How to Generate Random Numbers in Java

Java offers several ways to generate random numbers, each suited to a different situation. This guide covers all four. Prefer no code? Use our Random Number Generator.

Math.random()

The simplest option returns a double between 0.0 (inclusive) and 1.0 (exclusive):

double d = Math.random();

For a whole number from 1 to 100:

int n = (int)(Math.random() * 100) + 1;

java.util.Random

More flexible and reusable. Create one instance and call its methods:

Random rand = new Random();

int n = rand.nextInt(100) + 1; // 1 to 100

nextInt(bound) returns 0 to bound-1, nextDouble() returns a float, and nextBoolean() returns true or false.

ThreadLocalRandom (for concurrency)

In multi-threaded code, ThreadLocalRandom is faster and avoids contention:

int n = ThreadLocalRandom.current().nextInt(1, 101); // 1 to 100

SecureRandom (for security)

For passwords, tokens and cryptographic keys, always use SecureRandom, which is cryptographically strong:

SecureRandom secure = new SecureRandom();

int n = secure.nextInt(100);

Never use Math.random() or a plain Random for security-sensitive values.

Which should you use?

• Quick one-off — Math.random()
• General purpose — java.util.Random
• Multi-threaded — ThreadLocalRandom
• Security — SecureRandom

For a fast, no-setup result you can copy anywhere, try the Random Number Generator or the Password Generator on Toolssy.