UID (Java Platform SE 8 )
Keywords: java.rmi.server.UID class
Date: 2026-07-22

compact2, compact3
java.rmi.server

## Class UID

- java.lang.Object
- - java.rmi.server.UID

- All Implemented Interfaces:
    Serializable

      public final class UID
      extends Object
      implements Serializable

  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 `UID`s 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

- - ### Constructor Summary

    Constructors
    Constructor and Description
    `UID()` Generates a `UID` that is unique over time with respect to the host that it was generated on.
    `UID(short num)` Creates a "well-known" `UID`.

  - ### Method Summary

    All Methods Static Methods Instance Methods Concrete Methods
    Modifier and Type | Method and Description
    --- | ---
    `boolean` | `equals(Object obj)` Compares the specified object with this `UID` for equality.
    `int` | `hashCode()` Returns the hash code value for this `UID`.
    `static UID` | `read(DataInput in)` Constructs and returns a new `UID` instance by unmarshalling a binary representation from an `DataInput` instance.
    `String` | `toString()` Returns a string representation of this `UID`.
    `void` | `write(DataOutput out)` Marshals a binary representation of this `UID` to a `DataOutput` instance.

    - ### Methods inherited from class java.lang.Object

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

- - ### Constructor Detail

    - #### 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 `UID`s generated via the no-arg constructor.

      Parameters:
        `num` - number for well-known `UID`

  - ### Method Detail

    - #### 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)`, `System.identityHashCode(java.lang.Object)`

    - #### 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()`, `HashMap`

    - #### 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

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