JDK 9 RFR of JDK-8030942: Explicitly state floating-point summation requirements on non-finite inputs

[Paul Sandoz]

> In response to previous feedback, I propose this revised change to the specification:
> 

+1

Paul.

> --- a/src/share/classes/java/util/DoubleSummaryStatistics.java    Sat Jul 19 11:22:08 2014 +0800
> +++ b/src/share/classes/java/util/DoubleSummaryStatistics.java    Mon Jul 21 18:02:54 2014 -0700
> @@ -1,5 +1,5 @@
>  /*
> - * Copyright (c) 2012, 2013, Oracle and/or its affiliates. All rights reserved.
> + * Copyright (c) 2012, 2014, Oracle and/or its affiliates. All rights reserved.
>   * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
>   *
>   * This code is free software; you can redistribute it and/or modify it
> @@ -129,9 +129,6 @@
>       * Returns the sum of values recorded, or zero if no values have been
>       * recorded.
>       *
> -     * If any recorded value is a NaN or the sum is at any point a NaN
> -     * then the sum will be NaN.
> -     *
>       * <p> The value of a floating-point sum is a function both of the
>       * input values as well as the order of addition operations. The
>       * order of addition operations of this method is intentionally
> @@ -143,6 +140,44 @@
>       * numerical sum compared to a simple summation of {@code double}
>       * values.
>       *
> +     * Because of the unspecified order of operations and the
> +     * possibility of using differing summation schemes, the output of
> +     * this method may vary on the same input values.
> +     *
> +     * <p>Various conditions can result in a non-finite sum being
> +     * computed. This can occur even if the all the recorded values
> +     * being summed are finite. If any recorded value is non-finite,
> +     * the sum will be non-finite:
> +     *
> +     * <ul>
> +     *
> +     * <li>If any recorded value is a NaN, then the final sum will be
> +     * NaN.
> +     *
> +     * <li>If the recorded values contain one or more infinities, the
> +     * sum will be infinite or NaN.
> +     *
> +     * <ul>
> +     *
> +     * <li>If the recorded values contain infinities of opposite sign,
> +     * the sum will be NaN.
> +     *
> +     * <li>If the recorded values contain infinities of one sign and
> +     * an intermediate sum overflows to an infinity of the opposite
> +     * sign, the sum may be NaN.
> +     *
> +     * </ul>
> +     *
> +     * </ul>
> +     *
> +     * It is possible for intermediate sums of finite values to
> +     * overflow into opposite-signed infinities; if that occurs, the
> +     * final sum will be NaN even if the recorded values are all
> +     * finite.
> +     *
> +     * If all the recorded values are zero, the sign of zero is
> +     * <em>not</em> guaranteed to be preserved in the final sum.
> +     *
>       * @apiNote Values sorted by increasing absolute magnitude tend to yield
>       * more accurate results.
>       *
> @@ -193,15 +228,9 @@
>       * Returns the arithmetic mean of values recorded, or zero if no
>       * values have been recorded.
>       *
> -     * If any recorded value is a NaN or the sum is at any point a NaN
> -     * then the average will be code NaN.
> -     *
> -     * <p>The average returned can vary depending upon the order in
> -     * which values are recorded.
> -     *
> -     * This method may be implemented using compensated summation or
> -     * other technique to reduce the error bound in the {@link #getSum
> -     * numerical sum} used to compute the average.
> +     * <p> The computed average can vary numerically and have the
> +     * special case behavior as computing the sum; see {@link #getSum}
> +     * for details.
>       *
>       * @apiNote Values sorted by increasing absolute magnitude tend to yield
>       * more accurate results.
> 
> (With analogous changes in java/util/stream/DoubleStream.java.)
> 
> Thanks,
> 
> -Joe