Java Sorting Object According to Attribute

Question

I was having some problem when trying to sort array list of objects. Basically I have populated an object list and the sample data:

name: UNIFORMITY
date: Tue Oct 30 02:48:32 GMT+00:00 2018
name: ROI
date: Wed Oct 31 02:48:32 GMT+00:00 2018
name: UNIFORMITY
date: Wed Oct 31 02:48:32 GMT+00:00 2018

I populated the array list with the code below:

CalibrationHistoryEntity object3 = new CalibrationHistoryEntity();
    object3.setCalibrationName("ROI");
    try {
        object3.setCalibrationDate(dt.parse("Oct 31 02:48:32 GMT+00:00 2018"));
    } catch (ParseException e) {
        e.printStackTrace();
    }
    allCalibrationList.add(object3);

Then, what I wanted to do is to sort the array list such that for the object with same name, I only get the largest date and put it back to the array but I have no ideas how to do it. Any ideas?

My desired output will be like this:

name: ROI
date: Wed Oct 31 02:48:32 GMT+00:00 2018
name: UNIFORMITY
date: Wed Oct 31 02:48:32 GMT+00:00 2018

Thanks!

"Any ideas on how to do it?" Yes, but more importantly, do you have any ideas? And if so, how did it not work to your satisfaction. — Dexygen
– Dexygen, Commented Oct 29, 2018 at 5:54
What is CalibrationHistoryEntity ? can you provide the class. — SamHoque
– SamHoque, Commented Oct 29, 2018 at 6:17
@Rab It is an object class with CalibrationName and CalibrationDate as attributes! — dummygg
– dummygg, Commented Oct 29, 2018 at 6:19
Yes, I need the class, I want to know what Object they are. Provide the class and I will fiddle around with the code. — SamHoque
– SamHoque, Commented Oct 29, 2018 at 6:20

Sodala · Accepted Answer · 2018-10-29 07:22:33Z

1

A simple way to achieve this is :

grouping by name
for each groups get the maximum date value.
add each maximum to the new list

For example with stream

 final Map<String, List<CalibrationHistoryEntity>> groupingByName= allCalibrationList.stream()
            .collect(Collectors.groupingBy(CalibrationHistoryEntity::getCalibrationName));
 final List<CalibrationHistoryEntity> list = groupingByName.values()
            .stream().map(it -> Collections.max(it, Comparator.comparing(CalibrationHistoryEntity::getCalibrationDate)))
            .collect(Collectors.toList());

answered Oct 29, 2018 at 7:22

Sodala

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Comments

Samer Aamar · Accepted Answer · 2018-10-29 10:33:12Z

The following simple block will give you what you need. It is also efficient because it is O(n)

    Map<String, CalibrationHistoryEntity> unique = new HashMap<String, CalibrationHistoryEntity>();

    for (CalibrationHistoryEntity k : allCalibrationList) {
        CalibrationHistoryEntity temp = unique.getOrDefault(k.Name, null);

        if (temp == null || temp.time.before(k.time))
            unique.put(k.Name, k);
    }

In the following step we pull the results ... i am doing some sort by name as well (optional)

    // Optional but you can sort by name :
    Stream<CalibrationHistoryEntity> result = unique.values().stream()
            .sorted((a, b) -> {
                return a.Name.compareTo(b.Name);
            });

Now let's print results (or use as you wish):

    // Now you can print
    result.forEach(k -> {
        System.out.println("Name: " + k.Name + ", Time: " + k.time);
    });

SamHoque · Accepted Answer · 2018-10-29 06:49:35Z

Okay, I have done writing the method for you to sort the List here it is

    ListIterator<CalibrationHistoryEntity> listIterator = allCalibrationList.listIterator();
    while (listIterator.hasNext()) {
        CalibrationHistoryEntity calibrationHistoryEntity = listIterator.next();
        //Get all the index
        List<Integer> indexes = findAllByName(allCalibrationList, calibrationHistoryEntity.getCalibrationName());
        if (indexes.size() > 1) {
            List<CalibrationHistoryEntity> calibrationHistoryEntities = new ArrayList<>();
            for (int index : indexes) {
                calibrationHistoryEntities.add(allCalibrationList.get(index));
            }
            //Sort by biggest time
            calibrationHistoryEntities.sort(Comparator.comparing(CalibrationHistoryEntity::getCalibrationDate).reversed());
            for (int i = 1; i < calibrationHistoryEntities.size(); i++) {
                //Remove others
                listIterator.remove();
            }
        }
    }

and the method to get all the index

/**
 * gets all the index via the name
 */
public static List<Integer> findAllByName(List<CalibrationHistoryEntity> allCalibrationList, String name) {
    List<Integer> indexes = new ArrayList<>();
    for (CalibrationHistoryEntity calibrationHistoryEntity : allCalibrationList) {
        if (calibrationHistoryEntity.getCalibrationName().equals(name)) {
            indexes.add(allCalibrationList.indexOf(calibrationHistoryEntity));
        }
    }
    return indexes;
}

and I made the input via

    String[] names = {"UNIFORMITY", "Roi", "UNIFORMITY"};
    SimpleDateFormat dt = new SimpleDateFormat("MMM dd hh:mm:ss zzz yyyy");
    Date[] dates = new Date[0];
    try {
        dates = new Date[]{dt.parse("Oct 31 02:48:32 GMT+00:00 2018"),
                dt.parse("Oct 31 02:48:25 GMT+00:00 2018"),
                dt.parse("Oct 31 02:48:37 GMT+00:00 2018")};
    } catch (ParseException e) {
        e.printStackTrace();
    }
    List<CalibrationHistoryEntity> allCalibrationList = new ArrayList<>();
    for (int i = 0; i < names.length; i++) {
        String name = names[i];
        Date date = dates[i];
        CalibrationHistoryEntity calibrationHistoryEntity = new CalibrationHistoryEntity();
        calibrationHistoryEntity.setCalibrationName(name);
        calibrationHistoryEntity.setCalibrationDate(date);
        allCalibrationList.add(calibrationHistoryEntity);
    }

This is the output

Name: Roi Time:Wed Oct 31 05:48:25 GMT+03:00 2018
Name: UNIFORMITY Time:Wed Oct 31 05:48:37 GMT+03:00 2018

this is not good from performance point of view. you can give a solution with O(n) instead

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