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UNIQUE random number

Posted on 2004-10-03
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Last Modified: 2010-03-31
I have a constructor that needs to assign a unique random number to one of its variable everytime it is been constructed. Can someone tell me how can that be done? I mean, it has to be unique. hm~~~ Does normal random number generator guarantee unique number??
           
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Question by:jtcy
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11 Comments
 
LVL 92

Expert Comment

by:objects
ID: 12210652
no a random number generator will not guarantee uniquness.
you could store all the generated numbers in a Set, and check if it already exists using that (generating a new one if it did.
0
 

Author Comment

by:jtcy
ID: 12210670
so smth like...


------------------------------------


import java.util.Random;

public class Key
  {
     private int randomNumber;
     Set mySet = new Set();
   
     public Key()
      {
          Random generator = new Random();
          randomNumber = generator.nextInt();
          while (mySet.contains(randomNumber))
                randomNumber = generator.nextInt();
      }
}
               

-------------------------------------------------------------------
?

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LVL 92

Expert Comment

by:objects
ID: 12210682
yep
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Author Comment

by:jtcy
ID: 12210687
oops. Should be:

------------------------------------


import java.util.Random;

public class Key
  {
     private int randomNumber;
     Set mySet = new Set();
   
     public Key()
      {
          Random generator = new Random();
          randomNumber = generator.nextInt();
          while (mySet.contains(randomNumber))
                randomNumber = generator.nextInt();
          mySet.add(randomNumber);
      }
}
               

-------------------------------------------------------------------
0
 
LVL 92

Expert Comment

by:objects
ID: 12210689
you need to use the Integer wrapper class so you can add them to the set.
0
 

Author Comment

by:jtcy
ID: 12210694
Do u mean...

mySet.add((Integer)randomNumber);


0
 
LVL 86

Expert Comment

by:CEHJ
ID: 12210776
mySet.add(new Integer(randomNumber));
0
 
LVL 21

Accepted Solution

by:
MogalManic earned 20 total points
ID: 12211163
I all you want is a number with a high probability that it is unique, do the following:

   private static Random generator=new Random();
   public int getKey()
   {
          return generator.nextInt() ^ System.currentTimeMillis()
   }
  The current time in millis will be unique as long as two threads don't call getKey at the same time.  If so, the random number should ensure uniqueness.
0
 
LVL 86

Expert Comment

by:CEHJ
ID: 12211183
That would have to be:


return generator.nextInt() ^ (int)System.currentTimeMillis();
0
 
LVL 21

Expert Comment

by:MogalManic
ID: 12211314
no, Javas automatic conversion would be more correct  the following is what java does automatically:
    return (int) ((long) generator.nextInt() ^ System.currentTimeMillis());

OR if you did not want to drop bits
private static Random generator=new Random();
   public long getKey()
   {
         return generator.nextLong() ^ System.currentTimeMillis()
   }
0
 
LVL 86

Expert Comment

by:CEHJ
ID: 12211415
>>the following is what java does automatically

Certainly doesn't in the version of sdk i'm using (1.5.0-beta2-b51)
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