Generating Random Numbers in a Range in Java
Java provides a few methods to generate random numbers, but it is quite tricky to generate a random number in a specific range. In this tutorial, we will learn how to leverage the inbuilt Java methods to implement a random number generator method for a given range.
Using Math.random() Method
The random() method of the Math class is used to generate a decimal value between 0 and 1(0 inclusive, 1 exclusive).
public class RandomRangeDemo
{
public static void main(String[] args)
{
double r1 = Math.random();
double r2 = Math.random();
System.out.println("The first random number is: " + r1);
System.out.println("The second random number is: " + r2);
}
}
The first random number is: 0.8998047971282644
The second random number is: 0.9465494601371991
We can use this method to build our own random method which will take the minimum and the maximum range values and it will return a random number within that range.
- We will use the formula (Math.random() * (max-min)) + min in our method.
- This formula works because if Math.random() generates 0(the lowest value), then (0 * (max-min)) + min will be equal to min.
- And if Math.random() generates 1(the highest value), then the formula will give 1 * (max-min) + min, which is equal to max.
- But remember that Math.random() does not returns 1, but instead, a value that is slightly less than 1(say 0.999999). So the maximum returned value of our function will be slightly less than max.
- So, our method returns a value between min and max(min inclusive, max exclusive)
public class RandomRangeDemo
{
public static int randomNumberGenerator(int min, int max)
{
double r = Math.random();
int randomNum = (int)(r * (max - min)) + min;
return randomNum;
}
public static void main(String[] args)
{
int r1 = randomNumberGenerator(5, 105);//random number between 5 and 105
int r2 = randomNumberGenerator(2199, 2200);//random number between 2199 and 2200
System.out.println("The first random number is: " + r1);
System.out.println("The second random number is: " + r2);
}
}
The first random number is: 47
The second random number is: 2199
We can also use the nextDouble() and nextFloat() methods of the Random class in place of the Math.random() method. All these methods will generate a random decimal number between 0 and 1.
The above code using the nextDouble() method is shown below.
import java.util.Random;
public class RandomRangeDemo
{
public static int randomNumberGenerator(int min, int max)
{
Random r = new Random();
double randomNum = r.nextDouble();
int result = (int)(randomNum * (max - min)) + min;
return result;
}
public static void main(String[] args)
{
int r1 = randomNumberGenerator(5, 105);//random number between 5 and 105
int r2 = randomNumberGenerator(2199, 2200);//random number between 2199 and 2200
System.out.println("The first random number is: " + r1);
System.out.println("The second random number is: " + r2);
}
}
The first random number is: 53
The second random number is: 2199
Using nextInt() method of Random class
The nextInt() method can be used to generate a random integer. If we pass a positive parameter n to this method then it will return a random number in the range 0 to n(0 inclusive, n exclusive).
import java.util.Random;
public class RandomRangeDemo
{
public static void main(String[] args)
{
Random r = new Random();
int randomNum1 = r.nextInt(100);
int randomNum2 = r.nextInt(1);
System.out.println("The first random number is: " + randomNum1);
System.out.println("The second random number is: " + randomNum2);
}
}
The first random number is: 63
The second random number is: 0
We will use this method to build our own random number generator method. We will use the formula nextInt(max-min) + min. Let's try to understand this formula.
For example, if max = 31 and min = 7, then (max-min) will be equal to 24. So, nextInt(24) will generate a random number between 0 and 24. If it generates 0(the lowest value), then 0 + min will be equal to 7. If it generates 24(the largest value), then 24 + min will be equal to 31, which is the max value. Since nextInt(n) is exclusive of the n value, so our method will also be exclusive of the max value.
import java.util.Random;
public class RandomRangeDemo
{
public static int randomNumberGenerator(int min, int max)
{
Random r = new Random();
int randomNum = r.nextInt(max - min);
int result = randomNum + min;
return result;
}
public static void main(String[] args)
{
int randomNum1 = randomNumberGenerator(500, 2000);//random number between 500 and 2000
int randomNum2 = randomNumberGenerator(500, 501);//random number between 500 and 501
System.out.println("The first random number is: " + randomNum1);
System.out.println("The second random number is: " + randomNum2);
}
}
The first random number is: 846
The second random number is: 500
Using ints() method of Random Class
The java.util.Random.ints() method can return an infinite integer stream of randomly generated numbers. We can specify a range and all the random numbers will be generated within that range. We can also specify the stream size so that we get only a limited number of integers.
The following code runs infinitely as we have not specified a stream size to the ints() method. We have only shown a few of the numbers in the output.
import java.util.Random;
import java.util.stream.IntStream;
public class RandomRangeDemo
{
public static void main(String[] args)
{
Random r = new Random();
IntStream stream = r.ints(10, 20);
stream.forEach(s -> System.out.println(s));
}
}
11
16
16
14
12
13
Let's now generate just 5 random integers by setting the stream size.
import java.util.Random;
import java.util.stream.IntStream;
public class RandomRangeDemo
{
public static void main(String[] args)
{
Random r = new Random();
IntStream stream = r.ints(5, 100, 120);
stream.forEach(s -> System.out.println(s));
}
}
103
106
114
117
109
We can use the findFirst() and getAsInt() methods to get only the first random number from the stream.
import java.util.Random;
import java.util.stream.IntStream;
public class RandomRangeDemo
{
public static void main(String[] args)
{
int min = 150, max = 2000;//defining the range
Random r = new Random();
IntStream stream = r.ints(1, min, max);
int randomNum = stream.findFirst().getAsInt();
System.out.print("The random number is: " + randomNum);
}
}
The random number is: 1193
Summary
In this tutorial, we learned the different ways of generating a random number in a specified range. We implemented our methods using the inbuilt random() method of the Math class and the nextInt() of the Random class. We can also ints() method directly to generate a stream of random numbers in a given range.
