How to draw arrow JavaFX? (pane)

Question

I need to make directed graph from undirected. I can draw line-Edge, but I don't know how to make arrow:

public class Edge extends Group {

protected Cell source;
protected Cell target;

Line line;

public Edge(Cell source, Cell target) {

    this.source = source;
    this.target = target;

    source.addCellChild(target);
    target.addCellParent(source);

    line = new Line();

    line.startXProperty().bind(source.layoutXProperty().add(source.getBoundsInParent().getWidth() / 2.0));
    line.startYProperty().bind(source.layoutYProperty().add(source.getBoundsInParent().getHeight() / 2.0));

    line.endXProperty().bind(target.layoutXProperty().add( target.getBoundsInParent().getWidth() / 2.0));
    line.endYProperty().bind(target.layoutYProperty().add( target.getBoundsInParent().getHeight() / 2.0));

   getChildren().addAll(line);
}

Answer 1

You need to add 2 more lines to make an arrow head (or a Polygon with the same points for a filled arrow head).

Note that the direction of the arrow can be determined based on the difference between start and end of the line ends of the "main" connection. One end of each of the lines that make up the arrow head need to be at the same coordinates as the end of the main line. The other end can be calculated by combining a part in direction of the main line and a part ortogonal to the main line:

public class Arrow extends Group {

    private final Line line;

    public Arrow() {
        this(new Line(), new Line(), new Line());
    }

    private static final double arrowLength = 20;
    private static final double arrowWidth = 7;

    private Arrow(Line line, Line arrow1, Line arrow2) {
        super(line, arrow1, arrow2);
        this.line = line;
        InvalidationListener updater = o -> {
            double ex = getEndX();
            double ey = getEndY();
            double sx = getStartX();
            double sy = getStartY();

            arrow1.setEndX(ex);
            arrow1.setEndY(ey);
            arrow2.setEndX(ex);
            arrow2.setEndY(ey);

            if (ex == sx && ey == sy) {
                // arrow parts of length 0
                arrow1.setStartX(ex);
                arrow1.setStartY(ey);
                arrow2.setStartX(ex);
                arrow2.setStartY(ey);
            } else {
                double factor = arrowLength / Math.hypot(sx-ex, sy-ey);
                double factorO = arrowWidth / Math.hypot(sx-ex, sy-ey);

                // part in direction of main line
                double dx = (sx - ex) * factor;
                double dy = (sy - ey) * factor;

                // part ortogonal to main line
                double ox = (sx - ex) * factorO;
                double oy = (sy - ey) * factorO;

                arrow1.setStartX(ex + dx - oy);
                arrow1.setStartY(ey + dy + ox);
                arrow2.setStartX(ex + dx + oy);
                arrow2.setStartY(ey + dy - ox);
            }
        };

        // add updater to properties
        startXProperty().addListener(updater);
        startYProperty().addListener(updater);
        endXProperty().addListener(updater);
        endYProperty().addListener(updater);
        updater.invalidated(null);
    }

    // start/end properties

    public final void setStartX(double value) {
        line.setStartX(value);
    }

    public final double getStartX() {
        return line.getStartX();
    }

    public final DoubleProperty startXProperty() {
        return line.startXProperty();
    }

    public final void setStartY(double value) {
        line.setStartY(value);
    }

    public final double getStartY() {
        return line.getStartY();
    }

    public final DoubleProperty startYProperty() {
        return line.startYProperty();
    }

    public final void setEndX(double value) {
        line.setEndX(value);
    }

    public final double getEndX() {
        return line.getEndX();
    }

    public final DoubleProperty endXProperty() {
        return line.endXProperty();
    }

    public final void setEndY(double value) {
        line.setEndY(value);
    }

    public final double getEndY() {
        return line.getEndY();
    }

    public final DoubleProperty endYProperty() {
        return line.endYProperty();
    }

}

Use

@Override
public void start(Stage primaryStage) {
    Pane root = new Pane();
    Arrow arrow = new Arrow();
    root.getChildren().add(arrow);

    root.setOnMouseClicked(evt -> {
        switch (evt.getButton()) {
            case PRIMARY:
                // set pos of end with arrow head
                arrow.setEndX(evt.getX());
                arrow.setEndY(evt.getY());
                break;
            case SECONDARY:
                // set pos of end without arrow head
                arrow.setStartX(evt.getX());
                arrow.setStartY(evt.getY());
                break;
        }
    });

    Scene scene = new Scene(root, 400, 400);

    primaryStage.setScene(scene);
    primaryStage.show();
}