使用自定义比较器将流收集到TreeSet中

Question

在Java 8中工作,我有这样的TreeSet定义:

private TreeSet<PositionReport> positionReports = 
        new TreeSet<>(Comparator.comparingLong(PositionReport::getTimestamp));

PositionReport 是一个相当简单的类定义如下:

public static final class PositionReport implements Cloneable {
    private final long timestamp;
    private final Position position;

    public static PositionReport create(long timestamp, Position position) {
        return new PositionReport(timestamp, position);
    }

    private PositionReport(long timestamp, Position position) {
        this.timestamp = timestamp;
        this.position = position;
    }

    public long getTimestamp() {
        return timestamp;
    }

    public Position getPosition() {
        return position;
    }
}

这很好用.

现在我想删除TreeSet positionReportswhere timestamp比某个值更旧的条目.但我无法弄清楚正确的Java 8语法来表达这一点.

这个尝试实际上是编译的,但是给了我一个TreeSet带有未定义比较器的新东西:

positionReports = positionReports
        .stream()
        .filter(p -> p.timestamp >= oldestKept)
        .collect(Collectors.toCollection(TreeSet::new))

我如何表达,我想收集TreeSet比较像Comparator.comparingLong(PositionReport::getTimestamp)？

我会想到类似的东西

positionReports = positionReports
        .stream()
        .filter(p -> p.timestamp >= oldestKept)
        .collect(
            Collectors.toCollection(
                TreeSet::TreeSet(Comparator.comparingLong(PositionReport::getTimestamp))
            )
        );

但是这不会编译/看起来是方法引用的有效语法.

Answer 1

TreeSet<PositionReport> toTreeSet(Stream<PositionReport> reports, long timestamp) {
    return reports().filter(report -> report.getTimestamp() >= timestamp).collect(
        Collectors.toCollection(
            () -> new TreeSet<>(
                Comparator.comparingLong(PositionReport::getTimestamp)
            )
        )
    );
}

Answer 2

这很容易使用下一个代码:

    positionReports = positionReports
        .stream()
        .filter(p -> p.timestamp >= oldestKept)
        .collect(
            Collectors.toCollection(()->new TreeSet<>(Comparator.comparingLong(PositionReport::getTimestamp)
)));

Answer 3

您可以在最后转换为SortedSet(假设您不介意其他副本).

positionReports = positionReports
                .stream()
                .filter(p -> p.getTimeStamp() >= oldestKept)
                .collect(Collectors.toSet());

return new TreeSet(positionReports);

Answer 4

Collection上有一个方法,无需使用流:default boolean removeIf(Predicate<? super E> filter).见Javadoc.

所以你的代码可能看起来像这样:

positionReports.removeIf(p -> p.timestamp < oldestKept);