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Random Numbers in Java


In this post we will go through the basics of generating random numbers and also see how they can be generated within a range along with best practices.
Java provides support to either generate random numbers through java.lang.Math package or java.util.Random.

  • Using java.lang.Math

    There exists a static method inside this class, Math.random. It is very convenient to use and returns a double between the range 0.0 and 1.0 i.e. it returns a floating point number between 0 and 1. 
    
public static void main(String[] args){
    double randNum = Math.random();
    System.out.println(randNum);
}

    Random number within a range -

    To generate a random number within a range we need to further process the number returned by Math.random()
    
 public static void main (String[] args){
     double min = 10.0;
     double max = 100.0;
     double randNum = randRange(min, max);
     System.out.println(randNum);
 }
 
 public static double randRange(double min, double max){
     double x = (Math.random()*((max-min)+1))+min;
     return x;
 }
    This code produces a random double between the range 10 to 100.
    With slight modification it can be made to return an integer. 
    
     public static void main (String[] args){
         int min = 10;
         int max = 100;
         int randNum = randRange(min, max);
         System.out.println(randNum);
     }
 
 public static int randRange(int min, int max){
     int x = (int) (Math.random()*((max-min)+1))+min;
     return x;
 }

  • Using java.util.Random

    As java.util.Random is a class, we need to create an object before using it. This class has multiple methods like nextDouble(), nextInt() etc. which makes it easy to generate random numbers to our requirement.
    The following snippet produces a random double between 0 and 1 similar to Math.random()
    
     public static void main (String[] args){
         Random rand = new Random(System.currentTimeMillis());
         System.out.println(rand.nextDouble());
     }
    Usually, the constructor, i.e. Random rand = new Random(System.currentTimeMillis()); can also be used without the System.currentTimeMillis(). This is an optional argument and acts as a seed. It's fine if you just use Random rand = new Random(); but it's better to use with the seed. Theoretically, the seed could be any number.

    Generating a random integer - 

    
     public static void main (String[] args){
         Random rand = new Random(System.currentTimeMillis());
         int randNum = rand.nextInt();
         System.out.println(randNum);
     }

    Random integer between 0 and another number - 

    
     public static void main (String[] args){
         Random rand = new Random(System.currentTimeMillis());
         int max = 1000;
         int randNum = rand.nextInt(max);
         System.out.println(randNum);
     }

    Random integer between a range - 

    
     public static void main (String[] args){
         Random rand = new Random(System.currentTimeMillis());
         int max = 1000;
         int min = 500;
         int randNum = r.nextInt((max - min) + 1) + min;
         System.out.println(randNum);
     }

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