Back to index...

	/*
	* 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.
	*
	* Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA
	* or visit www.oracle.com if you need additional information or have any
	* questions.
	*/

	/*
	* 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
	* {@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.
	*/
	public DoubleAdder() {
	}

	/**
	* 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 <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.
	*/
	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 {@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 {@link #sum}.
	* @return the String representation of the {@link #sum}
	*/
	public String toString() {
	return Double.toString(sum());
	}

	/**
	* Equivalent to {@link #sum}.
	*
	* @return the sum
	*/
	public double doubleValue() {
	return sum();
	}

	/**
	* Returns the {@link #sum} as a {@code long} after a
	* narrowing primitive conversion.
	*/
	public long longValue() {
	return (long)sum();
	}

	/**
	* Returns the {@link #sum} as an {@code int} after a
	* narrowing primitive conversion.
	*/
	public int intValue() {
	return (int)sum();
	}

	/**
	* 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
	*/
	private static class SerializationProxy implements Serializable {
	private static final long serialVersionUID = 7249069246863182397L;

	/**
	* The current value returned by sum().
	* @serial
	*/
	private final double value;

	SerializationProxy(DoubleAdder a) {
	value = a.sum();
	}

	/**
	* 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
	* <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);
	}

	/**
	* @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");
	}

	}

Back to index...