GETTING A RANDOM NUMBER
A frequently asked Java forum question is, "How do I get a random 
integer?" For something that is so simple in other languages, it's 
remarkable how much effort it requires in Java, both in finding the 
classes and using them.

The Java Random API spreads across packages and has grown with each 
Java version. As each new version is released, the best way of dealing 
with a random number changes.

The first and simplest way of obtaining a random value is through the 
static random() method of java.lang.Math. This method returns a double, 
so it's typical to do:

    int n   = ... 
    int rnd = (int)(Math.random()*n);

Another class existed for handling random numbers, but it failed to 
consider the ability to pass in an 'n.' This class, java.util.Random, 
used the OS clock to seed the random algorithm. Otherwise, a seed could 
be passed. If the same seed is used twice, the random numbers are 
guaranteed to be the same. For example:

    Random rnd = new Random(42L); 
    int i = rnd.nextInt(); 
 
    Random rnd2 = new Random(42L); 
    int j = rnd.nextInt(); 
 
    // i == j is true

Java 1.1 added a nextBytes(byte[]) method to the Random class and an 
extension to Random named SecureRandom. The nextBytes method allows an 
empty byte array to be passed, then filled with random byte-values. The 
java.security.SecureRandom class allowed a cryptographic-quality 
pluggable algorithm to be used to create the random values. By default, 
the JDK comes with an algorithm named "SHA1PRNG." 

    SecureRandom secrnd = SecureRandom("SHA1PRNG"); 
    byte[] bytes = new byte[128]; 
    secrnd.nextBytes(bytes);

In 1.2, java.util.Random introduced a nextInt(n) method. The 
implementation is more complex than the simple code, and this allows 
for numbers that are more random.

This method allows a random number to be gained without need for much 
thought. Though not having an easy accessor in java.lang.Math means 
there will still be posts to forums, the answer can now be simple; 
see java.util.Random.nextInt(int).

Using this is easy:

    int n = 13; 
    Random rnd = new Random(); 
 
    // random number between 0 and 12 
    int r = rnd.nextInt(n); 
 
    // random number between -5 and 7 
    int x = rnd.nextInt(n) - 5;

REFERENCES:

* java.util.Random javadoc
http://click.online.com/Click?q=52-LUXoIrJGIRbzUtcMW7S2tqQfQoZR 

* java.lang.Math javadoc
http://click.online.com/Click?q=c3-x1yCQJLeNf0PEEMZ57eU-udMxcRR 

* java.security.SecureRandom javadoc
http://click.online.com/Click?q=a6-9zw4QCUWLIW6RtQOvxoQngjNsCFR 

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