How does the seed value passed to Java's random number generator effect its output?

Question

How does the seed value passed to Java's random number generator effect its output? How would i go about figuring out which numbers it will output if i know the seed value?

Also are some seed values better than others at producing more psudo-randomness than others?

Answer 1

You cannot generate truly random numbers in software, because software is deterministic: given some input, it will in principle always generate a predictable output.

So, to get random numbers, a number of algorithms have been invented that generate sequences of numbers that look random (but are not really - that's why they are called pseudo-random numbers).

Such an algorithm starts with some start value, the seed, and then does some calculations with it to generate the next pseudo-random number.

If the algorithm is any good, then there should be no difference in seed values: one seed value should not be better than any other in generating random numbers.

Often, the current time is taken as the seed value, so that each time you generate a sequence of numbers, you get a different sequence. Note that if you use the same seed value, you will get the same sequence of pseudo-random numbers each time you run it.

If you use pseudo-random numbers for cryptographic purposes, you should be very careful, because if an attacker knows the seed value then he can re-generate the sequence of random numbers which might compromise the security of your system. For really secure systems, people use special hardware-based random number generators, which can generate truly random numbers. Java has a class java.security.SecureRandom to interface with such systems.

See Random number generation on Wikipedia for a lot more detail and information on different algorithms.