Question

I’ve been playing around with ArrayLists. What I’m trying to achieve is a method to do something like this:

Item 1
Item 2
Item 3
Item 4

I’m trying to be able to move items up in the list, unless it is already at the top in which case it will stay the same. For example, if item 3 was moved the list would be:

Item 1
Item 3
Item 2
Item 4

From my small understanding at the moment then I would want something along the lines of:

IF arrayname index is not equal to 0
THEN move up
ELSE do nothing

The part I’m struggling with is the “move up” part. Any tips or code samples of how this could be achieved are much appreciated.

Answer 1

I came across this old question in my search for an answer, and I thought I would just post the solution I found in case someone else passes by here looking for the same.

For swapping 2 elements, Collections.swap is fine. But if we want to move more elements, there is a better solution that involves a creative use of Collections.sublist and Collections.rotate that I hadn’t thought of until I saw it described here:

http://docs.oracle.com/javase/6/docs/api/java/util/Collections.html#rotate%28java.util.List,%20int%29

Here’s a quote, but go there and read the whole thing for yourself too:

Note that this method can usefully be applied to sublists to move one
or more elements within a list while preserving the order of the
remaining elements. For example, the following idiom moves the element
at index j forward to position k (which must be greater than or equal
to j):

Collections.rotate(list.subList(j, k+1), -1);