Thread: Retrieving arrays

Retrieving arrays

From

Bendik Rognlien Johansen

Date:

10 October 2006, 13:15:33

Hello,
I wrote this method to read arrays from my result set.


public class MyClass {
   public <T> T[] getArray(ResultSet rs, String column) throws
Exception {
     if(rs.getArray(column) != null) {
       return (T[]) rs.getArray(column).getArray();
     }
     return null;
   }
}



I call i like this:

String[] values = MyClass.<String>getArray(rs, "myStringArrayColumn"));

  This works fine. ( "myStringArrayColumn" is of type character
varying(64)[])


But, when I call:

Integer[] values = MyClass.<Integer>getArray(rs,
"myIntegerArrayColumn"));
( "myIntegerArrayColumn" is of type integer[])


I get a:
java.lang.ClassCastException: [I


Any ideas?

Thanks

Re: Retrieving arrays

From

Mark Lewis

Date:

10 October 2006, 18:00:42

Java 5 doesn't support auto-boxing an int[] as an Integer[].

-- Mark Lewis

On Tue, 2006-10-10 at 15:15 +0200, Bendik Rognlien Johansen wrote:
> Hello,
> I wrote this method to read arrays from my result set.
>
>
> public class MyClass {
>    public <T> T[] getArray(ResultSet rs, String column) throws
> Exception {
>      if(rs.getArray(column) != null) {
>        return (T[]) rs.getArray(column).getArray();
>      }
>      return null;
>    }
> }
>
>
>
> I call i like this:
>
> String[] values = MyClass.<String>getArray(rs, "myStringArrayColumn"));
>
>   This works fine. ( "myStringArrayColumn" is of type character
> varying(64)[])
>
>
> But, when I call:
>
> Integer[] values = MyClass.<Integer>getArray(rs,
> "myIntegerArrayColumn"));
> ( "myIntegerArrayColumn" is of type integer[])
>
>
> I get a:
> java.lang.ClassCastException: [I
>
>
> Any ideas?
>
> Thanks
>
> ---------------------------(end of broadcast)---------------------------
> TIP 4: Have you searched our list archives?
>
>                http://archives.postgresql.org

Re: Retrieving arrays

From

Bendik Rognlien Johansen

Date:

11 October 2006, 11:54:23

Ok, thanks! So, that probably means I can not do this sort of thing
with Generics?

Is there any way the driver could get the same behavior as the driver
used in PL/Java? In PL/Java i can do:
return (String[]) rs.getObject(column);

On Oct 10, 2006, at 7:59 PM, Mark Lewis wrote:

> Java 5 doesn't support auto-boxing an int[] as an Integer[].
>
> -- Mark Lewis
>
> On Tue, 2006-10-10 at 15:15 +0200, Bendik Rognlien Johansen wrote:
>> Hello,
>> I wrote this method to read arrays from my result set.
>>
>>
>> public class MyClass {
>>    public <T> T[] getArray(ResultSet rs, String column) throws
>> Exception {
>>      if(rs.getArray(column) != null) {
>>        return (T[]) rs.getArray(column).getArray();
>>      }
>>      return null;
>>    }
>> }
>>
>>
>>
>> I call i like this:
>>
>> String[] values = MyClass.<String>getArray(rs,
>> "myStringArrayColumn"));
>>
>>   This works fine. ( "myStringArrayColumn" is of type character
>> varying(64)[])
>>
>>
>> But, when I call:
>>
>> Integer[] values = MyClass.<Integer>getArray(rs,
>> "myIntegerArrayColumn"));
>> ( "myIntegerArrayColumn" is of type integer[])
>>
>>
>> I get a:
>> java.lang.ClassCastException: [I
>>
>>
>> Any ideas?
>>
>> Thanks
>>
>> ---------------------------(end of
>> broadcast)---------------------------
>> TIP 4: Have you searched our list archives?
>>
>>                http://archives.postgresql.org
>
> ---------------------------(end of
> broadcast)---------------------------
> TIP 3: Have you checked our extensive FAQ?
>
>                http://www.postgresql.org/docs/faq

Re: Retrieving arrays

From

Markus Schaber

Date:

11 October 2006, 12:42:33

Hi, bendik,

Bendik Rognlien Johansen wrote:

> I wrote this method to read arrays from my result set.
>
> public class MyClass {
>   public <T> T[] getArray(ResultSet rs, String column) throws Exception {
>     if(rs.getArray(column) != null) {
>       return (T[]) rs.getArray(column).getArray();
>     }
>     return null;
>   }
> }

It seems that you misunderstand the concepts of Generics.

Your method will call the getArray() method, and then try to cast (not
convert) whatever this method returns into an T[]. And, due to erasure,
this cast is not even done in MyClass.getArray(), but in the code that
calls MyClass.getArray().

So getArray() does not even know about T, it just returns what its
mapping of the PostgreSQL types to java tells it to.

http://www.angelikalanger.com/GenericsFAQ/JavaGenericsFAQ.html should be
a good reading, and in case of any doubts,
http://java.sun.com/docs/books/jls/index.html

> I call i like this:
>
> String[] values = MyClass.<String>getArray(rs, "myStringArrayColumn"));
>
>  This works fine. ( "myStringArrayColumn" is of type character
> varying(64)[])

Yes, it works fine. ResultSet.getArray maps the varchar[] to a String[],
and then your 'String[] values=' assignment casts that to String[],
which works fine.

> But, when I call:
>
> Integer[] values = MyClass.<Integer>getArray(rs, "myIntegerArrayColumn"));
> ( "myIntegerArrayColumn" is of type integer[])
>
> I get a:
> java.lang.ClassCastException: [I

This one fails. ResultSet.getArray maps the integer[] to a int[], not to
an Integer[]. And so, the cast will fail.

HTH,
Markus

--
Markus Schaber | Logical Tracking&Tracing International AG
Dipl. Inf.     | Software Development GIS

Fight against software patents in Europe! www.ffii.org
www.nosoftwarepatents.org

Re: Retrieving arrays

From

Bendik Rognlien Johansen

Date:

11 October 2006, 13:39:56

Hello,
yes you are right, I don't really understand Generics that well. But
it is a little clearer now :-)

Anyway, I was hoping there would be a clean way to get arrays from
the result set, Generics or not, using a single method. I would like
to avoid:
getStringArray, getIntegerArray etc.

The reason I need to do this is that I use two different drivers for
PostgreSQL with the same code, and they handle arrays differently.

Thanks!


On Oct 11, 2006, at 2:42 PM, Markus Schaber wrote:

> Hi, bendik,
>
> Bendik Rognlien Johansen wrote:
>
>> I wrote this method to read arrays from my result set.
>>
>> public class MyClass {
>>   public <T> T[] getArray(ResultSet rs, String column) throws
>> Exception {
>>     if(rs.getArray(column) != null) {
>>       return (T[]) rs.getArray(column).getArray();
>>     }
>>     return null;
>>   }
>> }
>
> It seems that you misunderstand the concepts of Generics.
>
> Your method will call the getArray() method, and then try to cast (not
> convert) whatever this method returns into an T[]. And, due to
> erasure,
> this cast is not even done in MyClass.getArray(), but in the code that
> calls MyClass.getArray().
>
> So getArray() does not even know about T, it just returns what its
> mapping of the PostgreSQL types to java tells it to.
>
> http://www.angelikalanger.com/GenericsFAQ/JavaGenericsFAQ.html
> should be
> a good reading, and in case of any doubts,
> http://java.sun.com/docs/books/jls/index.html
>
>> I call i like this:
>>
>> String[] values = MyClass.<String>getArray(rs,
>> "myStringArrayColumn"));
>>
>>  This works fine. ( "myStringArrayColumn" is of type character
>> varying(64)[])
>
> Yes, it works fine. ResultSet.getArray maps the varchar[] to a
> String[],
> and then your 'String[] values=' assignment casts that to String[],
> which works fine.
>
>> But, when I call:
>>
>> Integer[] values = MyClass.<Integer>getArray(rs,
>> "myIntegerArrayColumn"));
>> ( "myIntegerArrayColumn" is of type integer[])
>>
>> I get a:
>> java.lang.ClassCastException: [I
>
> This one fails. ResultSet.getArray maps the integer[] to a int[],
> not to
> an Integer[]. And so, the cast will fail.
>
>
> HTH,
> Markus
>
>
> --
> Markus Schaber | Logical Tracking&Tracing International AG
> Dipl. Inf.     | Software Development GIS
>
> Fight against software patents in Europe! www.ffii.org
> www.nosoftwarepatents.org

Re: Retrieving arrays

From

Markus Schaber

Date:

11 October 2006, 13:43:18

Hi, Bendik,

Bendik Rognlien Johansen wrote:

> yes you are right, I don't really understand Generics that well. But it
> is a little clearer now :-)
>
> Anyway, I was hoping there would be a clean way to get arrays from the
> result set, Generics or not, using a single method. I would like to avoid:
> getStringArray, getIntegerArray etc.

Usually, ResultSet.getArray() should do it.

> The reason I need to do this is that I use two different drivers for
> PostgreSQL with the same code, and they handle arrays differently.

So you say that the the other one (pljava, IIRC) maps a postgresql
integer[] to an java.lang.Integer[] in java, and not an int[] like pgjdbc?

Looks strange.

Does the jdbc spec promote one or the other behaviour?

However, I don't know a clean solution.

Markus
--
Markus Schaber | Logical Tracking&Tracing International AG
Dipl. Inf.     | Software Development GIS

Fight against software patents in Europe! www.ffii.org
www.nosoftwarepatents.org

Re: Retrieving arrays

From

Dave Cramer

Date:

11 October 2006, 16:33:24

On 11-Oct-06, at 9:43 AM, Markus Schaber wrote:

> Hi, Bendik,
>
> Bendik Rognlien Johansen wrote:
>
>> yes you are right, I don't really understand Generics that well.
>> But it
>> is a little clearer now :-)
>>
>> Anyway, I was hoping there would be a clean way to get arrays from
>> the
>> result set, Generics or not, using a single method. I would like
>> to avoid:
>> getStringArray, getIntegerArray etc.
>
> Usually, ResultSet.getArray() should do it.
>
>> The reason I need to do this is that I use two different drivers for
>> PostgreSQL with the same code, and they handle arrays differently.
>
> So you say that the the other one (pljava, IIRC) maps a postgresql
> integer[] to an java.lang.Integer[] in java, and not an int[] like
> pgjdbc?
>
> Looks strange.
>
> Does the jdbc spec promote one or the other behaviour?
Yes, the spec, and the desire to not create a huge number of objects
does promote one method over another

pp 285 from JDBC API Tutorial and Reference 3rd Edition states

"The data type for the elements is determined by the standard mapping
from JDBC Types to Objects ... There is one very important exception
to this rule. If the base type of the array maps to a primitive type
in the Java programming language, the base type of the returned array
will be the primitive type rather than the Object type"

So I'd suggest you file a bug with pl/java

Dave
>
> However, I don't know a clean solution.
>
> Markus
> --
> Markus Schaber | Logical Tracking&Tracing International AG
> Dipl. Inf.     | Software Development GIS
>
> Fight against software patents in Europe! www.ffii.org
> www.nosoftwarepatents.org
>
> ---------------------------(end of
> broadcast)---------------------------
> TIP 3: Have you checked our extensive FAQ?
>
>                http://www.postgresql.org/docs/faq
>

Re: Retrieving arrays

From

Bendik Rognlien Johansen

Date:

13 October 2006, 08:59:03

Hello,

In PL/Java:
String[] values = (String[]) rs.getObject(column);

Regular driver:
String[] values = (String[]) rs.getArray(column).getArray();

Strange..

On Oct 11, 2006, at 3:43 PM, Markus Schaber wrote:

> Hi, Bendik,
>
> Bendik Rognlien Johansen wrote:
>
>> yes you are right, I don't really understand Generics that well.
>> But it
>> is a little clearer now :-)
>>
>> Anyway, I was hoping there would be a clean way to get arrays from
>> the
>> result set, Generics or not, using a single method. I would like
>> to avoid:
>> getStringArray, getIntegerArray etc.
>
> Usually, ResultSet.getArray() should do it.
>
>> The reason I need to do this is that I use two different drivers for
>> PostgreSQL with the same code, and they handle arrays differently.
>
> So you say that the the other one (pljava, IIRC) maps a postgresql
> integer[] to an java.lang.Integer[] in java, and not an int[] like
> pgjdbc?
>
> Looks strange.
>
> Does the jdbc spec promote one or the other behaviour?
>
> However, I don't know a clean solution.
>
> Markus
> --
> Markus Schaber | Logical Tracking&Tracing International AG
> Dipl. Inf.     | Software Development GIS
>
> Fight against software patents in Europe! www.ffii.org
> www.nosoftwarepatents.org

Re: Retrieving arrays

From

Markus Schaber

Date:

13 October 2006, 09:29:33

Hi, Bendik,

Bendik Rognlien Johansen wrote:

> In PL/Java:
> String[] values = (String[]) rs.getObject(column);

What does getArray() return here?

> Regular driver:
> String[] values = (String[]) rs.getArray(column).getArray();

And what does getObject return here?

Thanks,
Markus
--
Markus Schaber | Logical Tracking&Tracing International AG
Dipl. Inf.     | Software Development GIS

Fight against software patents in Europe! www.ffii.org
www.nosoftwarepatents.org

Re: Retrieving arrays

From

Dave Cramer

Date:

13 October 2006, 10:35:47

Patches are welcome

I haven't looked at the spec, but I also think this is outside of the
spec, but we have discussed it and feel it is intuitive enough to
accept.

Dave
On 13-Oct-06, at 4:58 AM, Bendik Rognlien Johansen wrote:

> Hello,
>
> In PL/Java:
> String[] values = (String[]) rs.getObject(column);
>
> Regular driver:
> String[] values = (String[]) rs.getArray(column).getArray();
>
> Strange..
>
> On Oct 11, 2006, at 3:43 PM, Markus Schaber wrote:
>
>> Hi, Bendik,
>>
>> Bendik Rognlien Johansen wrote:
>>
>>> yes you are right, I don't really understand Generics that well.
>>> But it
>>> is a little clearer now :-)
>>>
>>> Anyway, I was hoping there would be a clean way to get arrays
>>> from the
>>> result set, Generics or not, using a single method. I would like
>>> to avoid:
>>> getStringArray, getIntegerArray etc.
>>
>> Usually, ResultSet.getArray() should do it.
>>
>>> The reason I need to do this is that I use two different drivers for
>>> PostgreSQL with the same code, and they handle arrays differently.
>>
>> So you say that the the other one (pljava, IIRC) maps a postgresql
>> integer[] to an java.lang.Integer[] in java, and not an int[] like
>> pgjdbc?
>>
>> Looks strange.
>>
>> Does the jdbc spec promote one or the other behaviour?
>>
>> However, I don't know a clean solution.
>>
>> Markus
>> --
>> Markus Schaber | Logical Tracking&Tracing International AG
>> Dipl. Inf.     | Software Development GIS
>>
>> Fight against software patents in Europe! www.ffii.org
>> www.nosoftwarepatents.org
>
>
> ---------------------------(end of
> broadcast)---------------------------
> TIP 5: don't forget to increase your free space map settings
>

Re: Retrieving arrays

From

Bendik Rognlien Johansen

Date:

26 October 2006, 15:31:06

Hello again. I have been away for a while.

Column "stringarray" is VARCHAR[]
Column "integerarray" is INTEGER[]

System.out.println("getObject: " + rs.getObject
("integerarray").getClass());
System.out.println("getObject: " + rs.getObject
("stringarray").getClass());

System.out.println("getArray: " + rs.getArray("integerarray").getClass
());
System.out.println("getArray: " + rs.getArray("stringarray").getClass
());

Regular driver:

getObject: class org.postgresql.jdbc3.Jdbc3Array
getObject: class org.postgresql.jdbc3.Jdbc3Array
getArray: class org.postgresql.jdbc3.Jdbc3Array
getArray: class org.postgresql.jdbc3.Jdbc3Array


PL/Java
getObject: class [Ljava.lang.Integer;
getObject: class [Ljava.lang.String;
ERROR:  java.sql.SQLException: Cannot derive a value of class
java.sql.Array from an object of class [Ljava.lang.Integer;
ERROR:  java.sql.SQLException: Cannot derive a value of class
java.sql.Array from an object of class [Ljava.lang.String;

Thanks!


On Oct 13, 2006, at 11:29 AM, Markus Schaber wrote:

> Hi, Bendik,
>
> Bendik Rognlien Johansen wrote:
>
>> In PL/Java:
>> String[] values = (String[]) rs.getObject(column);
>
> What does getArray() return here?



>
>> Regular driver:
>> String[] values = (String[]) rs.getArray(column).getArray();
>
> And what does getObject return here?
>
> Thanks,
> Markus
> --
> Markus Schaber | Logical Tracking&Tracing International AG
> Dipl. Inf.     | Software Development GIS
>
> Fight against software patents in Europe! www.ffii.org
> www.nosoftwarepatents.org