# Java Notes

# Random numbers - Introduction

## When to use random numbers

There are many types of programs that use random numbers. Game programs use them to create situations that are not always the same every time a player is in the same situation, or a deck of cards must be shuffled. Simulations usually use random numbers. You might want to use random numbers to change the appearance of something -- screen savers are an example of use. There are many other uses also.

## Type, Range, and Distribution

When you generate random numbers, you need to decide
which type you need (eg, int or double), what range
you need (minimum to maximum), and how they are
distributed. There are two common types of distributions:
*uniform* and *normal* (equivalent to *Gaussian*).
In a uniform distribution all numbers in the range are
equally likely to be produced.

## How can a computer generate a random number?

Computers are deterministic -- they do the same thing whenever
they have the same input. Therefore, in some way it is
impossible for a computer to compute a truly random number.
But the methods that are used generate sequences of numbers
that meet the statistical requirements of randomness.
Because the numbers are not truly random, they are
sometimes referred to as **pseudo random** numbers.
These random number generates will produce exactly the same
sequence of pseudo random numbers if they start with the
same initial value. This initial value is called the **seed**.

If we start a program with the same random seed, we will get the same sequence of numbers. The default seed is the current time in milliseconds since 1970. This works very well, but you might want to set the seed so that the sequence can be reproduced when you are testing your program.

## Related pages

- Random numbers - API
- Random Numbers - shuffling
- java.sun.com/docs/books/effective/excursion-random.html,
*Excursion 1: Random Thoughts*by Joshua Bloch, author of*Effective Java*.