Java in a Nutshell, 5th Edition [Electronic resources]

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UUIDjava.util

Java 5.0serializable comparable

This class is an immutable representation of 128-bit Universal Unique Identifier, or UUID, which serves as an identifier that is (with very high probability) globally unique. Create a UUID based on random bits with the randomUUID( ) factory method. Create a UUID based on the MD5 hash code of an array of bytes with the nameUUIDFromBytes( ) factory method. Or create a UUID by parsing a string with the fromString( ) factory method. The standard string format of a UUID is 32 hexadecimal digits, broken into five hyphen-separated groups of 8, 4, 4, 4, and 12 digits. For example:

7cbf3e1a-d521-40ac-87f1-e28b17530f60

Both lowercase and uppercase hex digits are allowed. The toString( ) method converts a UUID object to a string using this standard format. You can also create a UUID object by explicitly passing the 128 bits in the form of two long values to the UUID( ) constructor, but this option should be used only if you are intimately familiar with the relevant UUID standards.

The toString( ) and equals( ) methods define the most common operations on a UUID. The UUID class implements the Comparable interface and defines an ordering for UUID objects. Note, however, that the ordering does not represent any meaningful property, such as generation order, of the underlying bits.

Various accessor methods provide details about the bits of a UUID, but these details are rarely useful. getLeastSignificantBits( ) and getMostSignificantBits( ) return the bits of a UUID as two long values. version( ) and variant( ) return the version and variant of the UUID, which specify the type (random, name-based, time-based) and bit layout of the UUID. timestamp( ), clockSequence( ), and node( ) return values only for time-based UUIDs that have a version( ) of 1. Note that the UUID class does not provide a factory method for creating a time-based UUID.

Figure 16-67. java.util.UUID

public final class

UUID implements Serializable, Comparable<UUID> { // Public Constructors public

UUID (long

mostSigBits , long

leastSigBits ); // Public Class Methods public static UUID

fromString (String

name ); public static UUID

nameUUIDFromBytes (byte[ ]

name ); public static UUID

randomUUID ( ); // Public Instance Methods public int

clockSequence ( ); public long

getLeastSignificantBits ( ); public long

getMostSignificantBits ( ); public long

node ( ); public long

timestamp ( ); public int

variant ( ); public int

version ( ); // Methods Implementing Comparable public int

compareTo (UUID

val ); // Public Methods Overriding Object public boolean

equals (Object

obj ); public int

hashCode ( ); public String

toString ( ); }