Looping and sorting ojbects in a class

Question

This is a noob question, sorry, but I am totally confused.

I had created a class called CalendarItems.

The fields are:

int index
String description
String textdate
int daysaway

The constructor uses the first three fields. And for the moment I have created four objects within the class.

I now want to do two things:

Loop through the CalendarItem's For each CalendarItem calculate daysaway, which is the difference between the text date and today. I have figured out how to calculate this and can manually add do it by calling my method for item1, then for item2, etc

But i'd like to understand how i can do this with a loop. If i was in javascript i'd have an multi-dimensional array and could easy create a loop to cycle through it, but in the scary new world of java I am lost

Sort my CalendarItems on the daysaway field I want to do this so that I can display them as CardViews in order of soonest-first.

[Inflating CardViews has proved blissfully straight-forward, and I can (again, with a manual 'loop') inflate a card for each CalendarItem and add the various fields to it. And creating the class and fields and adding my 4 sample objects has also worked, so it hasn't been a total lost weekend.]

I think where I am going wrong is that I keep thinking in terms of multi-dimensional arrays from my experience with javascript and Excel VBA, and however much I read about objects and iterator interfaces and arraylists in java, my brain is just refusing to take it in.

I have Google'd a lot on this, but every 'simple guide to iterators' has left me more confused than when I started.

Thanks in advance to anyone patient enough to help me.

Instead of describing your code, could you show what you currently have and some sort of demonstration of what you are wanting for your result? — Zannith, Jun 04 '18 at 19:37

geco17 · Answer 1 · 2018-06-04T20:16:47.297

For (1), what have you tried? If you need a multi-dimensional array, nothing stops you from using one. Consider:

int dim1 = 5;
int dim2 = 10; // whatever you need
CalendarItem[][] items = new CalendarItem[dim1][dim2];

Im this case you would iterate your array the same way as with JavaScript.

for (int i = 0; i < dim1; i++) {
     for (int j = 0; j < dim2; j++) {
         // do whatever you need to do
     }
}

You can also read about the enhanced for loop if you don't actually need a numerical index.

For (2) have a look at the Comparator interface in the javadocs as well as the methods in the Collections class, in particolar the sort method.

score 1 · Answer 2 · answered Jun 04 '18 at 19:53

For 1) you can either create array in java or you can use ArrayList and store your objects in that arraylist and iterate over arraylist.

ArrayList<CalendarItem> list = new ArrayList();
list.add(obj1); // obj1 is the instance you created for the calendar item.

for(CalendarItem c: list){
// this is loop , do whatever you want here
}

For sorting, Either write a comparator class or implement comparable interface to your CalenderItem and override method compareTo. In this method you can provide comparison logic based on requirede field.

Many thanks for this. I now have part one successfully working and am adding CardViews for each CalendarItem. — CFO, Jun 05 '18 at 06:19

score 1 · Accepted Answer · answered Jun 04 '18 at 20:10

I post my solution, as I already wrote it, it should be a complete one:

public class CalendarItem implements Comparable<CalendarItem> {
    private int index;
    private String description;
    private ZonedDateTime textdate;
    private long daysAway = 0;

    public CalendarItem(int index, String description, ZonedDateTime textdate) {
        this.index = index;
        this.description = description;
        this.textdate = textdate;
        this.calculateDaysAway();
    }

    private void calculateDaysAway() {
        this.daysAway = ChronoUnit.DAYS.between(textdate, ZonedDateTime.now(ZoneId.systemDefault()));
    }

    public long getDaysAway() {
        return daysAway;
    }

    @Override
    public int compareTo(CalendarItem o) {
        if (this.daysAway < o.daysAway) {
            return -1;
        } else if (this.daysAway > o.daysAway) {
            return 1;
        } else {
            return 0;
        }
    }

    public static void main(String[] args) {
        CalendarItem c1 = 
                new CalendarItem(0, "One", ZonedDateTime.of(2018, 5, 21, 0, 0, 0, 0, ZoneId.systemDefault()));
        CalendarItem c2 = 
                new CalendarItem(0, "Two", ZonedDateTime.of(2018, 5, 4, 0, 0, 0, 0, ZoneId.systemDefault()));
        CalendarItem c3 = 
                new CalendarItem(0, "Three",ZonedDateTime.of(2018, 5, 11, 0, 0, 0, 0, ZoneId.systemDefault()));
        CalendarItem c4 = 
                new CalendarItem(0, "Four", ZonedDateTime.of(2018, 5, 1, 0, 0, 0, 0, ZoneId.systemDefault()));

        List<CalendarItem> calendarItems = new ArrayList<>(Arrays.asList(c1, c2, c3, c4));

        Collections.sort(calendarItems);

        for (CalendarItem item : calendarItems) {
            System.out.println("Item " + item.getDescription() + " is " + item.getDaysAway() + " days away.");
        }
    }
}

Note: I am using ZonedDateTime from java 8 to represent the date.

Nailed it. Thank you so much for your help. – CFO Jun 05 '18 at 20:23 — CFO, Jun 05 '18 at 20:23

Looping and sorting ojbects in a class

3 Answers3