Miscellaneous Utilities - The Java Programming Language - page 974

22.8. UUID
The UUID class provides immutable objects that represent universally
unique identifiers. These are 128-bit values that are generated in such a
way that they are guaranteed to be unique. There are four different ver-
sions of UUID values, generally known as types 1, 2, 3, and 4. You can
ask a UUID object for its version by using the version method.
The usual way to create a type 4 (random) UUID is to use the static factory
method randomUUID . A type 3 (name-based) UUID can be obtained from the
static factory method nameUUIDFromBytes , which takes an array of bytes
representing the name.
You can directly construct an arbitrary form UUID by supplying two long
values that represent the upper and lower halves. Of course, there are
no guarantees about uniqueness for such a constructed UUID it is up to
you to supply appropriate values. The two halves of a UUID object can be
retrieved as long values from the getMostSignificantBits and getLeastSig-
nificantBits methods.
The layout of a UUID is determined by which variant is used to implement
it. There are four variants: 0, 2, 6, and 7. The UUID class always generates
variant 2 UUID valuesknown as the Leach-Salz variant. The details of
these variants are not important for this discussion. You can ask a UUID
object for its variant by using the variant method. Two UUID objects are
equal only if they have the same variant and 128-bit value.
The toString method returns a string representation of a UUID that can be
passed into the static fromString factory method to get a UUID object.
The remaining methods are applicable only to type 1 (time-based) UUID
values, and throw UnsupportedOperationException if invoked on other
types:
public long timestamp()
Returns the 60-bit timestamp associated with this UUID .