Java sort list containing list on index value of list?

Question

I have a nested list like below (but it has 1,000's of the holder lists within the one main list). Say I need to sort the main list listEmailData by the value for each of its holder lists on the holder.get(2) index. I can't seem to figure out how to do this any advice is appreciated.

ArrayList listEmailData;

ArrayList holder = new ArrayList();

listEmailData.add(3)
listEmailData.add(323)
listEmailData.add(2342)

listEmailData.add(holder)

EDIT: To clarify, I have a list where each list entry contains a sub-list, within this sub-list a specific index contains a value that is a ranking. I need to sort the main list based on this ranking value within each sub-list.

2ND EDIT: Thanks for the help on this, got it working but its seems that its putting larger numbers first and large numbers later, I was hoping to reverse this so it goes from largest to smallest as I am

Jon Skeet · Accepted Answer · 2011-03-17 07:56:29Z

6

You should implement Comparator<T> to compare lists, then call

Collections.sort(listEmailData, comparator);

Your comparator would have to compare any two "sublists" - e.g. by fetching a particular value. For example:

public class ListComparator implements Comparator<List<Integer>>
{
    private final int indexToCompare;

    public ListComparator(int indexToCompare)
    {
        this.indexToCompare = indexToCompare;
    }

    public int compare(List<Integer> first, List<Integer> second)
    {
        // TODO: null checking
        Integer firstValue = first.get(indexToCompare);
        Integer secondValue = second.get(indexToCompare);
        return firstValue.compareTo(secondValue);
    }
}

Note that this is using generics - hopefully your real code is too.

edited Mar 17, 2011 at 7:56

answered Mar 17, 2011 at 7:05

Jon Skeet

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14 Comments

Rick Over a year ago

sorry if I'm missing something obvious but I am not quite sure how to make this compare the thousands of entries in a list and then sort the list accordingly, I can see that calling compare() would sort between 2 of the list inputs

Jon Skeet Over a year ago

@Rick: The comparator compares any two lists. You use Collections.sort on the list of lists using this comparator, as per the first part of the answer.

Nico Huysamen Over a year ago

@Rick - You would not call compare() directly, that is handled by Collections.sort(listEmailData, new ListComparator()) for you.

Rick Over a year ago

ok thanks, I think the c in Collections needs to be capitalized, I'm fairly ignorant of collections in Java so that threw me off at first I think as I thought it was a regular variable object name

Jon Skeet Over a year ago

@Rick: Sorry, yes, that was a typo. Oops.

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