将部分哈希图重复项合并到Java中的新arraylist

Question

I have Stream<Map<String, String>> where each map is like a separate record and contains two entries:

1. Car Id ("ID", "1003")
2. Kilometers ("KMI", "500")

This list might contain maps which are duplicate in a way that 2 or more maps could have the same value of car id entry. Basically this: entry1.get("ID") == entry2.get("ID"). what i want to do is to remove maps with duplicate ids but then also to merge KMI values together. this:

{"ID":"1003","KMI":"500"}, {"ID":"1003","KMI":"400"}, {"ID":"1004","KMI":"200"}

should become this:

{"ID":"1003","KMI":"900"}, {"ID":"1004","KMI":"200"}

I have tried doing it with streams API alone, but I can't wrap my head around this. I tried modifying a similar example which was having List of objects here is what I got so far:

List<Map<String, String>> result = new ArrayList<>(
    queryKmAll.collect(
        Collectors.toMap(a1 -> a1.get("ID")), Function.identity(), (Map<String, String> m2, Map<String, String> m1) -> {
            m1.put("KMI", String.valueOf(Double.parseDouble(m1.get("KMI")) + Double.parseDouble(m2.get("KMI"))));

            return m1;
        })
    )
);

Answer 1

I have picked up from where OP left. I have modified your logic a bit to return what you wanted. Have a look at it. Hope it helps

Collectors.toMap will return a map which will have ID as key and sum of KMI for Function.identity() cases. So the return would be Map<Object,Map<String,String>> . Because expected output is Stream<Map<String,String> , I added .values().stream() .

Stream<Map<String, String>> result = queryKmAll.collect(Collectors.toMap(a1 -> a1.get("ID"),
            Function.identity(), (Map<String, String> m2, Map<String, String> m1) -> {
                m1.put("KMI",
                        String.valueOf(Double.parseDouble(m1.get("KMI")) + Double.parseDouble(m2.get("KMI"))));

                return m1;
            })).values().stream();

result.forEach(System.out::println);

Answer 2

What you're looking for is Collectors.groupingBy :

    Map<String, Long> summary = Stream
        .of(
            new HashMap<String, String>() {{
              put("ID", "1003");
              put("KMI", "500");
            }},
            new HashMap<String, String>() {{
              put("ID", "1003");
              put("KMI", "400");
            }},
            new HashMap<String, String>() {{
              put("ID", "1004");
              put("KMI", "200");
            }}
        )
        .collect(Collectors.groupingBy(
            m -> m.get("ID"),
            Collectors.summingLong(m -> Long.valueOf(m.get("KMI")))
        ));

    System.out.println(summary);

Also, you could replace the map with an class, say Summary :

  public class Summary {

    public Summary(String id, Long kmi) {
      this.id = id;
      this.kmi = kmi;
    }

    private String id;
    private Long kmi;

    public String getId() {
      return id;
    }

    public Long getKmi() {
      return kmi;
    }
  }

And then use:

    Map<String, Long> summary = Stream
        .of(
            new Summary("1003", 500L),
            new Summary("1003", 400L),
            new Summary("1004", 200L)
        )
        .collect(Collectors.groupingBy(
            s -> s.getId(),
            Collectors.summingLong(s -> s.getKmi())
        ));

    System.out.println(summary);

Prints: {1004=200, 1003=900}