Class UUID

java.lang.Object
java.util.UUID
All Implemented Interfaces:
Serializable, Comparable<UUID>
public final class UUID extends Object implements Comparable<UUID>, Serializable

A 128 bit identifier: its canonical text form, random (version 4) generation and name based (version 3) generation.

randomUUID() draws its bits from the platform's secure random source through JdkHost. Where the platform installed none it falls back to a Random seeded from the clock, which keeps identifiers unique for every practical purpose but NOT unpredictable; an application that uses one as a secret should check that its platform provides secure random bytes.

nameUUIDFromBytes(byte[]) needs an MD5 digest from the platform and throws UnsupportedOperationException where there is none. The time based accessors (timestamp, clockSequence, node) are not provided.

See Also:

  • Constructor Summary

    Constructors
    Constructor
    Description
    UUID(long mostSigBits, long leastSigBits)
    An identifier from its two halves.

  • Method Summary

    Modifier and Type
    Method
    Description
    int
    compareTo(UUID val)
    Orders by the most significant half, then the least, each as a signed number -- the order the JDK defines.
    boolean
    equals(Object obj)
    Indicates whether some other object is "equal to" this one.
    static UUID
    fromString(String name)
    Parses the text form toString() writes: five groups of hexadecimal digits separated by -.
    long
    getLeastSignificantBits()
    The least significant 64 bits.
    long
    getMostSignificantBits()
    The most significant 64 bits.
    int
    hashCode()
    Returns a hash code value for the object.
    static UUID
    nameUUIDFromBytes(byte[] name)
    The version 3 identifier of name: its MD5 digest, with the version and variant bits set.
    static UUID
    randomUUID()
    A new version 4 identifier; see the class description for where its randomness comes from.
    String
    toString()
    Returns a string representation of the object.
    int
    variant()
    The variant number: 2 for the IETF layout every identifier made here has.
    int
    version()
    The version number: 4 for one made by randomUUID(), 3 for one made by nameUUIDFromBytes(byte[]).

    Methods inherited from class Object

    clone, finalize, getClass, notify, notifyAll, wait, wait, wait

  • Constructor Details

    • UUID

      public UUID(long mostSigBits, long leastSigBits)
      An identifier from its two halves.

  • Method Details

    • randomUUID

      public static UUID randomUUID()
      A new version 4 identifier; see the class description for where its randomness comes from.

    • nameUUIDFromBytes

      public static UUID nameUUIDFromBytes(byte[] name)

      The version 3 identifier of name: its MD5 digest, with the version and variant bits set.

      Throws
      • UnsupportedOperationException: if the platform has no MD5 digest

    • fromString

      public static UUID fromString(String name)

      Parses the text form toString() writes: five groups of hexadecimal digits separated by -. As in the JDK, a group may be shorter than its full width, and one that is longer is not refused either: the digits past its width are dropped, so 100000000-0-0-0-0 is the nil UUID there and here. Only a group of more than sixteen digits, which no long holds, is an error.

      Throws
      • IllegalArgumentException: if name is not in that form

    • getMostSignificantBits

      public long getMostSignificantBits()
      The most significant 64 bits.

    • getLeastSignificantBits

      public long getLeastSignificantBits()
      The least significant 64 bits.

    • version

      public int version()
      The version number: 4 for one made by randomUUID(), 3 for one made by nameUUIDFromBytes(byte[]).

    • variant

      public int variant()
      The variant number: 2 for the IETF layout every identifier made here has.

    • toString

      public String toString()
      Description copied from class: Object
      Returns a string representation of the object. In general, the toString method returns a string that "textually represents" this object. The result should be a concise but informative representation that is easy for a person to read. It is recommended that all subclasses override this method. The toString method for class Object returns a string consisting of the name of the class of which the object is an instance, the at-sign character `@', and the unsigned hexadecimal representation of the hash code of the object. In other words, this method returns a string equal to the value of: getClass().getName() + '@' + Integer.toHexString(hashCode())
      Overrides:
      toString in class Object

    • hashCode

      public int hashCode()
      Description copied from class: Object
      Returns a hash code value for the object. This method is supported for the benefit of hashtables such as those provided by java.util.Hashtable. The general contract of hashCode is: Whenever it is invoked on the same object more than once during an execution of a Java application, the hashCode method must consistently return the same integer, provided no information used in equals comparisons on the object is modified. This integer need not remain consistent from one execution of an application to another execution of the same application. If two objects are equal according to the equals(Object) method, then calling the hashCode method on each of the two objects must produce the same integer result. It is not required that if two objects are unequal according to the equals(java.lang.Object) method, then calling the hashCode method on each of the two objects must produce distinct integer results. However, the programmer should be aware that producing distinct integer results for unequal objects may improve the performance of hashtables. As much as is reasonably practical, the hashCode method defined by class Object does return distinct integers for distinct objects. (This is typically implemented by converting the internal address of the object into an integer, but this implementation technique is not required by the JavaTM programming language.)
      Overrides:
      hashCode in class Object

    • equals

      public boolean equals(Object obj)
      Description copied from class: Object
      Indicates whether some other object is "equal to" this one. The equals method implements an equivalence relation: It is reflexive: for any reference value x, x.equals(x) should return true. It is symmetric: for any reference values x and y, x.equals(y) should return true if and only if y.equals(x) returns true. It is transitive: for any reference values x, y, and z, if x.equals(y) returns true and y.equals(z) returns true, then x.equals(z) should return true. It is consistent: for any reference values x and y, multiple invocations of x.equals(y) consistently return true or consistently return false, provided no information used in equals comparisons on the object is modified. For any non-null reference value x, x.equals(null) should return false. The equals method for class Object implements the most discriminating possible equivalence relation on objects; that is, for any reference values x and y, this method returns true if and only if x and y refer to the same object (x==y has the value true).
      Overrides:
      equals in class Object

    • compareTo

      public int compareTo(UUID val)
      Orders by the most significant half, then the least, each as a signed number -- the order the JDK defines.
      Specified by:
      compareTo in interface Comparable<UUID>
      Returns:

      a negative integer if this instance is less than another; a positive integer if this instance is greater than another; 0 if this instance has the same order as another.

      Throws
      • ClassCastException: @throws ClassCastException if another cannot be converted into something comparable to this instance.