Java8 convert list to map handling duplicate keys and manipulating values

Question

I have the following data structure

public class Zones {

   private List<Zone> zones;
}

public class Zone {

  private int id;
  private String name;
  private List<Part> parts;
}

public class Part {
  private int id;
  private String name;
}

THis is my problem. I have with me an instance of Zones, say z.

I want to stream z and do the following: construct a map out of z with the following conditions: if the key (based on the "Id" of the Zone) is new, then create an entry in the map with the key and the Zone. If the key is a duplicate, then append all "parts" of this duplicate zone into the existing zone's parts list. In the end I should have a map with "Id" of the zone as the key and the zone as the value.

How can I do this in Java8 using streams?

If the key is a duplicate, then append all "parts" of this duplicate zone into the existing zone's parts list. ... what happens to the name in such a case for say z1 and z2 being Zone with the same id? — Naman
– Naman, Commented Mar 25, 2019 at 6:49

Amardeep Bhowmick · Accepted Answer · 2019-03-25 06:14:51Z

8

You can use Collectors.toMap(), to make a new Map with zone id as the key and Zone as the value, if there is a duplicate then take the List<Part> from the second Zone and append it to the first one:

  Map<Integer, Zone> map = z.getZones().stream()
                            .collect(Collectors.toMap(Zone::getId, Function.identity(), 
                             (zone1, zone2) -> {
                               zone1.getParts().addAll(zone2.getParts());
                               return zone1;
                             }));

edited Mar 25, 2019 at 6:14

answered Mar 25, 2019 at 5:59

Amardeep Bhowmick

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6 Comments

Ram Viswanathan Over a year ago

Thanks Amardeep. While this helps to group by Id, what I need is to have the parts of the same zone merged so that I end up with one zone with aggregated parts in it's "parts" attribute. With the current approach, I end up with a key with 2 zone values whcih have the same zone Id.

Amardeep Bhowmick Over a year ago

@RamViswanathan yeah, I have updated my answer. Now you would have the Zones with the same zone Ids having merged Parts if there is an Id match between two zones.

Ram Viswanathan Over a year ago

Thanks Amardeep, this is exactly what I was looking for. Works like a charm!

Ram Viswanathan Over a year ago

how do i accept an answer ? It did solve my problem.

Samuel Philipp Over a year ago

How does accepting an answer work?

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