Java: Random numbers - API

Java provides the original Math.random method as well as the newer, and more flexible, java.util.Random class.

Math.random

The Math.random() method returns random numbers as double values in the range >=0.0 and <1.0 . It returns the same result as the Random nextDouble() method (see below).

double x;
x = Math.random(); // assigns random number to x

java.util.Random class

To use the Random class create an object of this class (giving a seed to the constructor if you wish), then call one of the methods below whenever you want a new random number. You must use one of the following import statements:

   import java.util.Random; // Only the Random class
   import java.util.*;      // All classes in the java.util package

Random constructors

   Random x = new Random();  // default seed is time in milliseconds
   Random x = new Random(long seed); // for reproducible testing

Random methods

The most common methods are those which get the next random number. These methods all return a uniform distribution of values, except nextGaussian(). In all of these examples, x is a Random object.

   int     x.nextInt(int n) // returns random int >= 0 and < n
   int     x.nextInt()      // returns random int (full range)
   long    x.nextLong()     // returns random long (full range)
   float   x.nextFloat()    // returns random float >= 0.0 and < 1.0
   double  x.nextDouble()   // returns random double >=0.0 and < 1.0
   boolean x.nextBoolean()  //returns random double (true or false)
   double  x.nextGaussian() //returns random number with mean 0.0
                            // and standard deviation 1.0

Example: Generating a random Color

You can create any color with values for red, green, and blue (the RGB system) between 0-255. You could do it this way:

Random r = new Random();
Color c = new Color(r.nextInt(256), r.nextInt(256), r.nextInt(256));

Although the three calls to nextInt(256) look the same, each will return an independent random number.

Random numbers - intro, Random Numbers - shuffling