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A UID represents an identifier that is unique over time with respect to the host it is generated on, or one of 2¹⁶ "well-known" identifiers.

The UID() constructor can be used to generate an identifier that is unique over time with respect to the host it is generated on. The UID(short) constructor can be used to create one of 2¹⁶ well-known identifiers.

A UID instance contains three primitive values:

• unique, an int that uniquely identifies the VM that this UID was generated in, with respect to its host and at the time represented by the time value (an example implementation of the unique value would be a process identifier), or zero for a well-known UID
• time, a long equal to a time (as returned by System.currentTimeMillis()) at which the VM that this UID was generated in was alive, or zero for a well-known UID
• count, a short to distinguish UIDs generated in the same VM with the same time value

An independently generated UID instance is unique over time with respect to the host it is generated on as long as the host requires more than one millisecond to reboot and its system clock is never set backward. A globally unique identifier can be constructed by pairing a UID instance with a unique host identifier, such as an IP address.

Since:

JDK1.1

See Also:

Serialized Form

UID

public UID()

Generates a UID that is unique over time with respect to the host that it was generated on.

UID

public UID(short num)

Creates a "well-known" UID. There are 2¹⁶ possible such well-known ids.

A UID created via this constructor will not clash with any UIDs generated via the no-arg constructor.

Parameters:

num - number for well-known UID

hashCode

public int hashCode()

Returns the hash code value for this UID.

Overrides:

hashCode in class Object

Returns:

the hash code value for this UID

See Also:

Object.equals(java.lang.Object), Hashtable

equals

public boolean equals(Object obj)

Compares the specified object with this UID for equality. This method returns true if and only if the specified object is a UID instance with the same unique, time, and count values as this one.

Overrides:

equals in class Object

Parameters:

obj - the object to compare this UID to

Returns:

true if the given object is equivalent to this one, and false otherwise

See Also:

Object.hashCode(), Hashtable

toString

public String toString()

Returns a string representation of this UID.

Overrides:

toString in class Object

Returns:

a string representation of this UID

write

public void write(DataOutput out)
           throws IOException

Marshals a binary representation of this UID to a DataOutput instance.

Specifically, this method first invokes the given stream's DataOutput.writeInt(int) method with this UID's unique value, then it invokes the stream's DataOutput.writeLong(long) method with this UID's time value, and then it invokes the stream's DataOutput.writeShort(int) method with this UID's count value.

Parameters:

out - the DataOutput instance to write this UID to

Throws:

IOException - if an I/O error occurs while performing this operation

read

public static UID read(DataInput in)
                throws IOException

Constructs and returns a new UID instance by unmarshalling a binary representation from an DataInput instance.

Specifically, this method first invokes the given stream's DataInput.readInt() method to read a unique value, then it invoke's the stream's DataInput.readLong() method to read a time value, then it invoke's the stream's DataInput.readShort() method to read a count value, and then it creates and returns a new UID instance that contains the unique, time, and count values that were read from the stream.

Parameters:

in - the DataInput instance to read UID from

Returns:

unmarshalled UID instance

Throws:

IOException - if an I/O error occurs while performing this operation