Generation of Random Number

[Srinivas Kalvala]

Hello,

Can any one help me in generating random number sequence for real time application.

with following constraint,
the routine should not access the current time stamp or db time stamp.

I have tried with util.Random and math.random() they are not generated unique random numbers, after a period of time , say after every 100 or 150 numbers , they are generating again same set of numbers.

Help required.

[Paul Sturrock]

unique random numbers

There is nothing in either of those classes that guarentees uniqueness. If this is a single JVM application, you might look at the UID class

[Jesper de Jong]

The difficulty is in the fact that you want unique numbers.

You could do this by first making a list of all possible numbers, and then call Collections.shuffle(...) on the list to put them in random order. Then you just iterate the list to get random numbers. This solution might not be practical if the range of all possible numbers is large, or when you need a lot of random numbers.

import java.util.ArrayList;
import java.util.Collections;
import java.util.Iterator;
import java.util.List;
 
public class UniqueRandomNumberGenerator {
 
    private List<Integer> numbers;
    private Iterator<Integer> iterator;
 
    public UniqueRandomNumberGenerator(int min, int max) {
        numbers = new ArrayList<Integer>();
 
        // First make a list of all the numbers from min to max
        for (int i = min; i <= max; i++) {
            numbers.add(i);
        }
 
        // Now put the numbers in a random order
        Collections.shuffle(numbers);
 
        iterator = numbers.iterator();
    }
 
    public boolean hasMoreNumbers() {
        return iterator.hasNext();
    }
 
    public int getNumber() {
        if (!hasMoreNumbers()) {
            throw new IllegalStateException("The generator is out of numbers!");
        }
 
        return iterator.next();
    }
}