Android: 比较方法违反了它的一般契约

Question

我发现我的应用程序在某些中国 Android 手机上经常崩溃，并出现错误：比较方法违反了它的一般合同！

我了解到这与 Collections.sort 调用有关。

我不太确定是不是因为我的自定义比较器。

这里是错误发生的地方：

            if (!airports.isEmpty()) {
            Collections.sort(airports, new DistanceToAirportComparator(location));
            return airports.get(0);
        }

比较器看起来像这样：

public class DistanceToAirportComparator implements Comparator<Airport> {

private final Location location;

public DistanceToAirportComparator(Location location) {
    this.location = location;
}

@Override
public int compare(Airport airport1, Airport airport2) {
    Location loc1 = getLocation(airport1);
    Location loc2 = getLocation(airport2);
    return Double.compare(location.distanceTo(loc1), location.distanceTo(loc2));
}

private Location getLocation(Airport airport){
    Location location = new Location(LocationManager.GPS_PROVIDER);
    location.setLatitude(airport.getLocation().getLatitude());
    location.setLongitude(airport.getLocation().getLongitude());
    return location;
}

所以我不太确定比较器是否搞砸了，或者我是否需要以不同的方式进行 collections.sort 调用

感谢任何帮助

Answer 1

异常是这样说的：

比较器应该是正确的，因为如果 A > B 且 B > C，则 A > C 必须为真。

所以，归结为为什么会出现这样的问题。我的猜测是，由于 Location 的性质，距离的计算是 unstable

我建议您预先计算距离并对结果使用比较器。

编辑：

示例

private Airport sortByDistance(Location location) {
    List<Airport> airports;
    if (!airports.isEmpty()) {
        ArrayList<AirportWithDistance> distanceArrayList = new ArrayList<AirportWithDistance>(airports.size());
        for (Airport airport : airports) {
            distanceArrayList.add(new AirportWithDistance(airport, getDistance(airport, location)));
        }
        // replace start
        Collections.sort(distanceArrayList, new DistanceToAirportComparator());
        return airports.get(0).airport;
        // replace end
        //also seems u should replace the above to the line below instead, as you only wan min
        //return Collections.min(distanceArrayList, new DistanceToAirportComparator()).airport;
    }
    return null;
}


private static double getDistance(Airport airport, Location location){
    Location airportLocation = new Location(LocationManager.GPS_PROVIDER);
    airportLocation.setLatitude(airport.getLocation().getLatitude());
    airportLocation.setLongitude(airport.getLocation().getLongitude());
    return location.distanceTo(airportLocation);
}

public static class AirportWithDistance {
    Airport airport;
    double distance;

    AirportWithDistance(Airport airport, double distance) {
        this.airport = airport;
        this.distance = distance;
    }

}

public static class DistanceToAirportComparator implements Comparator<AirportWithDistance> {

    public DistanceToAirportComparator() {
    }

    @Override
    public int compare(AirportWithDistance airport1, AirportWithDistance airport2) {
        return Double.compare(airport1.distance, airport2.distance);
    }

}

Android: 比较方法违反了它的一般契约

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