Seeding the random generator

Question

I have program that uses random but each time i start the program i get the same sequence. I need to seed and start the rnd-generator but java doesnt support true random. Can anyone tell where i can find such a generator?

Answer 1

True randomness requires a hardware solution that takes input from physical processes (such as radio "white noise"). I think somebody said that certain PCs or chipsets have such hardware, but in general you won't have that option available to you.
java.util.Random and Math.random use a seed you provide, or the system clock if you don't provide a seed. If you use the same seed twice, you'll get the same sequence of numbers both times.
One common error when generating random numbers is to reseed the generator each time in a loop. For instance for (int ix  0; ix  < 100; ix++) {
    Random rand = new Random();
    int i = rand.nextInt(10);
} This will probably produce the same number 100 times, or maybe one number N times and another number 100 - N times. The reason is that every time you create a new Random that way, you're seeding it with the system clock and starting from that sequence's first number. The system clock doesn't have time to change, or maybe changes once, during the time it takes to run through one loop iteration, so you keep starting the same sequence over, or maybe get the first number of two different sequences. The solution is to move the new Random() outside the loop. If it were a member variable, you'd probably want to make it static.
The above doesn't sound like your problem, but I present it because the solution to the problem you are having could lead to the above situation.
As someone already said, if the problem is that you get the same sequence on multiple runs of the program, then just use the system time to seed the PRNG, either explicitly or by not specifying a seed. As long as it takes at least 10 ms between one program startup and the next, you'll get different sequences.
Finally, you might want to look into [url http://java.sun.com/j2se/1.4.2/docs/api/java/security/SecureRandom.html]SecureRandom. It has better random properties than java.util.Random or Math.random, and I think if your computer does have true random hardware attached, it will use that, or can be told to use it.
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