RFR: 8261862: Expand discussion of rationale for BigDecimal equals/compareTo semantics

[Brian Burkhalter]

On Wed, 3 Mar 2021 07:20:03 GMT, Joe Darcy <darcy at openjdk.org> wrote:

> I considered @stuart-marks previous suggestion during the code review of JDK-8261123 to include a more explicit discussion of why, say, different representations of 2 should not be regarded as equivalent. After contemplating several alternatives, I didn't find anything simpler than Stuart's 2/3 example so I used that as seen in the diff.
> 
> A short digression, BigDecimal supports both fixed-point style and floating-point style rounding. Floating-point rounding primarily replies on the number of precision digits, regards of their scale, while fixed-point style rounding prioritizes the scale. The number of digits of eventual output is a function of number number of digits in the inputs and the number of precision digits in a floating-point style rounding. A floating-point style rounding has a preferred scale, rather than a fixed scale based on the inputs. The fixed-point style divide method used in the example has a scale based on the dividend, allowing a relatively simple expression to show a distinction between 2.0 and 2.00.

src/java.base/share/classes/java/math/BigDecimal.java line 3146:

> 3144:      * method since the former has [{@code BigInteger},
> 3145:      * {@code scale}] components equal to [20, 1] while the latter has
> 3146:      * components equals to [200, 2].

s/equals/equal/

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PR: https://git.openjdk.java.net/jdk/pull/2804