What is random seed about?

Question

For example the code below. It has a random class. However it always produce the same output everywhere . In this case which item is the seed?

source: link

import java.util.Random;
public class RandomTest {
    public static void main(String[] s) {
        Random rnd1 = new Random(42);
        Random rnd2 = new Random(42);

        System.out.println(rnd1.nextInt(100)+" - "+rnd2.nextInt(100));
        System.out.println(rnd1.nextInt()+" - "+rnd2.nextInt());
        System.out.println(rnd1.nextDouble()+" - "+rnd2.nextDouble());
        System.out.println(rnd1.nextLong()+" - "+rnd2.nextLong());
    }
}

Answer 1

42 is the seed, as the very same Javadoc says. So, what is a seed? A random number is seldom truly random - often it's a pseudo-random instead. This means it's generated from a function, which is said PRNG (pseudo random number genrator). Being generated from a function, in turn, means that the output is not random anymore, since it's predictable!

However, depending on your needs, this pseudo-randomness may be enough - I said enough because generating random bit is expensive, and I'm not talking about time or memory, but about money (see this link on wikipedia). So, for example, if you need a random value to place enemies in your game, a pseudo-random number is ok - but if your are building security-related software, you want to use a true random number, or at least a cryptographically secure PRNG.

How can we describe a PRNG, like the one used in Math.random()? It's a function, initialized with a seed S that returns an array of values A. Note that, for each integer S, is defined one and only one array A. For example (values are not actual):

                first call     second call     third call
seed: 14329            .18             .82             .5
seed:  3989             .7             .02            .93

So you seed you PRNG with some known value when you want its result to be predictable - for example for testing purposes or to ensure that, each time you run level 1 in your game, the enemies are always placed in the same (pseudo) random places - otherwise you don't need to explicitely pass a seed.

Answer 2

Random Seed on Wikipedia:

A random seed (or seed state, or just seed) is a number (or vector) used to initialize a pseudorandom number generator.

In other word, it is the number from which a seem-to-be-random sequence will be generated. Therefore, if you use the same number, the senquence will always be the same.

In practice, we usually use System Time as seed.