Jython, Java, and Iterators...

[Cliff Hill-3]

I'm attempting to make a Java class that emulates a Jython list, with some special conditions to it.

I have a java class called an Array, which contains a collection of another Java class called an Element.

I then have a Jython script set up so I can do the following:

arr[pos]

where arr is the Java Array object, and pos is the index in that Array object, the returned value will be the Element object.

Now, to my problem... I am unable to get an iterator to work correctly. What I mean is this code:

    for val in arr:
        count[val] += 1

Doesn't work. I get the following:

java.lang.NoSuchElementException()

Mind you, this method does work:

    for pos in range(len(arr)):
        count[arr[pos]] += 1

But the former is far cleaner than the latter, especially given Python's for syntax.

I took a look at the Jython 2.5.1 javadoc, which says to set up the __iternext__() method for my iterator, and I looked at the Python 2.5 documentation to see that there should be a next() method defined. In both cases, a null should be sent if there is no more elements to iterate over.

My iterator looks like this:

    /**
     * A forward-facing iterator for the Array.
     *
     * @author darkhelm
     */
    public class Iter implements Iterator<Element> {
        /**
         * The actual iterator, able to go forward or backward.
         */
        private ListIterator<Element> iter = arr.listIterator();
       
        /**
         * @return the actual iterator that is used.
         */
        protected ListIterator<Element> getIter() {
            return iter;
        }
       
        /* (non-Javadoc)
         * @see java.util.Iterator#hasNext()
         */
        @Override
        public boolean hasNext() {
            return getIter().hasNext();
        }

        /* (non-Javadoc)
         * @see java.util.Iterator#next()
         */
        @Override
        public Element next() {
            try {
                return getIter().next();
            } catch(NoSuchElementException e) {
                return null;
            }
        }

        /**
         * @return the next element from the iterator.
         */
        public Element __iternext__() {
            return next();
        }
       
        /* (non-Javadoc)
         * @see java.util.Iterator#remove()
         */
        @Override
        public void remove() {
            getIter().remove();
        }
    }

This produces the above "NoSuchElementException" exception being thrown when I do the first for loop I mentioned.

If I modify next() method as below:

        public Element next() {
            try {
                return getIter().next();
            } catch(NoSuchElementException e) {
                return null;
            }
        }

And then I run the first for loop, it changes the exception to a java.lang.NullPointerException(). This suggests to me that there is some other way to notify Jython that the iterator has reached the end. Because everything I've seen says that if it returns null, it should stop. Then again, for Java, the NoSuchElementException() exception should also stop the iterator, shouldn't it?

Anyway, I'd appreciate some help or at least a nod in the right direction for me to get this working. Thanks.

--
"I'm not responcabel fer my computer's spleling errnors" - Xlorep DarkHelm
Website: http://darkhelm.org
Sent from Santa Maria, California, United States

------------------------------------------------------------------------------
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[Leo Soto M.]

Hi Cliff,

On Tue, Oct 13, 2009 at 5:57 PM, Cliff Hill <xlorep@...> wrote:

> I'm attempting to make a Java class that emulates a Jython list, with some
> special conditions to it.
>
> I have a java class called an Array, which contains a collection of another
> Java class called an Element.
>
> I then have a Jython script set up so I can do the following:
>
> arr[pos]
>
> where arr is the Java Array object, and pos is the index in that Array
> object, the returned value will be the Element object.
>
> Now, to my problem... I am unable to get an iterator to work correctly. What
> I mean is this code:
>
>     for val in arr:
>         count[val] += 1
>
> Doesn't work. I get the following:
>
> java.lang.NoSuchElementException()
>
> Mind you, this method does work:
>
>     for pos in range(len(arr)):
>         count[arr[pos]] += 1
>
> But the former is far cleaner than the latter, especially given Python's for
> syntax.
>
> I took a look at the Jython 2.5.1 javadoc, which says to set up the
> __iternext__() method for my iterator, and I looked at the Python 2.5
> documentation to see that there should be a next() method defined. In both
> cases, a null should be sent if there is no more elements to iterate over.
>
> My iterator looks like this:
>
>     /**
>      * A forward-facing iterator for the Array.
>      *
>      * @author darkhelm
>      */
>     public class Iter implements Iterator<Element> {
>         /**
>          * The actual iterator, able to go forward or backward.
>          */
>         private ListIterator<Element> iter = arr.listIterator();
>
>         /**
>          * @return the actual iterator that is used.
>          */
>         protected ListIterator<Element> getIter() {
>             return iter;
>         }
>
>         /* (non-Javadoc)
>          * @see java.util.Iterator#hasNext()
>          */
>         @Override
>         public boolean hasNext() {
>             return getIter().hasNext();
>         }
>
>         /* (non-Javadoc)
>          * @see java.util.Iterator#next()
>          */
>         @Override
>         public Element next() {
>             try {
>                 return getIter().next();
>             } catch(NoSuchElementException e) {
>                 return null;
>             }
>         }
>
>         /**
>          * @return the next element from the iterator.
>          */
>         public Element __iternext__() {
>             return next();
>         }
>
>         /* (non-Javadoc)
>          * @see java.util.Iterator#remove()
>          */
>         @Override
>         public void remove() {
>             getIter().remove();
>         }
>     }
>
> This produces the above "NoSuchElementException" exception being thrown when
> I do the first for loop I mentioned.
>
> If I modify next() method as below:
>
>         public Element next() {
>             try {
>                 return getIter().next();
>             } catch(NoSuchElementException e) {
>                 return null;
>             }
>         }

I don't understand this part: the next() method looks the same as
above. What's the change?

> And then I run the first for loop, it changes the exception to a
> java.lang.NullPointerException(). This suggests to me that there is some
> other way to notify Jython that the iterator has reached the end. Because
> everything I've seen says that if it returns null, it should stop. Then
> again, for Java, the NoSuchElementException() exception should also stop the
> iterator, shouldn't it?

Probably the Python for syntax should work out of the box with java
Iterators. Can you please fill a bug in the tracker so it's not
forgotten?

> Anyway, I'd appreciate some help or at least a nod in the right direction
> for me to get this working. Thanks.

Sorry for the late response, but looks like everyone is quite busy these days.

Regards,
--
Leo Soto M.
http://blog.leosoto.com
------------------------------------------------------------------------------
Come build with us! The BlackBerry(R) Developer Conference in SF, CA
is the only developer event you need to attend this year. Jumpstart your
developing skills, take BlackBerry mobile applications to market and stay
ahead of the curve. Join us from November 9 - 12, 2009. Register now!
http://p.sf.net/sfu/devconference
_______________________________________________
Jython-users mailing list
Jython-users@...
https://lists.sourceforge.net/lists/listinfo/jython-users

[Cliff Hill-3]

On Wed, Oct 21, 2009 at 8:21 AM, Leo Soto M. <leo.soto@...> wrote:

Hi Cliff,

On Tue, Oct 13, 2009 at 5:57 PM, Cliff Hill <xlorep@...> wrote:
> I'm attempting to make a Java class that emulates a Jython list, with some
> special conditions to it.
>
> I have a java class called an Array, which contains a collection of another
> Java class called an Element.
>
> I then have a Jython script set up so I can do the following:
>
> arr[pos]
>
> where arr is the Java Array object, and pos is the index in that Array
> object, the returned value will be the Element object.
>
> Now, to my problem... I am unable to get an iterator to work correctly. What
> I mean is this code:
>
>     for val in arr:
>         count[val] += 1
>
> Doesn't work. I get the following:
>
> java.lang.NoSuchElementException()
>
> Mind you, this method does work:
>
>     for pos in range(len(arr)):
>         count[arr[pos]] += 1
>
> But the former is far cleaner than the latter, especially given Python's for
> syntax.
>
> I took a look at the Jython 2.5.1 javadoc, which says to set up the
> __iternext__() method for my iterator, and I looked at the Python 2.5
> documentation to see that there should be a next() method defined. In both
> cases, a null should be sent if there is no more elements to iterate over.
>
> My iterator looks like this:
>
>     /**
>      * A forward-facing iterator for the Array.
>      *
>      * @author darkhelm
>      */
>     public class Iter implements Iterator<Element> {
>         /**
>          * The actual iterator, able to go forward or backward.
>          */
>         private ListIterator<Element> iter = arr.listIterator();
>
>         /**
>          * @return the actual iterator that is used.
>          */
>         protected ListIterator<Element> getIter() {
>             return iter;
>         }
>
>         /* (non-Javadoc)
>          * @see java.util.Iterator#hasNext()
>          */
>         @Override
>         public boolean hasNext() {
>             return getIter().hasNext();
>         }
>
>         /* (non-Javadoc)
>          * @see java.util.Iterator#next()
>          */
>         @Override
>         public Element next() {
>             try {
>                 return getIter().next();
>             } catch(NoSuchElementException e) {
>                 return null;
>             }
>         }
>
>         /**
>          * @return the next element from the iterator.
>          */
>         public Element __iternext__() {
>             return next();
>         }
>
>         /* (non-Javadoc)
>          * @see java.util.Iterator#remove()
>          */
>         @Override
>         public void remove() {
>             getIter().remove();
>         }
>     }
>
> This produces the above "NoSuchElementException" exception being thrown when
> I do the first for loop I mentioned.
>
> If I modify next() method as below:
>
>         public Element next() {
>             try {
>                 return getIter().next();
>             } catch(NoSuchElementException e) {
>                 return null;
>             }
>         }

I don't understand this part: the next() method looks the same as
above. What's the change?

Silly me, I didn't make the correct adjustments:

        public Element next() {
            return getIter().next();
        }

vs

        public Element next() {
            try {
                return getIter().next();
            } catch(NoSuchElementException e) {
                return null;
            }
        }

The former produces "java.lang.NoSuchElementException()" while the latter produces "java.lang.NullPointerException()".
 

> And then I run the first for loop, it changes the exception to a
> java.lang.NullPointerException(). This suggests to me that there is some
> other way to notify Jython that the iterator has reached the end. Because
> everything I've seen says that if it returns null, it should stop. Then
> again, for Java, the NoSuchElementException() exception should also stop the
> iterator, shouldn't it?

Probably the Python for syntax should work out of the box with java
Iterators. Can you please fill a bug in the tracker so it's not
forgotten?

Ok, will do.
 

> Anyway, I'd appreciate some help or at least a nod in the right direction
> for me to get this working. Thanks.

Sorry for the late response, but looks like everyone is quite busy these days.

I understand completely. Been busy myself :P

--
"I'm not responcabel fer my computer's spleling errnors" - Xlorep DarkHelm
Website: http://darkhelm.org
Sent from Santa Maria, California, United States

------------------------------------------------------------------------------
Come build with us! The BlackBerry(R) Developer Conference in SF, CA
is the only developer event you need to attend this year. Jumpstart your
developing skills, take BlackBerry mobile applications to market and stay
ahead of the curve. Join us from November 9 - 12, 2009. Register now!
http://p.sf.net/sfu/devconference
_______________________________________________
Jython-users mailing list
Jython-users@...
https://lists.sourceforge.net/lists/listinfo/jython-users

[Cliff Hill-3]

Issue 1492 just made, hopefully it explains enough.

On Thu, Oct 22, 2009 at 10:05 AM, Cliff Hill <xlorep@...> wrote:

On Wed, Oct 21, 2009 at 8:21 AM, Leo Soto M. <leo.soto@...> wrote:

Hi Cliff,

On Tue, Oct 13, 2009 at 5:57 PM, Cliff Hill <xlorep@...> wrote:
> I'm attempting to make a Java class that emulates a Jython list, with some
> special conditions to it.
>
> I have a java class called an Array, which contains a collection of another
> Java class called an Element.
>
> I then have a Jython script set up so I can do the following:
>
> arr[pos]
>
> where arr is the Java Array object, and pos is the index in that Array
> object, the returned value will be the Element object.
>
> Now, to my problem... I am unable to get an iterator to work correctly. What
> I mean is this code:
>
>     for val in arr:
>         count[val] += 1
>
> Doesn't work. I get the following:
>
> java.lang.NoSuchElementException()
>
> Mind you, this method does work:
>
>     for pos in range(len(arr)):
>         count[arr[pos]] += 1
>
> But the former is far cleaner than the latter, especially given Python's for
> syntax.
>
> I took a look at the Jython 2.5.1 javadoc, which says to set up the
> __iternext__() method for my iterator, and I looked at the Python 2.5
> documentation to see that there should be a next() method defined. In both
> cases, a null should be sent if there is no more elements to iterate over.
>
> My iterator looks like this:
>
>     /**
>      * A forward-facing iterator for the Array.
>      *
>      * @author darkhelm
>      */
>     public class Iter implements Iterator<Element> {
>         /**
>          * The actual iterator, able to go forward or backward.
>          */
>         private ListIterator<Element> iter = arr.listIterator();
>
>         /**
>          * @return the actual iterator that is used.
>          */
>         protected ListIterator<Element> getIter() {
>             return iter;
>         }
>
>         /* (non-Javadoc)
>          * @see java.util.Iterator#hasNext()
>          */
>         @Override
>         public boolean hasNext() {
>             return getIter().hasNext();
>         }
>
>         /* (non-Javadoc)
>          * @see java.util.Iterator#next()
>          */
>         @Override
>         public Element next() {
>             try {
>                 return getIter().next();
>             } catch(NoSuchElementException e) {
>                 return null;
>             }
>         }
>
>         /**
>          * @return the next element from the iterator.
>          */
>         public Element __iternext__() {
>             return next();
>         }
>
>         /* (non-Javadoc)
>          * @see java.util.Iterator#remove()
>          */
>         @Override
>         public void remove() {
>             getIter().remove();
>         }
>     }
>
> This produces the above "NoSuchElementException" exception being thrown when
> I do the first for loop I mentioned.
>
> If I modify next() method as below:
>
>         public Element next() {
>             try {
>                 return getIter().next();
>             } catch(NoSuchElementException e) {
>                 return null;
>             }
>         }

I don't understand this part: the next() method looks the same as
above. What's the change?

Silly me, I didn't make the correct adjustments:

        public Element next() {
            return getIter().next();
        }

vs

        public Element next() {
            try {
                return getIter().next();
            } catch(NoSuchElementException e) {
                return null;
            }
        }

The former produces "java.lang.NoSuchElementException()" while the latter produces "java.lang.NullPointerException()".
 

> And then I run the first for loop, it changes the exception to a
> java.lang.NullPointerException(). This suggests to me that there is some
> other way to notify Jython that the iterator has reached the end. Because
> everything I've seen says that if it returns null, it should stop. Then
> again, for Java, the NoSuchElementException() exception should also stop the
> iterator, shouldn't it?

Probably the Python for syntax should work out of the box with java
Iterators. Can you please fill a bug in the tracker so it's not
forgotten?

Ok, will do.
 

> Anyway, I'd appreciate some help or at least a nod in the right direction
> for me to get this working. Thanks.

Sorry for the late response, but looks like everyone is quite busy these days.

I understand completely. Been busy myself :P

--
"I'm not responcabel fer my computer's spleling errnors" - Xlorep DarkHelm
Website: http://darkhelm.org
Sent from Santa Maria, California, United States

--
"I'm not responcabel fer my computer's spleling errnors" - Xlorep DarkHelm
Website: http://darkhelm.org
Sent from Santa Maria, California, United States

------------------------------------------------------------------------------
Come build with us! The BlackBerry(R) Developer Conference in SF, CA
is the only developer event you need to attend this year. Jumpstart your
developing skills, take BlackBerry mobile applications to market and stay
ahead of the curve. Join us from November 9 - 12, 2009. Register now!
http://p.sf.net/sfu/devconference
_______________________________________________
Jython-users mailing list
Jython-users@...
https://lists.sourceforge.net/lists/listinfo/jython-users