Java Lesson 7: Random Number Generator

Hello everybody,

It’s Michael, and I’ve decided to give you guys another series of java lessons. In this post, I’ll show you how to create your own random number generator, which is a tool that can generate random numbers in Java. Generating random numbers in Java is a common task that is used for a variety of purposes, such as gambling, statistical sampling, and any other scenario where you would need to simulate unpredictable behavior. Random number generators can also be used with arrays, which will be the topic of my next Java post.

Now let’s begin with an example of a simple random number generator:

package javalessons;
import java.util.Random;

public class JavaLessons
{

public static void main(String[] args)
{
Random generator = new Random ();
int random = generator.nextInt(50);
System.out.println(random);
}
}

 

And here are two sample outputs:

run:
46
BUILD SUCCESSFUL (total time: 0 seconds)

run:
12
BUILD SUCCESSFUL (total time: 1 second)

Before writing the code for your program, remember to import java.util.Random (include this package anytime you plan on using the random number generator).

So what does all of this code mean? First of all, the Random generator = new Random () line indicates your random number generator variable. You keep the parentheses empty since you will indicate the boundaries in a separate int variable, which in this case is found in the line int Random = generator.nextInt(50). This line indicates that you want your program to print out a random number between 0 and 50 (the number in the parentheses indicates the upper limit of your random number generator). System.out.println(random) prints out a random number between 0 and 50

But what if you wanted to print out several random numbers? We can do just that, with the help of our random number generator and a handy for-loop:

package javalessons;
import java.util.Random;

public class JavaLessons
{

public static void main(String[] args)
{
Random generator = new Random ();

for (int i = 0; i <= 8; i++)
{
int random = generator.nextInt(120);
System.out.print(random + ” , “);
}

}
}

And here are two sample outputs:

run:
6 , 17 , 20 , 98 , 82 , 46 , 43 , 5 , 19 , BUILD SUCCESSFUL (total time: 1 second)

run:
65 , 13 , 116 , 16 , 5 , 1 , 6 , 28 , 72 , BUILD SUCCESSFUL (total time: 2 seconds)

I kept the variables the same as the previous example; all I did here was add a for loop (for int = 0; i <= 8; i++) and changed the upper limit to 120. This program will print out 8 random numbers between 0 and 120, each separated by a comma.

So what if you want to use a custom range for your random number generator, as opposed to relying on the default of 0? Here’s how:

package javalessons;
import java.util.Random;

public class JavaLessons
{

public static void main(String[] args)
{
Random generator = new Random ();

for (int i = 0; i <= 8; i++)
{
int random = 200 + generator.nextInt(600);
System.out.print(random + ” , “);
}

}
}

And here are two sample outputs:

run:
555 , 211 , 587 , 707 , 545 , 653 , 594 , 785 , 520 , BUILD SUCCESSFUL (total time: 2 seconds)

run:
415 , 345 , 579 , 462 , 660 , 695 , 593 , 337 , 505 , BUILD SUCCESSFUL (total time: 2 seconds)

The program is the same as the previous example, save for changing the upper limit to 600 and adding 200 + in front of the generator.nextInt line. Remember how I said I was going to show you how to use a custom range for your random number generator? Well, the 200 + is exactly how you can do that because putting this (or any number for that matter) in front of the generator.nextInt(600) line ensures that the range of your random number generator will only be from 200 to 600.

And last but not least, let’s see what happens when you want to input the value for the upper limit:

package javalessons;
import java.util.Random;
import java.util.Scanner;

public class JavaLessons
{

public static void main(String[] args)
{
Random generator = new Random ();
Scanner sc = new Scanner (System.in);
System.out.println(“Please choose a number: “);
int custom = sc.nextInt();

for (int i = 0; i <= 8; i++)
{
int random = generator.nextInt(custom);
System.out.print(random + ” , “);
}

}
}

And here are two sample outputs (using 35 for the first example and 91 for the second):

run:
Please choose a number:
35
31 , 23 , 4 , 5 , 22 , 9 , 14 , 11 , 10 , BUILD SUCCESSFUL (total time: 4 seconds)

run:
Please choose a number:
91
78 , 45 , 63 , 82 , 89 , 58 , 12 , 8 , 61 , BUILD SUCCESSFUL (total time: 2 seconds)

In this program, I added a Scanner variable (along with importing the Scanner class) and a custom int variable, which allows you to choose the endpoint for your random number generator (the start-point is still 0). As you can see from each sample output, the program will print out 8 random numbers that are less than or equal to the number that was inputted.

Thanks for reading,

Michael