Java 8 Lambda Collectors.summingLong 多列？

Question

我的 POJO 定义如下:

class EmployeeDetails{
 private String deptName;
 private Double salary;
 private Double bonus;
 ...
}

目前，我有 Group By 'deptName' 的 lambda 表达式:

$set.stream().collect(Collectors.groupingBy(EmployeeDetails::getDeptName,
                                 Collectors.summingLong(EmployeeDetails::getSalary));

问题 是否可以对不止一列进行求和？我需要在一个表达式中而不是多次计算两个字段薪水和奖金的总和？

SQL 表示为:

SELECT deptName,SUM(salary),SUM(bonus)
FROM TABLE_EMP
GROUP BY deptName;

Answer 1

您需要创建一个额外的类来保存您的 2 个汇总数字(薪水和奖金)。还有一个自定义收集器。

假设你有

private static final class Summary {
    private double salarySum;
    private double bonusSum;

    public Summary() {
        this.salarySum = 0;
        this.bonusSum = 0;
    }

    @Override
    public String toString() {
        return "Summary{" +
                "salarySum=" + salarySum +
                ", bonusSum=" + bonusSum +
                '}';
    }
}

用于持有金额。那么你需要一个这样的收集器:

private static class EmployeeDetailsSummaryCollector implements Collector<EmployeeDetails, Summary, Summary> {
    @Override
    public Supplier<Summary> supplier() {
        return Summary::new;
    }

    @Override
    public BiConsumer<Summary, EmployeeDetails> accumulator() {
        return (summary, employeeDetails) -> {
            summary.salarySum += employeeDetails.salary;
            summary.bonusSum += employeeDetails.bonus;
        };
    }

    @Override
    public BinaryOperator<Summary> combiner() {
        return (summary, summary1) -> {
            summary.salarySum += summary1.salarySum;
            summary.bonusSum += summary1.bonusSum;
            return summary;
        };
    }

    @Override
    public Function<Summary, Summary> finisher() {
        return Function.identity();
    }

    @Override
    public Set<Characteristics> characteristics() {
        return EnumSet.of(Collector.Characteristics.IDENTITY_FINISH);
    }
}

通过这些类，您可以像这样收集结果

final List<EmployeeDetails> employees = asList(
        new EmployeeDetails(/* deptName */"A", /* salary */ 100d, /* bonus */ 20d),
        new EmployeeDetails("A", 150d, 10d),
        new EmployeeDetails("B", 80d, 5d),
        new EmployeeDetails("C", 100d, 20d)
);

final Collector<EmployeeDetails, Summary, Summary> collector = new EmployeeDetailsSummaryCollector();
final Map<String, Summary> map = employees.stream()
        .collect(Collectors.groupingBy(o -> o.deptName, collector));
System.out.println("map = " + map);

打印这个:

map = {A=[salary=250.0, bonus=30.0], B=[salary=80.0, bonus=5.0], C=[salary=100.0, bonus=20.0]}