减少java中的自定义对象列表

Question

public class OrthogonalPoint extends AbstractPoint<OrthogonalPoint> {
    private double x;
    private double y;

    // constructor, getters, ...
}

public class PolarPoint extends AbstractPoint<PolarPoint> {
    private double rho;
    private double theta;

    // constructor, getters, ...
}

public abstract class AbstractPoint<PointType extends AbstractPoint<PointType>> {

    public abstract double getX();

    public abstract double getY();

    public final double distance(PointType other) {
        // calculate distance between current point and other point
    }
}

public class Route<T extends AbstractPoint<T>> {

    private final List<T> points = new ArrayList<>();

    public void appendPoint(T point) {
        this.points.add(point);
    }

    public double routeDistance() {
        // calculate total distance
    }
}

public double routeDistance() {
    return this.points.stream().reduce(0d, (point, point2) -> point.distance(point2));
}

Bad return type in lambda expression: double cannot be converted to T

Answer 1

不能将点减少为双点。如果您需要使用ArrayList，那么您可以Maptodouble，然后sum()（减少的特例）：

points.stream().skip(1).mapToDouble(point ->
        points.get(points.indexOf(point) - 1).distance(point)).sum();

或

points.stream().limit(points.size() - 1).mapToDouble(point ->
        point.distance(points.get(points.indexOf(point) + 1))).sum();

使用μntstream可以执行以下操作：

IntStream.range(1, points.size()).mapToDouble(i ->
        points.get(i - 1).distance(points.get(i))).sum();

IntStream.range(0, points.size() - 1).mapToDouble(i ->
        points.get(i).distance(points.get(i + 1))).sum();