random numbers : Java Glossary

Anyone who considers arithmetical methods of producing random digits is, of course, in a state of sin. For, as has been pointed out several times, there is no such thing as a random number — there are only methods to produce random numbers and a strict arithmetic procedure, of course, is not such a method.
~ John von Neumann (1903-12-28 1957-02-08 age:53)

There are two kinds of random numbers, pseudo-random numbers that can be rapidly generated from mathematical formulae and true random numbers, generated from some random physical process such as radioactive decay. Pseudo-random numbers are useless for cryptography. You need true random numbers for that. You now probably want to follow the link to pseudo-random numbers.

Collections.shuffle will scramble the objects in an ArrayList.