Collections.sort(arraylist)

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Bruce Lee - 07 Dec 2005 08:25 GMT

If I use Collections.sort(arraylist), the elements in the arraylist will be
sorted by natural order. What happens when the arraylist is actually an
arraylist of arraylists - how can you sort against an object?

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Thomas Weidenfeller - 07 Dec 2005 09:12 GMT

> If I use Collections.sort(arraylist), the elements in the arraylist will be
> sorted by natural order. What happens when the arraylist is actually an
> arraylist of arraylists - how can you sort against an object?

"natural order" requires that the to-be-sorted elements in the List have
the Comparable interface implemented. If you use ArrayLists as elements
in the List you are out of luck, since ArrayList does not implement the
interface. Prepare for getting a ClassCastException.

/Thomas

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Roedy Green - 07 Dec 2005 09:46 GMT

On Wed, 07 Dec 2005 10:12:29 +0100, Thomas Weidenfeller
<nobody@ericsson.invalid> wrote, quoted or indirectly quoted someone
who said :

>If you use ArrayLists as elements
>in the List you are out of luck, since ArrayList does not implement the
>interface. Prepare for getting a ClassCastException.

you would have to extend your ArrayList and have it implement
Comparable and write some Comparators.

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http://mindprod.com Java custom programming, consulting and coaching.

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Viator - 07 Dec 2005 09:46 GMT

Or extend ArrayList to impelemnt Comparable interface to get the
desired behavior.

Amit :-)

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Thomas Hawtin - 07 Dec 2005 17:58 GMT

> "natural order" requires that the to-be-sorted elements in the List have
> the Comparable interface implemented. If you use ArrayLists as elements
> in the List you are out of luck, since ArrayList does not implement the
> interface. Prepare for getting a ClassCastException.

Or compilation errors in current Java.

Tom Hawtin

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Roedy Green - 07 Dec 2005 09:45 GMT

On Wed, 07 Dec 2005 08:25:18 GMT, "Bruce Lee"
<blah@blahbllbllahblah.com> wrote, quoted or indirectly quoted someone
who said :

>If I use Collections.sort(arraylist), the elements in the arraylist will be
>sorted by natural order. What happens when the arraylist is actually an
>arraylist of arraylists - how can you sort against an object?

you need to write a Comparator that compares entire ArrayLists in some
way e..g by number of elements.

see http://mindprod.com/jgloss/comparator.html

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Canadian Mind Products, Roedy Green.
http://mindprod.com Java custom programming, consulting and coaching.

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Bruce Lee - 07 Dec 2005 10:08 GMT

thanks a lot

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Collections.sort(arraylist)

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