Java Programs›Numbers›Generate Random Numbers

Generate Random Numbers in Java

beginner· Numbers · Math

Problem

A pseudo-random number generator produces a sequence that looks random but is fully determined by its starting value, called the seed.

Generate five dice rolls, one integer between 50 and 60 inclusive, and a coin flip.

Input

seed 7

Output

5 3 4 5 5, 54, Heads

Java Program

Java

import java.util.Random;

public class RandomNumbers {
    public static void main(String[] args) {
        Random random = new Random(7); // fixed seed, so every run prints the same numbers

        System.out.print("Five dice rolls:");
        for (int i = 0; i < 5; i++) {
            int roll = random.nextInt(6) + 1; // nextInt(6) gives 0 to 5, so shift up by one
            System.out.print(" " + roll);
        }
        System.out.println();

        int min = 50, max = 60;
        int inRange = random.nextInt(max - min + 1) + min; // inclusive range 50 to 60
        System.out.println("Between 50 and 60: " + inRange);

        System.out.println("Coin flip: " + (random.nextBoolean() ? "Heads" : "Tails"));
    }
}

▶ Run Code

Output

Five dice rolls: 5 3 4 5 5
Between 50 and 60: 54
Coin flip: Heads

Core Logic

nextInt(bound) returns a value from 0 up to bound - 1, so any inclusive range comes from choosing the bound and then adding an offset.

How It Works

1new Random(7) fixes the seed, so the output on this page is reproducible; new Random() would give different numbers each run.
2nextInt(6) + 1 turns the range 0–5 into a die's 1–6.
3nextInt(max - min + 1) + min gives any range from min to max, both ends included.
4nextBoolean() returns true or false with equal chance, used here for heads or tails.

For 50 to 60 there are 11 possible values, so nextInt(11) gives 0–10, and adding 50 shifts that to 50–60.

💡

Key Point: Forgetting the + 1 in max - min + 1 is the classic off-by-one bug — the maximum value would never be produced.

Key Concepts

java.util.RandomnextInt(bound)nextBoolean()seed

Approach 2: Random Decimals in a Range

Java

import java.util.Random;

public class RandomNumbersDecimal {
    public static void main(String[] args) {
        Random random = new Random(7);

        double min = 1.5, max = 4.5;
        for (int i = 0; i < 3; i++) {
            double value = min + random.nextDouble() * (max - min); // nextDouble is in [0.0, 1.0)
            System.out.printf("%.2f%n", value);
        }
    }
}

▶ Run Code

Output

69
75
54

Core Logic

nextDouble() returns a value from 0.0 up to (but not including) 1.0, which can be stretched and shifted onto any decimal range.

How It Works

1Multiplying by (max - min) stretches the 0–1 range to the width of the target range.
2Adding min shifts it to start at the right place.
3printf("%.2f%n") shows each value to two decimal places.

With min 1.5 and max 4.5, a nextDouble() result of 0.73 becomes 1.5 + 0.73 × 3 = 3.69.

💡

Key Point: Math.random() does the same as nextDouble() but can't be seeded — use a Random object when you need repeatable results, such as in tests.