Java - Grouping & Summing using Lambdas

Question

I am trying to achieve the grouping & summing of values in an array. In the data shown below the values under column Surcharge need to be summed for date - note combination & stored in another list.

I referred to Link1 Link2 & Link3 but unable to figure what mistake I am doing. Hope the issue faced by me is clear, await guidance.

Data (read from excel):

• Date | Note No. | Surcharge
• 29-Jan-2016 | 1234 | 12.2
• 29-Jan-2016 | 1234 | 10.5
• 28-Jan-2016 | 2468 | 9.25
• 28-Jan-2016 | 2468 | 14.4
• 28-Jan-2016 | 2468 | 13.3
• 27-Jan-2016 | 1357 | 17.2
• 27-Jan-2016 | 1357 | 18.7

Code Data Model:

public class ModelSurcharge {
private LocalDate cNoteDate;
private int cNote;
private double surcharge;

public ModelBrokerage(LocalDate cNoteDate, int cNote, double surcharge) {
    this.cNoteDate = cNoteDate;
    this.cNote = cNote;
    this.surcharge= surcharge;
}

public LocalDate getcNoteDate() {
    return cNoteDate;
}

public int getcNote() {
    return cNote;
}

public double getSurcharge() {
    return surcharge;
}
}

Code Main class:

File fileCNote = null;
try {
     fileCNote = new File("c:/surcharge.xlsx");
} catch (IOException e) {
     e.printStackTrace();
}
// Read data from excel file
List<List<XSSFCell>> dataSurcharge = iFaceXLSX.readData(fileCNote);
// Skip header line & filter out rows which do not have surcharge amount
dataSurcharge = dataSurcharge .stream().skip(1).filter(p -> p.get(6).getNumericCellValue() > 0).collect(Collectors.toList());

// Transfer data read from excel to the data model
List<ModelSurcharge> surcharges= new ArrayList<>();
dataSurcharge.forEach(e -> {
     List<ModelSurcharge> temp = Arrays.asList(new ModelSurcharge(
                e.get(0).getDateCellValue().toInstant().atZone(ZoneId.systemDefault()).toLocalDate(),
                (int) e.get(1).getNumericCellValue(),
                (e.get(3).getNumericCellValue() + e.get(4).getNumericCellValue()) * e.get(6).getNumericCellValue()));
        surcharges.addAll(temp);
    });

//Output to console
    surcharges.stream().collect(groupingBy(Function.identity(),
            ()->new TreeMap<>(
                    Comparator.<ModelSurcharge,LocalDate>comparing(p->p.getcNoteDate()).thenComparing(p->p.getcNote())),
            Collectors.summingDouble(foo->foo.getSurcharge())))
            .forEach((group,surcharge) ->
                    System.out.println(group.getcNoteDate()+"\t"+group.getcNote()+"\t"+surcharge));

// Collecting to another list does not work
List<ModelSurcharge> output = surcharges.stream()
            .collect(groupingBy(p -> p.getcNoteDate(), Collectors.summingDouble(p -> p.getSurcharge())));

error:

Cannot resolve method 'getcNoteDate()'
Cannot resolve method 'getSurcharge()'

Answer 1

Your ModelSurcharge class doesn't compile,because it has a constructor with the name ModelBrokerage. The beginning of your main code doesn't compile either: the File constructor never throws any IOException.

But the whole compilation problem can be reduced to the following code, which is much more concise and doesn't have any dependency of external libraries:

public class ModelSurcharge {
    private LocalDate cNoteDate;
    private int cNote;
    private double surcharge;

    public ModelSurcharge(LocalDate cNoteDate, int cNote, double surcharge) {
        this.cNoteDate = cNoteDate;
        this.cNote = cNote;
        this.surcharge = surcharge;
    }

    public LocalDate getcNoteDate() {
        return cNoteDate;
    }

    public int getcNote() {
        return cNote;
    }

    public double getSurcharge() {
        return surcharge;
    }

    public static void main(String[] args) {
        List<ModelSurcharge> surcharges= new ArrayList<>();
        List<ModelSurcharge> output = surcharges.stream()
                                                .collect(groupingBy(p -> p.getcNoteDate(), 
                                                                    Collectors.summingDouble(p -> p.getSurcharge())));
    }
}

The error message is misleading. But it's caused by the fact that the compiler tries inferring the type of p from the context, but it can't because you're trying to assign the right-hand-side expression to a List<ModelSurcharge>, although groupingBy produces a Map, not a List. Use the right type for the output variable (Map<LocalDate, Double>), and the error will disappear.