Why Does Java Random Generate the Same Number When Setting a Seed?

Why does the Java Random class always generate the same number when I set a seed?

Random random = new Random(12345);
int num1 = random.nextInt();
int num2 = random.nextInt(); // Will produce the same numbers every time.

The Java `Random` class generates pseudo-random numbers. When you set a seed value, it initializes the random number generator in a specific state, resulting in the same sequence of numbers every time. This is because random number generators are deterministic when seeded with the same value.

import java.util.Random;

public class RandomExample {
    public static void main(String[] args) {
        Random random = new Random(12345);
        System.out.println(random.nextInt()); // Always same output

        random.setSeed(System.nanoTime()); // Different on each run
        System.out.println(random.nextInt()); // Different output
    }
}

Causes

• The Random class in Java uses a linear congruential generator algorithm, which produces a sequence of numbers based on the initial seed.
• Setting a fixed seed ensures that the sequence of generated numbers is predictable and reproducible, which can be useful for testing.

Solutions

• If you want different sequences in each run, don't set a seed or use `new Random()` without arguments to generate a seed based on the current time.
• To create a truly random sequence each time, consider using `SecureRandom` instead of `Random` for cryptographic purposes, but note that `SecureRandom` behaves differently.

Mistake: Not understanding that setting the same seed results in repeatable sequences.

Solution: When needing different outputs, avoid setting the seed or set it dynamically.

Mistake: Assuming that Random generates true random numbers.

Solution: Understand that Random generates pseudo-random numbers based on algorithms.