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From Shreyas Kaushik <Shreyas.Kaus...@Sun.COM>
Subject [PATCH] (DERBY-123) Derby incorrectly rounds double values down during insert into NUMERIC
Date Thu, 03 Feb 2005 03:47:44 GMT
Patch for this issue.

~ Shreyas

Shreyas Kaushik (JIRA) wrote:

>     [ http://issues.apache.org/jira/browse/DERBY-123?page=comments#action_58294 ]
>Shreyas Kaushik commented on DERBY-123:
>The javadoc for BigDecimal constructor that takes a double as argument is as below:
>public BigDecimal(double val)
>    Translates a double into a BigDecimal which is the exact decimal representation of
the double's binary floating-point value. The scale of the returned BigDecimal is the smallest
value such that (10scale × val) is an integer.
>    Notes:
>       1. The results of this constructor can be somewhat unpredictable. One might assume
that writing new        BigDecimal(0.1) in Java creates a BigDecimal which is exactly equal
to 0.1 (an unscaled value of 1, with a scale of 1), but it is actually equal to 0.1000000000000000055511151231257827021181583404541015625.
This is because 0.1 cannot be represented exactly as a double (or, for that matter, as a binary
fraction of any finite length). Thus, the value that is being passed in to the constructor
is not exactly equal to 0.1, appearances notwithstanding.
>       2. The String constructor, on the other hand, is perfectly predictable: writing
new BigDecimal("0.1") creates a BigDecimal which is exactly equal to 0.1, as one would expect.
Therefore, it is generally recommended that the String constructor be used in preference to
this one.
>       3. When a double must be used as a source for a BigDecimal, note that this constructor
provides an exact conversion; it does not give the same result as converting the double to
a String using the Double.toString(double) method and then using the BigDecimal(String) constructor.
To get that result, use the static valueOf(double) method. 
>    Parameters:
>        val - double value to be converted to BigDecimal. 
>    Throws:
>        NumberFormatException - if val is infinite or NaN.
>>From the *Notes* section it is clear that double values cannto be stored precisely
as the value that is represented since it might not be possible to represent it as a fraction
of finite length, hence the behaviour here.
>So it suggested we construct the BigDecimal value by using Double.toString() method and
passing the value to BigDecimal constructor to build the exact value.
>>Derby incorrectly rounds double values down during insert into NUMERIC
>>         Key: DERBY-123
>>         URL: http://issues.apache.org/jira/browse/DERBY-123
>>     Project: Derby
>>        Type: Bug
>>  Components: SQL
>>    Versions:
>> Environment: Linux JDK 1.4.2
>>    Reporter: Geoff Soutter
>>When inserting a double value into a field defined as NUMERIC(a,b) using PreparedStatement.setDouble(),
Derby may incorrectly round the number down. 
>>For example, a NUMERIC(5,2) field with a double = 4.64 ends up with a value of 4.63
in the database. This works fine in Oracle and other databases.
>>The problem occurs because double cannot represent 4.64 exactly, so the actual value
is 4.6399999... SQLDecimal.setCoreValue uses BigDecimal(double) which constructs a BigDecimal
of 4.6399999... and then SQLDecimal.setWidth uses value.setScale(desiredScale, BigDecimal.ROUND_DOWN);
Note that BigDecimal javadoc recommends that the double constructor be avoided for this reason.
>>One fix appears to be to change SQLDecimal.setCoreValue(double) to avoid using the
double constructor of BigDecimal. Using Double.toString() and BigDecimal(String) looks as
if it would work as expected, because Double.toString() has "special rounding abilities" and
converts 4.639999... back to 4.64.

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