DoubleAccumulator

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. The function is applied with the current value as its first argument, and the given update as the second 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.

Public Constructor Summary

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

Public Method Summary

void
accumulate(double x)
Updates with the given value.
double
doubleValue()
Equivalent to get().
float
floatValue()
Returns the {@linkplain #get 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 {@linkplain #get current value} as an int after a narrowing primitive conversion.
long
longValue()
Returns the {@linkplain #get 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.

Inherited Method Summary

Public Constructors

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

Public Methods

public void accumulate (double x)

Updates with the given value.

Parameters
x the value

public double doubleValue ()

Equivalent to get().

Returns
  • the current value

public float floatValue ()

Returns the {@linkplain #get current value} as a float after a narrowing primitive conversion.

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

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

public int intValue ()

Returns the {@linkplain #get current value} as an int after a narrowing primitive conversion.

public long longValue ()

Returns the {@linkplain #get current value} as a long after a narrowing primitive conversion.

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.

public String toString ()

Returns the String representation of the current value.

Returns
  • the String representation of the current value