Java 8过滤器考虑流的多个项目

Question

考虑这个类：-

import java.sql.Timestamp;

public class Report {
    private short value;
    private Timestamp created;
    //Getters, Setters
}

我有一个List 使用ORDER BY created DESC 从数据库获得的报告。

任务是获取每个月的最新报告。我知道它可以在 SQL 级别完成，但由于某种原因我需要在 Java 中完成。

我就是这样解决的：-

    /**
     * Assuming that the reports are sorted with <code>ORDER BY created DESC</code>, this method filters the list so
     * that it contains only the latest report for any month.
     *
     * @param reports Sorted list of reports
     * @return List containing not more than one report per month
     */
    public static List<Report> oneReportPerMonthFilter(List<Report> reports) {
        Map<String, Report> monthlyReports = new HashMap<>();
        reports.forEach(report -> {
            String yearMonth = getCreatedYearMonth(report);
            if (!monthlyReports.containsKey(yearMonth)) {
                monthlyReports.put(yearMonth, report);
            }
        });
        return new ArrayList<>(monthlyReports.values());
    }

    private static String getCreatedYearMonth(Report report) {
        return YearMonth
                .from(ZonedDateTime.of(report.getCreated().toLocalDateTime(), ZoneOffset.UTC))
                .toString();
    }

问题 1

虽然按预期工作，但我必须创建一个Map，然后将values 转换回List。使用 Java 8 Stream API 有更好的方法吗？也许是更“实用”的方式？

问题 2

将Timestamp 转换为YearMonth 的方法getCreatedYearMonth(Report report) 可以简化吗？目前它将Timestamp 更改为LocalDateTime，然后更改为ZonedDateTime，然后更改为YearMonth。

单元测试：-

@Test
public void shouldFilterOutMultipleReportsPerMonth() {
    Report report1 = new Report();
    report1.setCreated(Timestamp.from(Instant.EPOCH));
    report1.setValue((short) 100);

    Report report2 = new Report();
    report2.setCreated(Timestamp.from(Instant.EPOCH.plus(10, ChronoUnit.DAYS)));
    report2.setValue((short) 200);

    Report report3 = new Report();
    report3.setCreated(Timestamp.from(Instant.EPOCH.plus(40, ChronoUnit.DAYS)));
    report3.setValue((short) 300);

    List<Report> reports = Stream.of(report3, report2, report1).collect(Collectors.toList());

    List<Report> filteredReportList = ExampleClass.oneReportPerMonthFilter(reports);
    Assert.assertEquals(2, filteredReportList.size());
    Assert.assertEquals((short) 300, (short) filteredReportList.get(0).getValue());
    Assert.assertEquals((short) 200, (short) filteredReportList.get(1).getValue());
}

编辑 1

回答

感谢大家的回答。使用 Amith 和 Johannes 的答案，我想出了这个简单易读的版本：-

public static List<Report> oneReportPerMonthFilter(List<Report> reports) {
    Set<YearMonth> found = new HashSet<>();
    return reports.stream()
            .filter(r -> found.add(getCreatedYearMonth(r)))
            .collect(Collectors.toList());
}

private static YearMonth getCreatedYearMonth(Report report) {
    return YearMonth.from(
            report.getCreated()
                    .toInstant()
                    .atZone(ZoneOffset.UTC));
}

似乎没有快速将时间戳转换为年月的方法。我们可以从 Timestamp 中获取年月的字符串表示形式，如 Amith 所示。

Answer 1

您不应将YearMonth 转换为字符串。只需省略 toString() 部分。此外，我能够将其简化为：

private static YearMonth getCreatedYearMonth(Report report) {
    return YearMonth.from(report.getCreated().toInstant().atZone(ZoneOffset.UTC));
}

为了得到你想要的结果，你必须链接一些收集器：

Map<YearMonth, Report> last = reports.stream()
        .collect(Collectors.groupingBy(r -> getCreatedYearMonth(r),
                Collectors.collectingAndThen(
                        Collectors.maxBy(Comparator.comparing(Report::getCreated)),
                        Optional::get)));

外部Collector 是groupingBy：我们想要每个YearMonth 的东西。下游收集器在同一个月只会看到Reports。
下游Collector 是collectingAndThen，因为maxBy 收集器将产生Optional<Report>。但是我们已经知道每个月至少有一个Report，所以我们只是打开它。
最里面的收集器只是通过Timestamp 获得最大值。

Answer 2

您可以使用带有如下状态谓词的 Java 流来按月过滤第一份报告。

注意： - 不要使用 parallelStream() 运行它，因为它不是线程安全，还假设列表按日期排序以获得按月和年顺序选择第一个的所需结果。

亮点

    public static List<Report> oneReportPerMonthFilter(List<Report> reports) {
            Set<String> found = new HashSet<>();
            return reports.stream().filter(r -> found.add(getCreatedYearMonth(r))).collect(Collectors.toList());    
    }

    public static String getCreatedYearMonth(Report report) {
            //Or you can use SimpleDateFormat to extract Year & Month
            Calendar cal = Calendar.getInstance();
            cal.setTime(report.getCreated());
            return "" + cal.get(Calendar.YEAR) + cal.get(Calendar.MONTH);
        }
    }

可测试（完整）代码

import java.sql.Timestamp;
import java.time.Instant;
import java.time.temporal.ChronoUnit;
import java.util.Arrays;
import java.util.Calendar;
import java.util.HashSet;
import java.util.List;
import java.util.Set;
import java.util.stream.Collectors;

public class ReportFilter {

    public static void main(String[] args) {
        Report report1 = new Report();      
        report1.setCreated(Timestamp.from(Instant.EPOCH));
        report1.setValue((short) 100);

        Report report2 = new Report();
        report2.setCreated(Timestamp.from(Instant.EPOCH.plus(10, ChronoUnit.DAYS)));
        report2.setValue((short) 200);

        Report report3 = new Report();
        report3.setCreated(Timestamp.from(Instant.EPOCH.plus(40, ChronoUnit.DAYS)));
        report3.setValue((short) 300);

        Report report4 = new Report();
        report4.setCreated(Timestamp.from(Instant.EPOCH.plus(40, ChronoUnit.DAYS)));
        report4.setValue((short) 400);

        List<Report> reports = Arrays.asList(report1, report2, report3, report4);
        List<Report> filteredReports = oneReportPerMonthFilter(reports); 
        System.out.println(filteredReports);
    }

    public static List<Report> oneReportPerMonthFilter(List<Report> reports) {
        Set<String> found = new HashSet<>();
        return reports.stream().filter(r -> found.add(getCreatedYearMonth(r))).collect(Collectors.toList());    
    }

    public static String getCreatedYearMonth(Report report) {
        //Or you can use SimpleDateFormat to extract Year & Month
        Calendar cal = Calendar.getInstance();
        cal.setTime(report.getCreated());
        return "" + cal.get(Calendar.YEAR) + cal.get(Calendar.MONTH);
    }
}

class Report {
    private Timestamp created;
    private short value;    

    public Timestamp getCreated() {
        return created;
    }

    public void setCreated(Timestamp created){
        this.created = created;
    }


    public short getValue() {
        return value;
    }

    public void setValue(short value) {
        this.value = value;
    }

    @Override
    public String toString() {
        return "Report [created=" + created + ", value=" + value + "]";
    }
}

Answer 3

回答你的第一个问题：

使用标准 API，如果不创建地图，实际上是没有办法做你想做的事的；但是，我确实重新编写了它以使其更惯用。

public static List<Report> oneReportPerMonthFilter2(List<Report> reports) {
    return reports.stream()
        .collect(Collectors.groupingBy(Q50938904::getCreatedYearMonth))
        .values().stream()
            .map(p-> p.get(0))
            .collect(Collectors.toList());
}

如果可以，请考虑查看 StreamEx。它是流 API 的扩展。