Question

I have a arraylist of a POJO and the data in it is of the form

id    time
2     467
3     403
4     602
3     529
5     398

The requirement is that first I need to sort the data by time and then after that the same IDs should be one after the other i.e

id     time
5      398
3      403
3      529
2      467
4      602.

Initially to sort by time , I’m using the following logic

Collections.sort(list, new Comparator<Asset>() {
                    @Override
                    public int compare(Asset o1, Asset o2) {

                        if (o1.getTime() > o2.getTime())

                            return -1;

                        else if (o1.getTime() < o2.getTime())

                            return 1;

                        else

                            return 0;

                    }

                });

Could some one help me in clubbing by IDs in the next stage?

Answer 1

To sort the data according to the example you gave, you probably need two passes over the list. (How else would you determine whether or not 3 504 should come before or after 5 315?)

1. Sort according to time.
2. Sort the list according to the first index of each id.

Here’s some sample code:

import java.util.*;

class Asset {
    public int id;
    public int time;

    public Asset(int id, int time) {
        this.id = id;
        this.time = time;
    }

    public String toString() {
        return id + "  " + time;
    }
}


class Test {
    public static void main(String[] args) {

        List<Asset> assets = new ArrayList<Asset>();
        assets.add(new Asset(2, 467));
        assets.add(new Asset(3, 403));
        assets.add(new Asset(4, 602));
        assets.add(new Asset(3, 529));
        assets.add(new Asset(5, 398));

        // Sort according to time.
        Collections.sort(assets, new Comparator<Asset>() {
            @Override
            public int compare(Asset o1, Asset o2) {
                return new Integer(o1.time).compareTo(o2.time);
            }
        });

        // Remember the original indexes of each asset.
        final List<Asset> assetsCopy = new ArrayList<Asset>(assets);

        // Sort the collection based on the index of the first asset
        // with the same id
        Collections.sort(assets, new Comparator<Asset>() {

            private int firstIndexOf(int id) {
                for (int i = 0; i < assetsCopy.size(); i++)
                    if (assetsCopy.get(i).id == id)
                        return i;
                return -1;
            }

            @Override
            public int compare(Asset o1, Asset o2) {
                return new Integer(firstIndexOf(o1.id))
                        .compareTo(firstIndexOf(o2.id));
            }
        });


        for (Asset a : assets)
            System.out.println(a);
    }
}

Output:

5  398
3  403
3  529
2  467
4  602