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DoubleAccumulator

public class DoubleAccumulator
extends Number implements Serializable

java.lang.Object
   ↳ java.lang.Number
     ↳ java.util.concurrent.atomic.DoubleAccumulator


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.

Summary

Public constructors

DoubleAccumulator(DoubleBinaryOperator accumulatorFunction, double identity)

Creates a new instance using the given accumulator function and identity element.

Public methods

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.

Inherited methods

Public constructors

DoubleAccumulator

added in API level 24
public DoubleAccumulator (DoubleBinaryOperator accumulatorFunction, 
                double identity)

Creates a new instance using the given accumulator function and identity element.

Parameters
accumulatorFunction DoubleBinaryOperator: a side-effect-free function of two arguments

identity double: identity (initial value) for the accumulator function

Public methods

accumulate

added in API level 24
public void accumulate (double x)

Updates with the given value.

Parameters
x double: the value

doubleValue

added in API level 24
public double doubleValue ()

Equivalent to get().

Returns
double the current value

floatValue

added in API level 24
public float floatValue ()

Returns the current value as a float after a narrowing primitive conversion.

Returns
float the numeric value represented by this object after conversion to type float.

get

added in API level 24
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
double the current value

getThenReset

added in API level 24
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
double the value before reset

intValue

added in API level 24
public int intValue ()

Returns the current value as an int after a narrowing primitive conversion.

Returns
int the numeric value represented by this object after conversion to type int.

longValue

added in API level 24
public long longValue ()

Returns the current value as a long after a narrowing primitive conversion.

Returns
long the numeric value represented by this object after conversion to type long.

reset

added in API level 24
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.

toString

added in API level 24
public String toString ()

Returns the String representation of the current value.

Returns
String the String representation of the current value