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 decimal point) is `431392914284627` . So, `randomNumber` will only hold the whole number part `2`.

You can read more about the `Math.random()` method in the Java documentation.

Using `Math.random()` is not the only way to generate random numbers in Java. Next, we'll consider how we can generate random numbers using the Random class.

## 2. Use the Random Class to Generate Integers

In the Random class, we have many instance methods which provide random numbers. In this section, we will consider two instance methods, `nextInt(int bound)`, and `nextDouble()`.

### How to use the nextInt(int bound) method

`nextInt(int bound)` returns an int type pseudo-random number, greater than or equal to zero and less than the bound value.

The `bound` parameter specifies the range. For example, if we specify the bound as 4, `nextInt(4)` will return an int type value, greater than or equal to zero and less than four. 0,1,2,3 are the possible outcomes of `nextInt(4)` .

As this is an instance method we should create a random object to access this method. Let's try it.

``````    public static void main(String[] args) {
// create Random object
Random random = new Random();
// generate random number from 0 to 3
int number = random.nextInt(4);
System.out.println(number);
}``````

### How to use the nextDouble() method

Similar to `Math.random()` , the `nextDouble()` returns a double type pseudo-random number, greater than or equal to zero and less than one.

``````    public static void main(String[] args) {
// create Random object
Random random = new Random();
// generates random number from 0.0 and less than 1.0
double number = random.nextDouble();
System.out.println(number);
}``````

## So which random number method should you use?

`Math.random()` uses the random class. If we only want double type pseudo-random numbers in our application, then we can use `Math.random()` .

Otherwise, we can use the random class as it provides various methods to generate pseudo-random numbers in different types such as `nextInt()`, `nextLong()`, `nextFloat()` and `nextDouble()`.