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8261366: Add discussion of IEEE 754 to BigDecimal
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Reviewed-by: bpb
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@jddarcy
jddarcy committed Mar 8, 2021
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79 changes: 69 additions & 10 deletions src/java.base/share/classes/java/math/BigDecimal.java
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import java.util.Objects;

/**
* Immutable, arbitrary-precision signed decimal numbers. A
* {@code BigDecimal} consists of an arbitrary precision integer
* <i>unscaled value</i> and a 32-bit integer <i>scale</i>. If zero
* or positive, the scale is the number of digits to the right of the
* decimal point. If negative, the unscaled value of the number is
* multiplied by ten to the power of the negation of the scale. The
* value of the number represented by the {@code BigDecimal} is
* therefore <code>(unscaledValue &times; 10<sup>-scale</sup>)</code>.
* Immutable, arbitrary-precision signed decimal numbers. A {@code
* BigDecimal} consists of an arbitrary precision integer
* <i>{@linkplain unscaledValue() unscaled value}</i> and a 32-bit
* integer <i>{@linkplain scale() scale}</i>. If zero or positive,
* the scale is the number of digits to the right of the decimal
* point. If negative, the unscaled value of the number is multiplied
* by ten to the power of the negation of the scale. The value of the
* number represented by the {@code BigDecimal} is therefore
* <code>(unscaledValue &times; 10<sup>-scale</sup>)</code>.
*
* <p>The {@code BigDecimal} class provides operations for
* arithmetic, scale manipulation, rounding, comparison, hashing, and
Expand Down Expand Up @@ -220,6 +221,64 @@
* Comparable}, {@link java.util.SortedMap} or {@link
* java.util.SortedSet} for more information.
*
* <h2>Relation to IEEE 754 Decimal Arithmetic</h2>
*
* Starting with its 2008 revision, the <cite>IEEE 754 Standard for
* Floating-point Arithmetic</cite> has covered decimal formats and
* operations. While there are broad similarities in the decimal
* arithmetic defined by IEEE 754 and by this class, there are notable
* differences as well. The fundamental similarity shared by {@code
* BigDecimal} and IEEE 754 decimal arithmetic is the conceptual
* operation of computing the mathematical infinitely precise real
* number value of an operation and then mapping that real number to a
* representable decimal floating-point value under a <em>rounding
* policy</em>. The rounding policy is called a {@linkplain
* RoundingMode rounding mode} for {@code BigDecimal} and called a
* rounding-direction attribute in IEEE 754-2019. When the exact value
* is not representable, the rounding policy determines which of the
* two representable decimal values bracketing the exact value is
* selected as the computed result. The notion of a <em>preferred
* scale/preferred exponent</em> is also shared by both systems.
*
* <p>For differences, IEEE 754 includes several kinds of values not
* modeled by {@code BigDecimal} including negative zero, signed
* infinities, and NaN (not-a-number). IEEE 754 defines formats, which
* are parameterized by base (binary or decimal), number of digits of
* precision, and exponent range. A format determines the set of
* representable values. Most operations accept as input one or more
* values of a given format and produce a result in the same format.
* A {@code BigDecimal}'s {@linkplain scale() scale} is equivalent to
* negating an IEEE 754 value's exponent. {@code BigDecimal} values do
* not have a format in the same sense; all values have the same
* possible range of scale/exponent and the {@linkplain
* unscaledValue() unscaled value} has arbitrary precision. Instead,
* for the {@code BigDecimal} operations taking a {@code MathContext}
* parameter, if the {@code MathContext} has a nonzero precision, the
* set of possible representable values for the result is determined
* by the precision of the {@code MathContext} argument. For example
* in {@code BigDecimal}, if a nonzero three-digit number and a
* nonzero four-digit number are multiplied together in the context of
* a {@code MathContext} object having a precision of three, the
* result will have three digits (assuming no overflow or underflow,
* etc.).
*
* <p>The rounding policies implemented by {@code BigDecimal}
* operations indicated by {@linkplain RoundingMode rounding modes}
* are a proper superset of the IEEE 754 rounding-direction
* attributes.
* <p>{@code BigDecimal} arithmetic will most resemble IEEE 754
* decimal arithmetic if a {@code MathContext} corresponding to an
* IEEE 754 decimal format, such as {@linkplain MathContext#DECIMAL64
* decimal64} or {@linkplain MathContext#DECIMAL128 decimal128} is
* used to round all starting values and intermediate operations. The
* numerical values computed can differ if the exponent range of the
* IEEE 754 format being approximated is exceeded since a {@code
* MathContext} does not constrain the scale of {@code BigDecimal}
* results. Operations that would generate a NaN or exact infinity,
* such as dividing by zero, throw an {@code ArithmeticException} in
* {@code BigDecimal} arithmetic.
*
* @see BigInteger
* @see MathContext
* @see RoundingMode
Expand Down Expand Up @@ -1681,7 +1740,7 @@ public BigDecimal divide(BigDecimal divisor, RoundingMode roundingMode) {
*
* @param divisor value by which this {@code BigDecimal} is to be divided.
* @throws ArithmeticException if the exact quotient does not have a
* terminating decimal expansion
* terminating decimal expansion, including dividing by zero
* @return {@code this / divisor}
* @since 1.5
* @author Joseph D. Darcy
Expand Down Expand Up @@ -1745,7 +1804,7 @@ public BigDecimal divide(BigDecimal divisor) {
* @throws ArithmeticException if the result is inexact but the
* rounding mode is {@code UNNECESSARY} or
* {@code mc.precision == 0} and the quotient has a
* non-terminating decimal expansion.
* non-terminating decimal expansion,including dividing by zero
* @since 1.5
*/
public BigDecimal divide(BigDecimal divisor, MathContext mc) {
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38 changes: 19 additions & 19 deletions src/java.base/share/classes/java/math/MathContext.java
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/*
* Copyright (c) 2003, 2020, Oracle and/or its affiliates. All rights reserved.
* Copyright (c) 2003, 2021, 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
Expand Down Expand Up @@ -69,39 +69,39 @@ public final class MathContext implements Serializable {

/* ----- Public Properties ----- */
/**
* A {@code MathContext} object whose settings have the values
* required for unlimited precision arithmetic.
* The values of the settings are:
* <code>
* precision=0 roundingMode=HALF_UP
* </code>
* A {@code MathContext} object whose settings have the values
* required for unlimited precision arithmetic.
* The values of the settings are: {@code precision=0 roundingMode=HALF_UP}
*/
public static final MathContext UNLIMITED =
new MathContext(0, RoundingMode.HALF_UP);

/**
* A {@code MathContext} object with a precision setting
* matching the IEEE 754R Decimal32 format, 7 digits, and a
* rounding mode of {@link RoundingMode#HALF_EVEN HALF_EVEN}, the
* IEEE 754R default.
* A {@code MathContext} object with a precision setting
* matching the precision of the IEEE 754-2019 decimal32 format, 7 digits, and a
* rounding mode of {@link RoundingMode#HALF_EVEN HALF_EVEN}.
* Note the exponent range of decimal32 is <em>not</em> used for
* rounding.
*/
public static final MathContext DECIMAL32 =
new MathContext(7, RoundingMode.HALF_EVEN);

/**
* A {@code MathContext} object with a precision setting
* matching the IEEE 754R Decimal64 format, 16 digits, and a
* rounding mode of {@link RoundingMode#HALF_EVEN HALF_EVEN}, the
* IEEE 754R default.
* A {@code MathContext} object with a precision setting
* matching the precision of the IEEE 754-2019 decimal64 format, 16 digits, and a
* rounding mode of {@link RoundingMode#HALF_EVEN HALF_EVEN}.
* Note the exponent range of decimal64 is <em>not</em> used for
* rounding.
*/
public static final MathContext DECIMAL64 =
new MathContext(16, RoundingMode.HALF_EVEN);

/**
* A {@code MathContext} object with a precision setting
* matching the IEEE 754R Decimal128 format, 34 digits, and a
* rounding mode of {@link RoundingMode#HALF_EVEN HALF_EVEN}, the
* IEEE 754R default.
* A {@code MathContext} object with a precision setting
* matching the precision of the IEEE 754-2019 decimal128 format, 34 digits, and a
* rounding mode of {@link RoundingMode#HALF_EVEN HALF_EVEN}.
* Note the exponent range of decimal64 is <em>not</em> used for
* rounding.
*/
public static final MathContext DECIMAL128 =
new MathContext(34, RoundingMode.HALF_EVEN);
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26 changes: 13 additions & 13 deletions src/java.base/share/classes/java/math/RoundingMode.java
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/*
* Copyright (c) 2003, 2020, Oracle and/or its affiliates. All rights reserved.
* Copyright (c) 2003, 2021, 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
Expand Down Expand Up @@ -29,12 +29,12 @@
package java.math;

/**
* Specifies a <i>rounding behavior</i> for numerical operations
* capable of discarding precision. Each rounding mode indicates how
* the least significant returned digit of a rounded result is to be
* calculated. If fewer digits are returned than the digits needed to
* represent the exact numerical result, the discarded digits will be
* referred to as the <i>discarded fraction</i> regardless the digits'
* Specifies a <i>rounding policy</i> for numerical operations capable
* of discarding precision. Each rounding mode indicates how the least
* significant returned digit of a rounded result is to be calculated.
* If fewer digits are returned than the digits needed to represent
* the exact numerical result, the discarded digits will be referred
* to as the <i>discarded fraction</i> regardless the digits'
* contribution to the value of the number. In other words,
* considered as a numerical value, the discarded fraction could have
* an absolute value greater than one.
Expand Down Expand Up @@ -89,7 +89,7 @@
*
* @apiNote
* Five of the rounding modes declared in this class correspond to
* rounding direction attributes defined in the <cite>IEEE Standard
* rounding-direction attributes defined in the <cite>IEEE Standard
* for Floating-Point Arithmetic</cite>, IEEE 754-2019. Where present,
* this correspondence will be noted in the documentation of the
* particular constant.
Expand Down Expand Up @@ -137,7 +137,7 @@ public enum RoundingMode {
* Rounding mode to round towards zero. Never increments the digit
* prior to a discarded fraction (i.e., truncates). Note that this
* rounding mode never increases the magnitude of the calculated value.
* This mode corresponds to the IEEE 754-2019 rounding
* This mode corresponds to the IEEE 754-2019 rounding-direction
* attribute roundTowardZero.
*
*<p>Example:
Expand Down Expand Up @@ -168,7 +168,7 @@ public enum RoundingMode {
* result is positive, behaves as for {@code RoundingMode.UP};
* if negative, behaves as for {@code RoundingMode.DOWN}. Note
* that this rounding mode never decreases the calculated value.
* This mode corresponds to the IEEE 754-2019 rounding
* This mode corresponds to the IEEE 754-2019 rounding-direction
* attribute roundTowardPositive.
*
*<p>Example:
Expand Down Expand Up @@ -199,7 +199,7 @@ public enum RoundingMode {
* result is positive, behave as for {@code RoundingMode.DOWN};
* if negative, behave as for {@code RoundingMode.UP}. Note that
* this rounding mode never increases the calculated value.
* This mode corresponds to the IEEE 754-2019 rounding
* This mode corresponds to the IEEE 754-2019 rounding-direction
* attribute roundTowardNegative.
*
*<p>Example:
Expand Down Expand Up @@ -232,7 +232,7 @@ public enum RoundingMode {
* fraction is &ge; 0.5; otherwise, behaves as for
* {@code RoundingMode.DOWN}. Note that this is the rounding
* mode commonly taught at school.
* This mode corresponds to the IEEE 754-2019 rounding
* This mode corresponds to the IEEE 754-2019 rounding-direction
* attribute roundTiesToAway.
*
*<p>Example:
Expand Down Expand Up @@ -301,7 +301,7 @@ public enum RoundingMode {
* chiefly used in the USA. This rounding mode is analogous to
* the rounding policy used for {@code float} and {@code double}
* arithmetic in Java.
* This mode corresponds to the IEEE 754-2019 rounding
* This mode corresponds to the IEEE 754-2019 rounding-direction
* attribute roundTiesToEven.
*
*<p>Example:
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