tbsalling tbsalling - 2 years ago 403
Java Question

Using streams to collect into TreeSet with custom comparator

Working in Java 8, I have a TreeSet defined like this:

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

PositionReport is a rather simple class defined like this:

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;

This works fine.

Now I want to remove entries from the TreeSet positionReports where timestamp is older than some value. But I cannot figure out the correct Java 8 syntax to express this.

This attempt actually compiles, but gives me a new TreeSet with an undefined comparator:

positionReports = positionReports
.filter(p -> p.timestamp >= oldestKept)

How do I express, that I want to collect into a TreeSet with a comparator like

I would have thought something like

positionReports = positionReports
.filter(p -> p.timestamp >= oldestKept)

But this does not compile / appear to be valid syntax for method references.

Answer Source
Comparator<PositionReport> byTimestamp =

Supplier<TreeSet<PositionReport>> supplier =
        () -> new TreeSet<PositionReport>(byTimestamp);

positionReports = positionReports
                    .filter(p -> p.getTimeStamp() >= oldestKept)
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