/*
* DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
*
* This code is free software; you can redistribute it and/or modify it
* under the terms of the GNU General Public License version 2 only, as
* published by the Free Software Foundation. Oracle designates this
* particular file as subject to the "Classpath" exception as provided
* by Oracle in the LICENSE file that accompanied this code.
*
* This code is distributed in the hope that it will be useful, but WITHOUT
* ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
* FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License
* version 2 for more details (a copy is included in the LICENSE file that
* accompanied this code).
*
* You should have received a copy of the GNU General Public License version
* 2 along with this work; if not, write to the Free Software Foundation,
* Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.
*
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/*
* This file is available under and governed by the GNU General Public
* License version 2 only, as published by the Free Software Foundation.
* However, the following notice accompanied the original version of this
* file:
*
* Written by Doug Lea with assistance from members of JCP JSR-166
* Expert Group and released to the public domain, as explained at
* http://creativecommons.org/publicdomain/zero/1.0/
*/
package java.util.concurrent.atomic;
import java.io.Serializable;
One or more variables that together maintain an initially zero double sum. When updates (method add) are contended across threads, the set of variables may grow dynamically to reduce contention. Method sum (or, equivalently doubleValue) returns the current total combined across the variables maintaining the sum. 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. 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.
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.
Author: Doug Lea Since: 1.8
/**
* One or more variables that together maintain an initially zero
* {@code double} sum. When updates (method {@link #add}) are
* contended across threads, the set of variables may grow dynamically
* to reduce contention. Method {@link #sum} (or, equivalently {@link
* #doubleValue}) returns the current total combined across the
* variables maintaining the sum. 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.
*
* <p>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.
*
* <p>This class extends {@link Number}, but does <em>not</em> define
* methods such as {@code equals}, {@code hashCode} and {@code
* compareTo} because instances are expected to be mutated, and so are
* not useful as collection keys.
*
* @since 1.8
* @author Doug Lea
*/
public class DoubleAdder extends Striped64 implements Serializable {
private static final long serialVersionUID = 7249069246863182397L;
/*
* Note that we must use "long" for underlying representations,
* because there is no compareAndSet for double, due to the fact
* that the bitwise equals used in any CAS implementation is not
* the same as double-precision equals. However, we use CAS only
* to detect and alleviate contention, for which bitwise equals
* works best anyway. In principle, the long/double conversions
* used here should be essentially free on most platforms since
* they just re-interpret bits.
*/
Creates a new adder with initial sum of zero.
/**
* Creates a new adder with initial sum of zero.
*/
public DoubleAdder() {
}
Adds the given value.
Params: - x – the value to add
/**
* Adds the given value.
*
* @param x the value to add
*/
public void add(double x) {
Cell[] cs; long b, v; int m; Cell c;
if ((cs = cells) != null ||
!casBase(b = base,
Double.doubleToRawLongBits
(Double.longBitsToDouble(b) + x))) {
boolean uncontended = true;
if (cs == null || (m = cs.length - 1) < 0 ||
(c = cs[getProbe() & m]) == null ||
!(uncontended = c.cas(v = c.value,
Double.doubleToRawLongBits
(Double.longBitsToDouble(v) + x))))
doubleAccumulate(x, null, uncontended);
}
}
Returns the current sum. 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 sum is being calculated might not be
incorporated. Also, because floating-point arithmetic is not
strictly associative, the returned result need not be identical
to the value that would be obtained in a sequential series of
updates to a single variable.
Returns: the sum
/**
* Returns the current sum. The returned value is <em>NOT</em> an
* atomic snapshot; invocation in the absence of concurrent
* updates returns an accurate result, but concurrent updates that
* occur while the sum is being calculated might not be
* incorporated. Also, because floating-point arithmetic is not
* strictly associative, the returned result need not be identical
* to the value that would be obtained in a sequential series of
* updates to a single variable.
*
* @return the sum
*/
public double sum() {
Cell[] cs = cells;
double sum = Double.longBitsToDouble(base);
if (cs != null) {
for (Cell c : cs)
if (c != null)
sum += Double.longBitsToDouble(c.value);
}
return sum;
}
Resets variables maintaining the sum to zero. This method may
be a useful alternative to creating a new adder, 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.
/**
* Resets variables maintaining the sum to zero. This method may
* be a useful alternative to creating a new adder, 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 void reset() {
Cell[] cs = cells;
base = 0L; // relies on fact that double 0 must have same rep as long
if (cs != null) {
for (Cell c : cs)
if (c != null)
c.reset();
}
}
Equivalent in effect to sum 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 sum
/**
* Equivalent in effect to {@link #sum} followed by {@link
* #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
* <em>not</em> guaranteed to be the final value occurring before
* the reset.
*
* @return the sum
*/
public double sumThenReset() {
Cell[] cs = cells;
double sum = Double.longBitsToDouble(getAndSetBase(0L));
if (cs != null) {
for (Cell c : cs) {
if (c != null)
sum += Double.longBitsToDouble(c.getAndSet(0L));
}
}
return sum;
}
Returns the String representation of the sum. Returns: the String representation of the sum
/**
* Returns the String representation of the {@link #sum}.
* @return the String representation of the {@link #sum}
*/
public String toString() {
return Double.toString(sum());
}
Equivalent to sum. Returns: the sum
/**
* Equivalent to {@link #sum}.
*
* @return the sum
*/
public double doubleValue() {
return sum();
}
Returns the sum as a long after a narrowing primitive conversion. /**
* Returns the {@link #sum} as a {@code long} after a
* narrowing primitive conversion.
*/
public long longValue() {
return (long)sum();
}
Returns the sum as an int after a narrowing primitive conversion. /**
* Returns the {@link #sum} as an {@code int} after a
* narrowing primitive conversion.
*/
public int intValue() {
return (int)sum();
}
Returns the sum as a float after a narrowing primitive conversion. /**
* Returns the {@link #sum} as a {@code float}
* after a narrowing primitive conversion.
*/
public float floatValue() {
return (float)sum();
}
Serialization proxy, used to avoid reference to the non-public
Striped64 superclass in serialized forms.
@serial include
/**
* Serialization proxy, used to avoid reference to the non-public
* Striped64 superclass in serialized forms.
* @serial include
*/
private static class SerializationProxy implements Serializable {
private static final long serialVersionUID = 7249069246863182397L;
The current value returned by sum().
@serial
/**
* The current value returned by sum().
* @serial
*/
private final double value;
SerializationProxy(DoubleAdder a) {
value = a.sum();
}
Returns a DoubleAdder object with initial state held by this proxy. Returns: a DoubleAdder object with initial state held by this proxy
/**
* Returns a {@code DoubleAdder} object with initial state
* held by this proxy.
*
* @return a {@code DoubleAdder} object with initial state
* held by this proxy
*/
private Object readResolve() {
DoubleAdder a = new DoubleAdder();
a.base = Double.doubleToRawLongBits(value);
return a;
}
}
Returns a
SerializationProxy
representing the state of this instance.
Returns: a SerializationProxy representing the state of this instance
/**
* Returns a
* <a href="../../../../serialized-form.html#java.util.concurrent.atomic.DoubleAdder.SerializationProxy">
* SerializationProxy</a>
* representing the state of this instance.
*
* @return a {@link SerializationProxy}
* representing the state of this instance
*/
private Object writeReplace() {
return new SerializationProxy(this);
}
Params: - s – the stream
Throws: - InvalidObjectException – always
/**
* @param s the stream
* @throws java.io.InvalidObjectException always
*/
private void readObject(java.io.ObjectInputStream s)
throws java.io.InvalidObjectException {
throw new java.io.InvalidObjectException("Proxy required");
}
}