Class DoubleAccumulator

java.lang.Object
java.lang.Number
java.util.concurrent.atomic.DoubleAccumulator
All Implemented Interfaces:
Serializable
public class DoubleAccumulator extends Number implements Serializable
One or more variables that together maintain a running double value updated using a supplied function. When updates (method accumulate(double)) are contended across threads, the set of variables may grow dynamically to reduce contention. Method get() (or, equivalently, doubleValue()) returns the current value across the variables maintaining updates.

This class is usually preferable to alternatives when multiple threads update a common value that is used for purposes such as summary statistics that are frequently updated but less frequently read.

The supplied accumulator function should be side-effect-free, since it may be re-applied when attempted updates fail due to contention among threads. For predictable results, the accumulator function should be commutative and associative within the floating point tolerance required in usage contexts. The function is applied with an existing value (or identity) as one argument, and a given update as the other argument. For example, to maintain a running maximum value, you could supply Double::max along with Double.NEGATIVE_INFINITY as the identity. The order of accumulation within or across threads is not guaranteed. Thus, this class may not be applicable if numerical stability is required, especially when combining values of substantially different orders of magnitude.

Class DoubleAdder provides analogs of the functionality of this class for the common special case of maintaining sums. The call new DoubleAdder() is equivalent to new DoubleAccumulator((x, y) -> x + y, 0.0).

This class extends Number, but does not define methods such as equals, hashCode and compareTo because instances are expected to be mutated, and so are not useful as collection keys.

Since:
1.8
See Also:

  • Constructor Summary

    Constructors
    Constructor
    Description
    DoubleAccumulator(DoubleBinaryOperator accumulatorFunction, double identity)
    Creates a new instance using the given accumulator function and identity element.

  • Method Summary

    Modifier and Type
    Method
    Description
    void
    accumulate(double x)
    Updates with the given value.
    double
    doubleValue()
    Equivalent to get().
    float
    floatValue()
    Returns the current value as a float after a narrowing primitive conversion.
    double
    get()
    Returns the current value.
    double
    getThenReset()
    Equivalent in effect to get() followed by reset().
    int
    intValue()
    Returns the current value as an int after a narrowing primitive conversion.
    long
    longValue()
    Returns the current value as a long after a narrowing primitive conversion.
    void
    reset()
    Resets variables maintaining updates to the identity value.
    String
    toString()
    Returns the String representation of the current value.

    Methods declared in class Number

    byteValue, shortValue
    Modifier and Type
    Method
    Description
    byte
    byteValue()
    Returns the value of the specified number as a byte.
    short
    shortValue()
    Returns the value of the specified number as a short.

    Methods declared in class Object

    clone, equals, finalize, getClass, hashCode, notify, notifyAll, wait, wait, wait
    Modifier and Type
    Method
    Description
    protected Object
    clone()
    Answers a new instance of the same class as the receiver, whose slots have been filled in with the values in the slots of the receiver.
    boolean
    equals(Object o)
    Compares the argument to the receiver, and answers true if they represent the same object using a class specific comparison.
    protected void
    finalize()
    Deprecated, for removal: This API element is subject to removal in a future version.
    May cause performance issues, deadlocks and hangs.
    final Class<? extends Object>
    getClass()
    Answers the unique instance of java.lang.Class which represents the class of the receiver.
    int
    hashCode()
    Answers an integer hash code for the receiver.
    final void
    notify()
    Causes one thread which is waiting on the receiver to be made ready to run.
    final void
    notifyAll()
    Causes all threads which are waiting on the receiver to be made ready to run.
    final void
    wait()
    Causes the thread which sent this message to be made not ready to run pending some change in the receiver (as indicated by notify or notifyAll).
    final void
    wait(long time)
    Causes the thread which sent this message to be made not ready to run either pending some change in the receiver (as indicated by notify or notifyAll) or the expiration of the timeout.
    final void
    wait(long time, int frac)
    Causes the thread which sent this message to be made not ready to run either pending some change in the receiver (as indicated by notify or notifyAll) or the expiration of the timeout.

  • Constructor Details

    • DoubleAccumulator

      public DoubleAccumulator(DoubleBinaryOperator accumulatorFunction, double identity)
      Creates a new instance using the given accumulator function and identity element.
      Parameters:
      accumulatorFunction - a side-effect-free function of two arguments
      identity - identity (initial value) for the accumulator function

  • Method Details

    • accumulate

      public void accumulate(double x)
      Updates with the given value.
      Parameters:
      x - the value

    • get

      public double get()
      Returns the current value. The returned value is NOT an atomic snapshot; invocation in the absence of concurrent updates returns an accurate result, but concurrent updates that occur while the value is being calculated might not be incorporated.
      Returns:
      the current value

    • reset

      public void reset()
      Resets variables maintaining updates to the identity value. This method may be a useful alternative to creating a new updater, but is only effective if there are no concurrent updates. Because this method is intrinsically racy, it should only be used when it is known that no threads are concurrently updating.

    • getThenReset

      public double getThenReset()
      Equivalent in effect to get() followed by reset(). This method may apply for example during quiescent points between multithreaded computations. If there are updates concurrent with this method, the returned value is not guaranteed to be the final value occurring before the reset.
      Returns:
      the value before reset

    • toString

      public String toString()
      Returns the String representation of the current value.
      Overrides:
      toString in class Object
      Returns:
      the String representation of the current value

    • doubleValue

      public double doubleValue()
      Equivalent to get().
      Specified by:
      doubleValue in class Number
      Returns:
      the current value

    • longValue

      public long longValue()
      Returns the current value as a long after a narrowing primitive conversion.
      Specified by:
      longValue in class Number
      Returns:
      the numeric value represented by this object after conversion to type long.

    • intValue

      public int intValue()
      Returns the current value as an int after a narrowing primitive conversion.
      Specified by:
      intValue in class Number
      Returns:
      the numeric value represented by this object after conversion to type int.

    • floatValue

      public float floatValue()
      Returns the current value as a float after a narrowing primitive conversion.
      Specified by:
      floatValue in class Number
      Returns:
      the numeric value represented by this object after conversion to type float.