How to Generate Random Numbers in Java

The Challenge

Generate random numbers for games, simulations, and testing!

Solutions

Method 1: Using Math.random()

java

// Random double between 0.0 and 1.0
double random = Math.random();
System.out.println(random); // e.g., 0.7234567

// Random int between 0 and 9
int randomInt = (int)(Math.random() * 10);
System.out.println(randomInt); // e.g., 7

// Random int between 1 and 100
int randomRange = (int)(Math.random() * 100) + 1;
System.out.println(randomRange); // e.g., 42

Method 2: Using Random Class

java

import java.util.Random;

Random rand = new Random();

// Random int
int randomInt = rand.nextInt();
System.out.println(randomInt);

// Random int between 0 and 99
int randomRange = rand.nextInt(100);
System.out.println(randomRange);

// Random int between 1 and 100
int randomBetween = rand.nextInt(100) + 1;
System.out.println(randomBetween);

// Random double
double randomDouble = rand.nextDouble();
System.out.println(randomDouble);

// Random boolean
boolean randomBool = rand.nextBoolean();
System.out.println(randomBool);

Method 3: Using ThreadLocalRandom (Java 7+)

java

import java.util.concurrent.ThreadLocalRandom;

// Random int between min (inclusive) and max (exclusive)
int random = ThreadLocalRandom.current().nextInt(1, 101);
System.out.println(random); // Between 1 and 100

Common Patterns

java

// Random number between min and max (inclusive)
public static int randomBetween(int min, int max) {
    return (int)(Math.random() * (max - min + 1)) + min;
}

// Usage
System.out.println(randomBetween(10, 20)); // e.g., 15

Did You Know?

Java's random numbers are "pseudorandom" - they're generated by an algorithm, not truly random. For cryptographic security, use SecureRandom!

Pro Tip

Use ThreadLocalRandom for better performance in multi-threaded applications:

java

// Faster in concurrent environments
int num = ThreadLocalRandom.current().nextInt(1, 101);

Real-World Use Cases

Games: Dice rolls, card shuffling, enemy spawning
Simulations: Monte Carlo simulations
Testing: Generate test data
Security: Generate tokens, passwords (use SecureRandom)

›Short versionOpen if you only have a minute.

Math.random() is in [0.0, 1.0). It never returns 1.
(int) (Math.random() * 100) is 0 through 99. Inclusive 0 through 100 needs * 101.
Random.nextInt(bound) is 0 inclusive through bound exclusive, and bound must be positive.
A random draw may repeat. It is not a shuffle and it is not a unique id.

Random number in a range

Generate a value in a specific range with clear inclusive or exclusive boundaries.

Working approach

For an inclusive int range, call nextInt(max - min + 1) + min.
Check max >= min before subtracting. A negative bound throws.
Use SecureRandom only when the value is a secret. Ordinary Random is the right tool for a dice roll.

Walk the sample

Inclusive range 1 to 6
The bound width is 6
nextInt(6) returns 0 through 5
Adding 1 returns 1 through 6

The trap

The upper bound of Math.random() and of nextInt is exclusive. Forgetting the + 1 drops the highest value the caller asked for.

java

static int inclusive(Random random, int min, int max) {
    if (max < min) {
        throw new IllegalArgumentException("max < min");
    }
    return random.nextInt(max - min + 1) + min;
}

Why does the index recipe multiply by 101 to get a number from 0 through 100?

Math.random() * 100 lands in [0, 100) and the cast keeps 0 through 99. 100 is never produced.

Multiplying by 101 lands in [0, 101) and the cast keeps 0 through 100. nextInt(101) says the same thing without the double.

Time O(1), space O(1).

Easy to get wrong

Using max - min as the bound when both ends are inclusive.
Creating a new Random on every call when one instance should be reused.