# Java Notes

# Random numbers - API

**Two classes.** Java provides the `Math.random()`

method as well as the
`java.util.Random`

class. The methods of the `Random`

class often produce random numbers in a more convenient form, but
requires creating an object, which sometimes is inconvenient. In constrast, the `Math.random()`

method produces a double value which must sometimes be translated and cast
into the form you need it. It's a tradeoff between the globalness of
`Math.random`

more directly useful numbers from the `Random`

class.

## 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 to get 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 r = new Random(); // Default seed comes from system time. Random r = new Random(long seed); // For reproducible testing

### Random methods

The most common methods are those which return a random number.
These methods return a uniform distribution of values, except
`nextGaussian()`

.
In these examples, *x* is a Random object.

Return type | Call | Description |
---|---|---|

`int i = ` | `r.nextInt(int n)` | Returns random int >= 0 and < n |

`int i = ` | `r.nextInt() ` | Returns random int (full range) |

`long l = ` | `r.nextLong() ` | Returns random long (full range) |

`float f = ` | `r.nextFloat() ` | Returns random float >= 0.0 and < 1.0 |

`double d = ` | `r.nextDouble() ` | Returns random double >=0.0 and < 1.0 |

`boolean b = ` | `r.nextBoolean() ` | Returns random double (true or false) |

`double d = ` | `xrnextGaussian()` | Returns random number with mean 0.0 and standard deviation 1.0 |

### Example: Generating a random Color

To create any color with values for red, green, and blue (the RGB system) between 0-255:

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 returns an independent random number.

### Example: Generating a number from 1 to 6

Because `nextInt(6)`

returns a number from
0-5, it's necessary to add 1 to scale the number
into the range 1-6,

static Random randGen = new Random(); int spots; . . . spots = randGen.nextInt(6) + 1;

## Math.random() method

The `Math.random()`

method
returns random double numbers in the range >=0.0 to <1.0 .
It returns the same result as the `Random nextDouble()`

method (see below).

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

Usually the range 0.0...0.999999+ isn't what is desired so it's necessary
to *scale* the range by multiplying and *translate* the values
by addition. Finally, an int is commonly desired, so casting is required.

For example, if you need an int int the range 1 to 10, the following code could be used.

int n = (int)(10.0 * Math.random()) + 1;

The multiplication scales the range to 0.0 to 9.9999+, casting it to int to gets it into the integer range 0 to 9 (truncation, not rounding), then addition of 1 translates it into the range 1 to 10.

The `java.util.Random`

class has convenient
methods which do this extra work for you, but you have to create a Random
object and make sure it's accessible where you need it (sometimes awkward).