Java8 Collecting maps

Nuñito de la Calzada :

I have this object:

public class MenuPriceByDay implements Serializable {

    private BigDecimal avgPrice;
    private BigDecimal minPrice;
    private BigDecimal maxPrice;
    private Date updateDate;

..
}

and this other one:

public class Statistics {

     double min;
     double max;
     double average;

     public Statistics() {
        super();
     }



    public Statistics(double min, double max, double average) {
        super();
        this.min = min;
        this.max = max;
        this.average = average;
    }



}

and also a list of prices:

List<MenuPriceByDay> prices = new ArrayList<MenuPriceByDay>();

that I want to convert to a map:

Map<LocalDate, Statistics> last30DPerDay =  

        prices
            .stream()
            .collect(Collectors.toMap(MenuPriceByDay::getUpdateDate, p -> new Statistics(   p.getAvgPrice().doubleValue(),
                    p.getMaxPrice().doubleValue(),
                    p.getMinPrice().doubleValue())));

but I got a compilation problem:

Type mismatch: cannot convert from Map<Date,Object> to 
 Map<LocalDate,Statistics>

YCF_L :

UpdateDate is of type Date and you try to collect as LocalDate, so you just need to convert the Date to LocalDate :

Map<LocalDate, Statistics> collect = prices.stream()
        .collect(Collectors.toMap(m -> m.getUpdateDate()
        .toInstant()
        .atZone(ZoneId.systemDefault())
        .toLocalDate(),
                 p -> new Statistics(p.getAvgPrice().doubleValue(),
                        p.getMaxPrice().doubleValue(),
                        p.getMinPrice().doubleValue())
        ));

More details about Convert Date to LocalDate or LocalDateTime and Back

Or you can create a method instead like so :

public LocalDate convertToLocalDateViaInstant(Date dateToConvert) {
    return dateToConvert.toInstant()
            .atZone(ZoneId.systemDefault())
            .toLocalDate();
}

and your code can be :

  Map<LocalDate, Statistics> collect = prices.stream()
            .collect(Collectors.toMap(
                    m -> this.convertToLocalDateViaInstant(m.getUpdateDate()),
                    p -> new Statistics(p.getAvgPrice().doubleValue(),
                            p.getMaxPrice().doubleValue(),
                            p.getMinPrice().doubleValue())
            ));

---

Better solution

the best solution is Just to change updateDate type to LocalDate :

private LocalDate updateDate;

and you can just use your code