Ad:

Java Discussion Thread
Marked Solved
Views: 2422

Page 1
2
Next

You are currently viewing page 1 of this multi-page discussion thread

Oct 10th, 2008

HashMap re-set question.

Expand Post »

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.

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.

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,

Similar Threads

Question about string pattern matching in Java
Database manipulation with AVL tree.. please help in Java

new_2_java

Reputation Points: 7

Solved Threads: 6

Junior Poster

Offline

127 posts
since Apr 2007

Permalink

Oct 10th, 2008

Re: HashMap re-set question.

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

Ezzaral

Moderator

Featured Poster

Reputation Points: 3239

Solved Threads: 838

Posting Genius

Offline

6,756 posts
since May 2007

Permalink

Oct 10th, 2008

Re: HashMap re-set question.

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:

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

Thanks

new_2_java

Reputation Points: 7

Solved Threads: 6

Junior Poster

Offline

127 posts
since Apr 2007

Permalink

Oct 10th, 2008

Re: HashMap re-set question.

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.

Ezzaral

Moderator

Featured Poster

Reputation Points: 3239

Solved Threads: 838

Posting Genius

Offline

6,756 posts
since May 2007

Permalink

Oct 10th, 2008

Re: HashMap re-set question.

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.

new_2_java

Reputation Points: 7

Solved Threads: 6

Junior Poster

Offline

127 posts
since Apr 2007

Permalink

Oct 10th, 2008

Re: HashMap re-set question.

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.

Ezzaral

Moderator

Featured Poster

Reputation Points: 3239

Solved Threads: 838

Posting Genius

Offline

6,756 posts
since May 2007

Permalink

Oct 10th, 2008

Re: HashMap re-set question.

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.

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...

new_2_java

Reputation Points: 7

Solved Threads: 6

Junior Poster

Offline

127 posts
since Apr 2007

Permalink

Oct 10th, 2008

Re: HashMap re-set question.

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.

Ezzaral

Moderator

Featured Poster

Reputation Points: 3239

Solved Threads: 838

Posting Genius

Offline

6,756 posts
since May 2007

Permalink

Oct 10th, 2008

Re: HashMap re-set question.

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

Ezzaral

Moderator

Featured Poster

Reputation Points: 3239

Solved Threads: 838

Posting Genius

Offline

6,756 posts
since May 2007

Permalink

Oct 13th, 2008

Re: HashMap re-set question.

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

new_2_java

Reputation Points: 7

Solved Threads: 6

Junior Poster

Offline

127 posts
since Apr 2007

Page 1
2
Next

This thread is solved

Either the thread starter or a moderator has marked this thread as solved. You can most likely trust the responses and answers given. There is most likely no reason for any further responses to be posted here. If you have a related question, please start a new thread in this forum instead.

This thread is more than three months old

No one has posted to this discussion for at least three months. Please let old threads die and do not reply to them unless you feel you have something new and valuable to contribute that absolutely must be added to make the discussion complete. Otherwise, please start a new thread in this forum instead.

Previous Thread in Java Forum Timeline: Find Code Generated

Next Thread in Java Forum Timeline: Help with JLabel

DaniWeb Message

Cancel Changes

User Name
Password