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.

Answer 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);
    }
}

Answer 2

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.

Answer 3

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);