List、Set集合通过Stream流求和

一、泛型为Integer、Long、Double、BigDecimal求和

Integer sum = scores.stream().reduce(Integer::sum).orElse(0);
Long sum = scores.stream().reduce(Long::sum).orElse(0L);
Double sum = scores.stream().reduce(Double::sum).orElse(0.00);
BigDecimal sum = scores.stream().reduce(BigDecimal::add).orElse(new BigDecimal(0.00));

二、泛型为实体类

• 对单个属性求和

 Integer sum = sales.stream().mapToInt(Sale::getOrderNum).sum();
 Long sum = sales.stream().mapToLong(Sale::getOrderNum).sum();
 Double sum = sales.stream().mapToDouble(Sale::getOrderNum).sum();
 BigDecimal sum = sales.stream().map(Sale::getAppleSale).reduce(BigDecimal.ZERO, BigDecimal::add);

• 对多个属性求和

// 类型为BigDecimal
Sale result = sales.stream().reduce((x, y) -> new Sale(x.getAppleSale().add(y.getAppleSale()),
                x.getBananaSale().add(y.getBananaSale()), x.getGrapeSale().add(y.getGrapeSale())))
                .orElse(new Sale(BigDecimal.ZERO, BigDecimal.ZERO, BigDecimal.ZERO));
// 类型为Integer、Long、Double（注：orElse中需输入对应类型初始值）
Sale sale = sales.stream().reduce((x, y) -> new Sale(x.getAppleSale() + y.getAppleSale(),
                x.getBananaSale() + y.getBananaSale(), x.getGrapeSale() + y.getGrapeSale()))
                .orElse(new Sale(0.00, 0.00,0.00));

注意：多属性求和需在对应实体中添加对应的有参构造方法