I have a piece of code that has list of objects as follows.

List<PivotMapEgModel> pivotMapList = List.of(new PivotMapEgModel(1L, "1"), new PivotMapEgModel(1L, "2"), new PivotMapEgModel(1L, "3"), new PivotMapEgModel(2L, "5"));

It is guaranteed that there will always be a maximum of 3 codes per value.

I have a class that looks like this:

@Data
@AllArgsConstructor
class ResultSet {
    long value;
    String code_1;
    String code_2;
    String code_3;
}

I am currently doing the stream operation in this way:

pivotMapList.stream().collect(Collectors.groupingBy(PivotMapEgModel::getValue, Collectors.mapping(PivotMapEgModel::getCode, Collectors.toList())))

This is producing the output in the following way: {1=[1, 2, 3], 2=[5]}

I need to perform stream operations on the pivotMapList to get the output to show in List<ResultSet> as follows:

[{value=1, code_1=1, code_2=2, code_3=3},
 {value=2, code_1=1, code_2=null, code_3=null}]

I am not sure how I can get List<ResultSet> from stream operations

Any help to achieve the desired output would be greatly appreciated! Thank you

CodePudding user response：

You have already mapped value to its codes. You can just continue by streaming the entry set of the resulting map and map entries to ResultSet.

List<ResultSet> result = pivotMapList.stream()
            .collect(Collectors.groupingBy(PivotMapEgModel::getValue, Collectors.mapping(PivotMapEgModel::getCode, Collectors.toList())))
            .entrySet()
            .stream()
            .map(entry -> new ResultSet(entry.getKey(), getCode(entry.getValue(), 0), getCode(entry.getValue(), 1), getCode(entry.getValue(), 2)))
            .collect(Collectors.toList());

getCode() is simple method taking care not to get exception when retrieving values from the list.

private static String getCode(List<String> codes, int index) {
  if (index >= codes.size()) {
    return null;
  }
  return codes.get(index);
}

CodePudding user response：

Here is one way.

class ResultSet {

    long value;
    String code_1;
    String code_2;
    String code_3;

    public ResultSet(long value, String code_1,String code_2, String code_3) {
        this.value = value;
        this.code_1 = code_1;
        this.code_2 = code_2;
        this.code_3 = code_3;
    }
    
    public String toString() {
        return "{%d, %s, %s, %s}".formatted(value, code_1, code_2, code_3);
    }
}

Use your existing map to build the list. Stream the entrySet of the map and use map to instantiate a ResultSet instance. The forloop will fill the list with nulls if it isn't fully populated.

List<ResultSet> resultSet = map.entrySet().stream()
        .<ResultSet>map(ent-> {
            List<String> lst = ent.getValue();
            for( int i = lst.size(); i < 3; i  ) {
                lst.add(null);
            }
            return new ResultSet(ent.getKey(), lst.get(0),
                    lst.get(1), lst.get(2));
        }).toList();


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

prints

{1, 1, 2, 3}
{2, 5, null, null}

Note that you could simply stream the existing entrySet from the original map to combine the process, returning the desired List<ResultSet>