Module java.base

Class DoubleAccumulator

All Implemented Interfaces:

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.

See Also:
Serialized Form
  • Constructor Details

    • DoubleAccumulator

      public DoubleAccumulator​(DoubleBinaryOperator accumulatorFunction, double identity)
      Creates a new instance using the given accumulator function and identity element.
      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.
      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.
      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.
      the value before reset
    • toString

      public String toString()
      Returns the String representation of the current value.
      toString in class Object
      the String representation of the current value
    • doubleValue

      public double doubleValue()
      Equivalent to get().
      Specified by:
      doubleValue in class Number
      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
      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
      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
      the numeric value represented by this object after conversion to type float.