# Java Random Number Generator — How to Generate Integers With Math Random

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Computer-generated random numbers are divided into two categories: true random numbers and pseudo-random numbers.

True random numbers are generated based on external factors. For example, generating randomness using surrounding noises.

But generating such true random number is a time-consuming task. Therefore, we can utilize pseudo-random numbers which are generated using an algorithm and a seed value.

These pseudo-random numbers are sufficient for most purposes. For example, you can use them in cryptography, in building games such as dice or cards, and in generating OTP (one-time-password) numbers.

In this article, we will learn how to generate pseudo-random numbers using `Math.random()`

in Java.

# 1. Use Math.random() to Generate Integers

`Math.random()`

returns a double type pseudo-random number, greater than or equal to zero and less than one.

Let’s try it out with some code:

`public static void main(String[] args) {`

double randomNumber = Math.random();

System.out.println(randomNumber);

}

// output #1 = 0.5600740702032417

// output #2 = 0.04906751303932033

`randomNumber`

will give us a different random number for each execution.

Let’s say we want to generate random numbers within a specified range, for example, zero to four.

`// generate random numbers between 0 to 4`

public static void main(String[] args) {

// Math.random() generates random number from 0.0 to 0.999

// Hence, Math.random()*5 will be from 0.0 to 4.999

double doubleRandomNumber = Math.random() * 5;

System.out.println("doubleRandomNumber = " + doubleRandomNumber);

// cast the double to whole number

int randomNumber = (int)doubleRandomNumber;

System.out.println("randomNumber = " + randomNumber);

}

/* Output #1

doubleRandomNumber = 2.431392914284627

randomNumber = 2

*/

When we cast a double to int, the int value keeps only whole number part.

For example, in the above code, `doubleRandomNumber`

is `2.431392914284627`

. `doubleRandomNumber`

's whole number part is `2`

and fractional part (numbers after the…