Generating Random Numbers

Java provides at least 2 ways to produce random numbers:

1. Using the Math.random method

   Random numbers are useful in many computer applications, particularly in games and simulations. For example, to simulate a roll of a single die, we can generate a number between 1 and 6 (inclusive). One way to generate random numbers in a Java program is to use the static random method of the Math class.

   The method random is a pseudorandom number generator because the number generated is not truly random, in a theoretical sense. However, we must make do with the tools available to us and the numbers are random enough for most purposes. The random method returns a number (type double) that is greater than or equal to 0.0 but less than 1.0.

   The random numbers we want to generate for most applications are whole numbers. For example, the sides of a die are represented by the whole numbers 1 through 6. So we must convert the double returned by the random method to an integer in the desired range. Suppose the range of integers we want is [min,max]. If X is a number returned by random, we can convert it into a number Y such that Y is in the range [min,max](i.e., min <= Y <= max), by applying the formula

       int randomNumber 
           = (int)(Math.floor(Math.random() * (max - min + 1)) + min);
   

   So suppose I wanted to generate a random number between 1 and 6, as in the roll of a die. The expression would be

       int randomRoll
          = (int)(Math.floor(Math.random() * 6 + 1));
   

   The cast to int is necessary because Math.floor returns a double.

2. Using the Random class

   There is a class called Random in java.util that contains a method called nextInt. This method is less complicated to use than the Math.random method. To generate a random number between 1 and 6 using this class, you could write the following code (assuming this code is written within some method body and that the class imports java.util):

        Random rollDie = new Random();
   
        int randomRoll = rollDie.nextInt(6) + 1;
   

   The nextInt method with one int parameter returns a pseudorandom, uniformly distributed int value between 0 (inclusive) and the specified value (exclusive) (which explains why we add 1 to the returned value).