Iterating a list inside a map of map using java 8 streams

Question

I have a map that contains date as key and (another map of string as key and list as the value) as value. Map<LocalDate, Map<String, List<FlexiServer>>>, I want to populate another map that has String as a key and double as a value. The contents of FlexiServer are

public class FlexiServer {
    private String serverNumber;
    private Double quantity;
    private Integer level;
    private String parentServer;
}

So, basically I want to iterate from first the exterior map to get the internal map and then iterate the internal map to get the list of FlexiServers and populate the new map having server number as key and list of quantities as values. How can I do this using java 8 streams?

I tried with for loop, but I will like to replace it using Java streams.

if(data.isPresent()) {
            for(Map.Entry<LocalDate, Map<String, List<FlexiServer>>> entry : data.get().entrySet()) {
                for(Map.Entry<String, List<FlexiServer>> innerEntry : entry.getValue().entrySet()) {
                    for(FlexiServer dto : innerEntry.getValue()) {
                        dailyRequirements.computeIfAbsent(dto.getserverNumber(),
                            k -> new ArrayList<>()).add(dto.getQuantity());
                    }
                }
            }
        }

I tried to do it using for loops but then it was 3 loops nested so I did not find it to be a good approach. — Loui
– Loui, Commented Mar 5, 2020 at 16:28

Ryuzaki L · Accepted Answer · 2020-03-05 17:28:06Z

3

Just get the values from the outer map and inner map and then use Collectors.groupingby

Map<String, List<Double>> result = map.values()
                                      .stream()
                                      .map(Map::values)
                                      .flatMap(Collection::stream)
                                      .flatMap(List::stream)
            .collect(Collectors.groupingBy(FlexiServer::getServerNumber, Collectors.mapping(FlexiServer::getQuantity, Collectors.toList())));

edited Mar 5, 2020 at 17:28

answered Mar 5, 2020 at 16:11

Ryuzaki L

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3 Comments

Loui Over a year ago

I get an error saying collect (java.util.stream.Collector<? super java.util.List<x.y.z.FlexiServer>,A,R>) in Stream cannot be applied to (java.util.stream.Collector<x.y.z.FlexiServer,capture<?>,java.util.Map<java.lang.String,java.util.List<java.lang.Double>>>) reason: no instance(s) of type variable(s) exist so that List<FlexiServer> conforms to FlexiServer

Loui Over a year ago

Thanks, it worked, do you know some good tutorial or post from where I can get my concepts clear about java streams, I know there are many over the internet, but just asking for some recommendations.

Ryuzaki L Over a year ago

This is the best have read in online baeldung.com/java-groupingby-collector @Loui

Falah H. Abbas · Accepted Answer · 2020-03-05 17:14:07Z

1

You can do it this way using Java8 StreamApi

    Map<LocalDate, Map<String, List<FlexiServer>>> localDateMapMap = new HashMap<>();
    localDateMapMap.put(LocalDate.now(), new HashMap<String, List<FlexiServer>>() {{
      put("1", new ArrayList<FlexiServer>() {{
        add(new FlexiServer() {{
          setQuantity(200D);
          setServerNumber("001");
        }});
        add(new FlexiServer() {{
          setQuantity(500D);
          setServerNumber("001");
        }});
        add(new FlexiServer() {{
          setQuantity(800D);
          setServerNumber("002");
        }});
      }});
      put("2", new ArrayList<FlexiServer>() {{
        add(new FlexiServer() {{
          setQuantity(200D);
          setServerNumber("003");
        }});
        add(new FlexiServer() {{
          setQuantity(500D);
          setServerNumber("001");
        }});
        add(new FlexiServer() {{
          setQuantity(800D);
          setServerNumber("001");
        }});
      }});
    }});
    Map<String, Double> map = new HashMap<>();
    localDateMapMap.values().stream()
        .map(Map::values)
        .flatMap(Collection::stream)
        .flatMap(Collection::stream)
        .forEach(flexiServer -> {
          if (map.containsKey(flexiServer.getServerNumber())) {
            map.replace(flexiServer.getServerNumber(),
                map.get(flexiServer.getServerNumber()) + flexiServer.getQuantity());
          } else {
            map.put(flexiServer.getServerNumber(), flexiServer.getQuantity());
          }
        });
    map.forEach((serverNumber, quantity) -> System.out
        .println("ServerNumber = " + serverNumber + "  Quantity = " + quantity));

answered Mar 5, 2020 at 17:14

Falah H. Abbas

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2 Comments

J-Alex Over a year ago

Don't try this at home.

Falah H. Abbas Over a year ago

What do you mean ?!

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