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  1. import java.math.BigDecimal;
  2. import java.time.LocalDate;
  3. import java.time.Month;
  4. import java.util.Arrays;
  5. import java.util.List;
  6. import java.util.Map;
  7. import java.util.stream.Collectors;
  8.  
  9.  
  10. class Ideone
  11. {
  12. 	public static void main (String[] args) 
  13. 	{
  14. 		List<Invoice> invoiceListForMonth = Arrays.asList(
  15. 		        new Invoice(LocalDate.of(2018, Month.JULY, 01), BigDecimal.valueOf(10_000)),
  16. 		        new Invoice(LocalDate.of(2018, Month.JULY, 28), BigDecimal.valueOf(500)),
  17. 		        new Invoice(LocalDate.of(2018, Month.JULY, 01), BigDecimal.valueOf(700)),
  18. 		        new Invoice(LocalDate.of(2018, Month.JULY, 28), BigDecimal.valueOf(4500)),
  19. 		        new Invoice(LocalDate.of(2018, Month.JULY, 15), BigDecimal.valueOf(8960)),
  20. 		        new Invoice(LocalDate.of(2018, Month.JULY, 15), BigDecimal.valueOf(14450))
  21. 		);
  22. 		Map<LocalDate, List<Invoice>> grouping =
  23. 		        invoiceListForMonth.stream()
  24. 		                .collect(Collectors.groupingBy(Invoice::getPurchaseDate));
  25.  
  26. 		Map<LocalDate, BigDecimal> result = grouping.entrySet().stream()
  27. 		        .collect(Collectors.toMap(Map.Entry::getKey,
  28. 		                e -> e.getValue().stream().map(Invoice::getPrice)
  29. 		                        .reduce(BigDecimal.ZERO, BigDecimal::add))
  30. 		        );
  31.  
  32. 		result.forEach((k, v) -> System.out.println(k + " : " + v));
  33. 	}
  34. }
  35.  
  36. class Invoice {
  37. 	    private LocalDate purchaseDate;
  38. 	    private BigDecimal price;
  39.  
  40. 	    public Invoice() {
  41. 	    }
  42.  
  43. 	    public Invoice(LocalDate purchaseDate, BigDecimal price) {
  44. 	        this.purchaseDate = purchaseDate;
  45. 	        this.price = price;
  46. 	    }
  47.  
  48. 	    public LocalDate getPurchaseDate() {
  49. 	        return purchaseDate;
  50. 	    }
  51.  
  52. 	    public void setPurchaseDate(LocalDate purchaseDate) {
  53. 	        this.purchaseDate = purchaseDate;
  54. 	    }
  55.  
  56. 	    public BigDecimal getPrice() {
  57. 	        return price;
  58. 	    }
  59.  
  60. 	    public void setPrice(BigDecimal price) {
  61. 	        this.price = price;
  62. 	    }
  63. 	}
Success #stdin #stdout 0.23s 2184192KB
comments (?)
stdin 
Standard input is empty
stdout 
2018-07-15 : 23410
2018-07-28 : 5000
2018-07-01 : 10700
https://ideone.com/oQkZVO
language:
Java (HotSpot 12)
created:
5 years ago
visibility:
public

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