java random generator

Question

I want to generate some random doubles and add them into an ArrayList, but it seems that the nextDouble() function returns a unique double every time, not a new one

Random r = new Random();
ArrayList<Pair> centers = new ArrayList<Pair>();  
ArrayList<ArrayList<Pair>> classes = new ArrayList<ArrayList<Pair>>();  
for (int i=0 ; i < 100; i++) {
    // Random r = new Random ();
    // System.out.println (r.nextDouble ()) ;
    double a = r.nextDouble () * 10;
    double b = r.nextDouble () * 10;
    centers.add (new Pair (a, b ));
    System.out.println (centers);
}

Can anyone help me with this? Is this a mistake of optimization?

What if you rewrite r = new Random(); after having calling it? Random r = new Random(); double a = r.nextDouble() * 10; r = new Random(); double b = r.nextDouble() * 10; — sp00m
– sp00m, Commented May 14, 2012 at 12:16
What do you mean with "not a new one"? nextDouble returns a unique double, just as it should.. — Marcus Hansson
– Marcus Hansson, Commented May 14, 2012 at 12:17
What do you mean when you say that nextDouble() returns a unique double and not a new one. Isn't that the same thing? (and the excpected behaviour) — Ludwig Magnusson
– Ludwig Magnusson, Commented May 14, 2012 at 12:18
If you want to reset the seed, you can do as sp00m wrote: double a = new Random().nextDouble * 10; etc. If you want to use a more than once, do double b = a;. — Marcus Hansson
– Marcus Hansson, Commented May 14, 2012 at 12:18
@Marcus: I want to have 100 different random generated numbers, but in this way, I get 100 numbers that all are equal — Masood Delfarah
– Masood Delfarah, Commented May 14, 2012 at 12:19

Marko Topolnik · Accepted Answer · 2012-05-14 12:22:27Z

4

I ran this code:

public static void main(String[] args) {
  Random r = new Random();
  ArrayList<Pair> centers = new ArrayList<Pair>();
  for(int i = 0; i < 100; i++ ) {
    double a = r.nextDouble() * 10;
    double b = r.nextDouble() * 10;
    centers.add( new Pair(a, b) );
  }
  System.out.println(centers);
}

This was the output:

[(8.08, 8.06), (9.97, 1.83), (3.83, 3.19), (2.97, 2.51), (9.40, 2.88), (7.78, 2.59), (1.67, 9.07) ...

Isn't that what you want? FYI, this is the Pair class I used:

class Pair {
  private final double a, b;
  Pair(double a, double b) { this.a = a; this.b = b; }
  @Override public String toString() { return String.format("(%.2f, %.2f)", a, b); }
}

answered May 14, 2012 at 12:22

Marko Topolnik

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