Return 在一个字符串中，最小值、最大值、平均值、总和、Stream 和 java8 的工资计数

Question

我有 List 名以薪水为特征的员工。为什么此代码不起作用？

String joined = employees.stream().collect(
    Collectors.summingInt(Employee::getSalary),
    Collectors.maxBy(Comparator.comparing(Employee::getSalary)),
    Collectors.minBy(Comparator.comparing(Employee::getSalary)),
    Collectors.averagingLong((Employee e) ->e.getSalary() * 2),
    Collectors.counting(),
    Collectors.joining(", "));

我正在使用一套收集器。

Answer 1

请注意，目前您尝试获取的不是 max/min 薪水，而是拥有该薪水的员工。如果您真的想要 max/min 工资本身（数字），那么可以使用 Collectors.summarizingInt():

立即计算这些特征

IntSummaryStatistics stats = employees.stream()
    .collect(Collectors.summarizingInt(Employee::getSalary));

如果你想将它们连接成字符串，你可以使用：

String statsString = Stream.of(stats.getSum(), stats.getMax(), stats.getMin(), 
                               stats.getAverage()*2, stats.getCount())
                           .map(Object::toString)
                           .collect(Collectors.joining(", "));

如果你真的想得到一名薪水 max/min 的员工，这里 IntSummaryStatistics 对你没有帮助。但是，您可以改为创建收集器流：

String result = Stream.<Collector<Employee,?,?>>of(
            Collectors.summingInt(Employee::getSalary),
            Collectors.maxBy(Comparator.comparing(Employee::getSalary)),
            Collectors.minBy(Comparator.comparing(Employee::getSalary)),
            Collectors.averagingLong((Employee e) ->e.getSalary() * 2),
            Collectors.counting())
        .map(collector -> employees.stream().collect(collector))
        .map(Object::toString)
        .collect(Collectors.joining(", "));

请注意，通过这种方式，您将得到如下输出（取决于 Employee.toString() 实现：

1121, Optional[Employee [salary=1000]], Optional[Employee [salary=1]], 560.5, 4

别忘了 maxBy/minBy return Optional.

如果您对第一个解决方案不满意并且出于某种原因不想多次迭代输入，您可以使用如下方法创建组合收集器：

/**
 * Returns a collector which joins the results of supplied collectors
 * into the single string using the supplied delimiter.
 */
@SafeVarargs
public static <T> Collector<T, ?, String> joining(CharSequence delimiter, 
        Collector<T, ?, ?>... collectors) {
    @SuppressWarnings("unchecked")
    Collector<T, Object, Object>[] cs = (Collector<T, Object, Object>[]) collectors;
    return Collector.<T, Object[], String>of(
        () -> Stream.of(cs).map(c -> c.supplier().get()).toArray(), 
        (acc, t) -> IntStream.range(0, acc.length)
            .forEach(idx -> cs[idx].accumulator().accept(acc[idx], t)), 
        (acc1, acc2) -> IntStream.range(0, acc1.length)
            .mapToObj(idx -> cs[idx].combiner().apply(acc1[idx], acc2[idx]))
            .toArray(), 
        acc -> IntStream.range(0, acc.length)
            .mapToObj(idx -> cs[idx].finisher().apply(acc[idx]).toString())
            .collect(Collectors.joining(delimiter)));
}

有了这样的方法就可以写

String stats = employees.stream().collect(joining(", ",
        Collectors.summingInt(Employee::getSalary),
        Collectors.maxBy(Comparator.comparing(Employee::getSalary)),
        Collectors.minBy(Comparator.comparing(Employee::getSalary)),
        Collectors.averagingLong((Employee e) ->e.getSalary() * 2),
        Collectors.counting()));

Answer 2

我终于找到了解决办法..感谢大家的尝试

String s = employees.stream().mapToDouble(a>a.getSalary()).summaryStatistics().toString();

这是输出：

 DoubleSummaryStatistics{count=21, sum=17200,000000, min=100,000000,
 average=819,047619, max=2100,000000}

Return 在一个字符串中，最小值、最大值、平均值、总和、Stream 和 java8 的工资计数

Return in one string the min, max, average, sum, count of salaries with Stream and java8

java

java-8

java-stream

collectors