Merge Lists + update items

Question

I have e.g. following Lists:

List<tst> listx; // tst object has properties: year, A, B, C, D year A B C D ------------ 2013 5 0 0 0 // list1 2014 3 0 0 0 2013 0 8 0 0 // list2 2014 0 1 0 0 2013 0 0 2 0 // list3 2014 0 0 3 0 2013 0 0 0 1 // list4 2014 0 0 0 5

if I use addAll method, the listTotal will be:

year A B C D ------------ 2013 5 0 0 0 // listTotal 2014 3 0 0 0 2013 0 8 0 0 2014 0 1 0 0 2013 0 0 2 0 2014 0 0 3 0 2013 0 0 0 1 2014 0 0 0 5

How to merge them to the listRequired which would be like this?

year A B C D ------------ 2013 5 8 2 1 // listRequired 2014 3 1 3 5

JB Nizet · Accepted Answer · 2014-10-20 11:07:38Z

Use a Map<Integer, Tst> containing, for each year (the key of the map) the Tst you want for this year as a result.

Iterate through your listTotal and, for each Tst:

if the year of the Tst isn't in the map yet, then store the Tst for this year
else, get the Tst from the map and merge it with the current Tst

In the end, the values() of the map is what you want in your listRequired.

Code:

Map<Integer, Tst> resultPerYear = new HashMap<>(); for (Tst tst : listTotal) { Tst resultForYear = resultPerYear.get(tst.getYear()); if (resultForYear == null) { resultPerYear.put(tst.getYear(), tst); } else { resultForYear.merge(tst); } } Set<Tst> result = resultPerYear.values();

yurib · Accepted Answer · 2014-10-20 11:07:45Z

use a map to maintain a mapping from year to the TST structure.
iterate each item of the lists, retrieve the corresponding TST structure and update the A/B/C/D attributes manually

Guy Bouallet · Accepted Answer · 2014-10-20 11:14:19Z

You should be able to do it by defining a custom merge method:

List<tst> merge (List<tst> listA, List<tst> listB)

which merges the properties of each element in listA with the element of the same index in listB.

Call this method iteratively on list1, ...list4 to get listRequired.

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