How do I merge two sorted object lists based on common attribute in Java

Question

I have two object lists both with a common attribute called timestamp and both are sorted by timestamp.

I want to broadcast these objects one by one based on the timestamp, example if first object of 1st list has timestamp < first object of 2nd list, broadcast first object of the 1st list and then compare second object of the 1st list with the first object of the 2nd list.

I am a newbie in java. This is what I have come up with:

 //Merge the 2 object lists based on timestamp
        ListIterator<x> xIterator = list1.listIterator();
        ListIterator<y> yIterator = list2.listIterator();
        while (xIterator.hasNext() && yIterator.hasNext()) {
            if (xIterator.next().timestamp <= yIterator.next().timestamp) {
                Bundle extra = new Bundle();
                extra.putParcelable(LocalBroadcastConstants.ACTION, xIterator.previous());
                MyBroadcastManager.getInstance(getTargetContext()).sendBroadcast(
                        new Intent(LocalBroadcastConstants.ACTION_SEND).putExtras(extra)
                );
                yIterator.previous();
            } else {
                Bundle extra = new Bundle();
                extra.putParcelable(LocalBroadcastConstants.ACTION,
                        yIterator.previous());
                MyBroadcastManager.getInstance(getTargetContext()).sendBroadcast(
                        new Intent(LocalBroadcastConstants.ACTION_SEND).putExtras(extra)
                );
                xIterator.previous();
            }

I know my logic is incorrect because for the first item in the Iterators, xIterator.previous and yIterator.previous will point to nothing. I can't seem to find the correct solution for this problem statement. Please help.

I am not familiar with Java. However, if this in python I would use an index to track and iterate the index only if that item is broadcasted. The above code seems inefficient as you are traversing up and down a lot. I could post a pseudo code if you like — Oasa
– Oasa, Commented Jul 9, 2017 at 12:43
Unrelated : write a helper method that does the broad cast. There is no point in duplicating that code. To the contrary - it makes your code harder to read and easier to insert bugs. Whenever you start repeating code - move that code into its own method. — GhostCat
– GhostCat, Commented Jul 9, 2017 at 12:46
Though unrelated @GhostCat raises a very valid point. You should make it a practice to make a helper class for any code that is reused. — Oasa
– Oasa, Commented Jul 9, 2017 at 12:51

GhostCat · Accepted Answer · 2017-07-09 12:51:54Z

1

A simple but easy to implement solution:

create a list that simply contains all objects from both lists
create a Comparator that knows how to extract timestamps from both different classes - to then compare them
use Collections .sort() to sort the merged list using that comparator

That's it. Now you loop the sorted list to broadcast objects in order.

answered Jul 9, 2017 at 12:51

GhostCat

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Guy · Accepted Answer · 2017-07-09 12:54:23Z

1

Use indexes instead of iterator. You can also use methods to prevent code redundancy

int xIndex = 0;
int yIndex = 0;

while (xIndex < list1.size() && yIndex < list2.size()) {
    if (list1[xIndex].timestamp <= list2[yIndex].timestamp) {
        broadcast(list1, xIndex);
        ++xIndex;
    }
    else {
        broadcast(list2, yIndex);
        ++yIndex;
    }
}

dealWithLeftovers(list1, xIndex);
dealWithLeftovers(list2, yIndex);

broadcast:

private void broadcast(List<> list, int index) {
    Bundle extra = new Bundle();
    extra.putParcelable(LocalBroadcastConstants.ACTION, list[index]);
    MyBroadcastManager.getInstance(getTargetContext()).sendBroadcast(
        new Intent(LocalBroadcastConstants.ACTION_SEND).putExtras(extra)
    );
}

dealWithLeftovers:

private void dealWithLeftovers(List<> list, int index) {
    for (int i = index; i < list.size() ; ++i) {
        broadcast(list, i);
    }
}

edited Jul 9, 2017 at 12:54

answered Jul 9, 2017 at 12:43

Guy

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1 Comment

Oasa Over a year ago

You can also incorporate helper class into your dealWithLeftOvers function.

sagar kulkarni · Accepted Answer · 2017-07-09 13:10:52Z

If you want to merge two lists with Sorting with one of the fields, the below example may help u. Using Java8 Streams

Obj p1 = new Obj();
    p1.setDept("EC");
    p1.setName("Sagar");
    p1.setJoinedDate(new Date(2007, 8, 2));

    Obj p2 = new Obj();
    p2.setDept("EE");
    p2.setName("Rang");
    p2.setJoinedDate(new Date(2007, 8, 3));

    Obj p3 = new Obj();
    p3.setDept("ME");
    p3.setName("Avadh");
    p3.setJoinedDate(new Date(2008, 8, 2));

    Obj p4 = new Obj();
    p4.setDept("IP");
    p4.setName("Pams");
    p4.setJoinedDate(new Date(2007, 8, 1));

    List<Obj> dept1 = new ArrayList<>();
    dept1.add(p1);
    dept1.add(p2);

    List<Obj> dept2 = new ArrayList<>();
    dept2.add(p3);
    dept2.add(p4);
    dept1.addAll(dept2);
    List<Obj> sortedList = dept1.stream().sorted(new Comparator<Obj>() {

        @Override
        public int compare(Obj o1, Obj o2) {
            if(Instant.ofEpochMilli(o1.getJoinedDate().getTime()).atZone(ZoneId.systemDefault())
            .toLocalDateTime().isBefore(Instant.ofEpochMilli(o2.getJoinedDate().getTime()).atZone(ZoneId.systemDefault())
            .toLocalDateTime())){
                return -1;
            }else if(Instant.ofEpochMilli(o1.getJoinedDate().getTime()).atZone(ZoneId.systemDefault())
                    .toLocalDateTime().isAfter(Instant.ofEpochMilli(o2.getJoinedDate().getTime()).atZone(ZoneId.systemDefault())
                            .toLocalDateTime())){
                return 1;
            }
            return 0;
        }
    }).collect(Collectors.toList());

    System.out.println(sortedList);

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