How to generate random integers in Java

1. java.util.Random

This Random().nextInt(int bound) will generate a random integer from 0 (inclusive) to bound (exclusive).

(1) Code snippet

For getRandomNumberInRange(5, 10), this will generate a random integer between 5 (inclusive) and 10 (inclusive).

private static int getRandomNumberInRange(int min, int max) {
		if (min >= max) {
			throw new IllegalArgumentException("max must be greater than min");
		}
		Random r = new Random();
		return r.nextInt((max - min) + 1) + min;
	}

(2) What is (max – min) 1) min?

The above formula will generate a random integer between min (inclusive) and max (inclusive) .

//Random().nextInt(int bound) = Random integer from 0 (inclusive) to bound (exclusive)	
	//1. nextInt(range) = nextInt(max - min)
	new Random().nextInt(5);  // [0...4] [min = 0, max = 4]
	new Random().nextInt(6);  // [0...5]
	new Random().nextInt(7);  // [0...6]
	new Random().nextInt(8);  // [0...7]
	new Random().nextInt(9);  // [0...8]
	new Random().nextInt(10); // [0...9]			
	new Random().nextInt(11); // [0...10]	
	//2. To include the last value (max value) = (range + 1)
	new Random().nextInt(5 + 1)  // [0...5] [min = 0, max = 5]
	new Random().nextInt(6 + 1)  // [0...6]
	new Random().nextInt(7 + 1)  // [0...7]
	new Random().nextInt(8 + 1)  // [0...8]
	new Random().nextInt(9 + 1)  // [0...9]
	new Random().nextInt(10 + 1) // [0...10]			
	new Random().nextInt(11 + 1) // [0...11]	
	//3. To define a start value (min value) in a range,
	//   For example, the range should start from 10 = (range + 1) + min
	new Random().nextInt(5 + 1)  + 10 // [0...5]  + 10 = [10...15]
	new Random().nextInt(6 + 1)  + 10 // [0...6]  + 10 = [10...16]
	new Random().nextInt(7 + 1)  + 10 // [0...7]  + 10 = [10...17]
	new Random().nextInt(8 + 1)  + 10 // [0...8]  + 10 = [10...18]
	new Random().nextInt(9 + 1)  + 10 // [0...9]  + 10 = [10...19]
	new Random().nextInt(10 + 1) + 10 // [0...10] + 10 = [10...20]
	new Random().nextInt(11 + 1) + 10 // [0...11] + 10 = [10...21]	
	// Range = (max - min)
	// So, the final formula is ((max - min) + 1) + min	
	//4. Test [10...30]
	// min = 10 , max = 30, range = (max - min)
	new Random().nextInt((max - min) + 1) + min
	new Random().nextInt((30 - 10) + 1) + 10
	new Random().nextInt((20) + 1) + 10
	new Random().nextInt(21) + 10    //[0...20] + 10 = [10...30]	
	//5. Test [15...99]
	// min = 15 , max = 99, range = (max - min)
	new Random().nextInt((max - min) + 1) + min
	new Random().nextInt((99 - 15) + 1) + 15
	new Random().nextInt((84) + 1) + 15
	new Random().nextInt(85) + 15    //[0...84] + 15 = [15...99]	
	//Done, understand?

(3) Generate a complete example of 10 random integers in the range between 5 (inclusive) and 10 (inclusive).

package com.mkyong.example.test;
import java.util.Random;
public class TestRandom {
	public static void main(String[] args) {
		for (int i = 0; i < 10; i++) {
			System.out.println(getRandomNumberInRange(5, 10));
		}
	}
	private static int getRandomNumberInRange(int min, int max) {
		if (min >= max) {
			throw new IllegalArgumentException("max must be greater than min");
		}
		Random r = new Random();
		return r.nextInt((max - min) + 1) + min;
	}
}

Output

7
6
10
8
9
5
7
10
8
5

2. Math.Random

This Math.random() gives a random double value from 0.0 (inclusive) to 1.0 (exclusive) .

(1) Code snippet. Referring to 1.2, it's more or less the same formula.

(int)(Math.random() * ((max - min) 1)) min

(2) generates 16 (inclusive) to 20 (inclusive Complete example of 10 random integers in the range ).

package com.mkyong.example.test;
public class TestRandom {
	public static void main(String[] args) {
		for (int i = 0; i < 10; i++) {
			System.out.println(getRandomNumberInRange(16, 20));
		}
	}
	private static int getRandomNumberInRange(int min, int max) {
		if (min >= max) {
			throw new IllegalArgumentException("max must be greater than min");
		}
		return (int)(Math.random() * ((max - min) + 1)) + min;
	}
}

Output

17
16
20
19
20
20
20
17
20
16

3. Java 8 Random.ints

In Java 8, a new method java.util.Random

public IntStream ints(int randomNumberOrigin, int randomNumberBound)
	public IntStream ints(long streamSize, int randomNumberOrigin, int randomNumberBound)

This Random. ints(int origin, int bound) or Random.ints(int min, int max) generates a random integer from origin (inclusive) to bound (exclusive).

(1) Code snippet.

private static int getRandomNumberInRange(int min, int max) {	
		Random r = new Random();
		return r.ints(min, (max + 1)).findFirst().getAsInt();
	}

(2) Generate a complete example of 10 random integers in the range 33 (inclusive) to 38 (inclusive).

package com.mkyong.form.test;
import java.util.Random;
public class TestRandom {
	public static void main(String[] args) {
		for (int i = 0; i < 10; i++) {
			System.out.println(getRandomNumberInRange(33, 38));
		}
	}
	private static int getRandomNumberInRange(int min, int max) {	
		Random r = new Random();
		return r.ints(min, (max + 1)).limit(1).findFirst().getAsInt();		
	}	
}

Output

34
35
37
33
38
37
34
35
36
37

(3) Extra, for your own reference.

Generate a random integer in the range between 33 (inclusive) and 38 (exclusive), with a stream size of 10. and print out the output with forEach.

//Java 8 only
	new Random().ints(10, 33, 38).forEach(System.out::println);

34
37
37
34
34
35
36
33
37
34

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