Question

Okay at first I thought this would be pretty straightforward. But I can’t think of an efficient way to solve this. I figured a brute force way to solve this but that’s not very elegant. I have an ArrayList. Contacts is a VO class that has multiple members – name, regions, id. There are duplicates in ArrayList because different regions appear multiple times. The list is sorted by ID. Here is an example:

Entry 0 – Name: John Smith; Region: N; ID: 1
Entry 1 – Name: John Smith; Region: MW; ID: 1
Entry 2 – Name: John Smith; Region: S; ID: 1
Entry 3 – Name: Jane Doe; Region: NULL; ID: 2
Entry 4 – Name: Jack Black; Region: N; ID: 3
Entry 6 – Name: Jack Black; Region: MW; ID: 3
Entry 7 – Name: Joe Don; Region: NE; ID: 4

I want to transform the list to below by combining duplicate regions together for the same ID. Therefore, the final list should have only 4 distinct elements with the regions combined.

So the output should look like this:-

Entry 0 – Name: John Smith; Region: N,MW,S; ID: 1
Entry 1 – Name: Jane Doe; Region: NULL; ID: 2
Entry 2 – Name: Jack Black; Region: N,MW; ID: 3
Entry 3 – Name: Joe Don; Region: NE; ID: 4

What are your thoughts on the optimal way to solve this? I am not looking for actual code but ideas or tips to go about the best way to get it done.

Thanks for your time!!!

Answer 1

You can iterate them while dumping them (and merging duplicates) into a TreeMap. Then create a list from the sorted view of the TreeMap’s values.

In the sample code I’m assuming you have an Entry class with id, name and regions fields this last one being a List of Region instances. This could easily be changed to a Set, and Region to Strings or whatever you’re using. The sample copies the entries before inserting them into the map since they will be modified when merged to other entries.

SortedMap<Integer, Entry> mergedEntriesMap = new TreeMap<Integer, Entry>();
for (Entry e : entries) {
  if (mergedEntriesMap.contains(e.id)) {
    Entry m = mergedEntriesMap.get(e);
    m.regions.addAll(e.regions);
  } else {
    Entry m = new Entry();
    // copy the entry to keep the original array clean
    m.id = e.id;
    m.name = e.name;
    m.regions = new ArrayList<Region>(e.regions);
    mergedEntriesMap.put(m.id, m);
  }
}

List<Entry> mergedEntries = new ArrayList<Entry>(mergedEntriesMap.values());