re: HashMap re-set question.

Oct 10th, 2008

No, you have absolutely no synchronization there and will have all kinds of concurrency problems.

There are a lot of ways to structure what you are wanting to do, but you have to understand some basics about threading first. Additionally, if you are running on a single processor, starting 10 threads won't make it go faster - it will just have to time-slice the processing of each thread anyway.

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Re: HashMap re-set question.

Oct 10th, 2008

In case you do want to pursue it, start with the Java concurrency tutorial trail: http://java.sun.com/docs/books/tutor...ncy/index.html

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Re: HashMap re-set question.

#10

Oct 13th, 2008

Ezzaral,

Thanks alot for the reference. I will deffinitly go over this tutorial.

Appreciate your help. I will do some reading and will update the thread accordingly,

Thanks