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.
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Answer
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:
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);