Java Random Class

The Random class lets your program make chance choices, like rolling dice. You can also pick numbers in a range or repeat the same pretend-random sequence.

In this page:

The Random Class
Random in a Range
Seeded Randomness
ThreadLocalRandom
Secure Randomness

Syntax

java

import java.util.Random;

Random random = new Random();
random.nextInt(bound);

The Random Class

Random generates pseudo-random numbers from a seed value — two Random instances created with the same seed will produce the exact same sequence of values, which is useful for reproducible tests.

Example: The Random Class

java

import java.util.Random;
public class Main {
	public static void main(String[] args) {
		Random r1 = new Random(42);
		Random r2 = new Random(42); // same seed
		System.out.println(r1.nextInt(100) == r2.nextInt(100)); // true
	}
}

Random in a Range

nextInt(bound) restricts output to a range starting at zero; to generate a value within an arbitrary range like 10-20, you typically compute min + random.nextInt(max - min + 1).

Example: Random in a Range

java

// Import java.util.Random so it can be used by its short name
import java.util.Random;
// Define the class Main
public class Main {
	// Program entry point: the JVM starts running here
	public static void main(String[] args) {
		// Create a new Random object and store it in random
		Random random = new Random(1);
		int min = 10, max = 20;
		int value = min + random.nextInt(max - min + 1);
		// Print a line to the console
		System.out.println(value >= 10 && value <= 20);
	}
}

Seeded Randomness

Explicitly seeding a Random instance (new Random(42)) makes its output sequence deterministic and repeatable, which is valuable for debugging or writing tests that need consistent random values.

Example: Seeded Randomness

java

import java.util.Random;
public class Main {
	public static void main(String[] args) {
		Random random = new Random(42); // deterministic sequence
		System.out.println(random.nextInt(100));
		System.out.println(random.nextInt(100));
	}
}

ThreadLocalRandom

ThreadLocalRandom.current() gives each thread its own independent random generator, avoiding the contention that happens when multiple threads share a single Random instance under heavy concurrent use.

Example: ThreadLocalRandom

java

import java.util.concurrent.ThreadLocalRandom;
public class Main {
	public static void main(String[] args) {
		int value = ThreadLocalRandom.current().nextInt(1, 10); // own generator per thread
		System.out.println(value >= 1 && value < 10);
	}
}

Secure Randomness

SecureRandom produces cryptographically strong randomness suitable for security-sensitive uses like generating tokens or keys — regular Random is predictable enough that it should never be used for that purpose.

Example: Secure Randomness

java

import java.security.SecureRandom;
public class Main {
	public static void main(String[] args) {
		SecureRandom secureRandom = new SecureRandom(); // cryptographically strong
		int token = secureRandom.nextInt(1000000);
		System.out.println(token >= 0);
	}
}

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Java Random Class

The Random Class

Random in a Range

Seeded Randomness

ThreadLocalRandom

Secure Randomness

Chapter Quiz — Complete all 10 topics to unlock

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