B4J Code Snippet A method for drawing paths of line shapes in B4J with B4XCanvas

This is not a problem you might say. Try it and you'll find that nothing appears to be drawn. If you do the same thing in B4A it works as expected.
The reason is that the JavaFx designers differentiated between lines and line shapes. The DrawLine method draws lines, the DrawPath method draws closed shapes.

In B4J if you draw a shape, the points in the path describe the outside edge of the shape. So the inside of a closed path of a line is empty, no matter how thick you make the stroke width.
The solution is to turn the line into a line shape by making it a thin rectangle. That is fairly easy if the line is horizontal or vertical, but if it is on an angle, it requires some trigonometry.

I knew that, but a recent question by @labcold https://www.b4x.com/android/forum/threads/b4x-canvas-path-differences-between-b4a-and-b4j.132810/
made me revisit the issue and wrote a sub ("listToPath") that adjusts the path of a shape if the path describes a line. Because the points in a loaded path are not available, the method works if you generate the points on the path as a List of points (2 element Arrays). The Sub converts the List into an path that makes an adjustment for lines.

Edit: I changed the name of the path in the subroutine, the earlier name was from a previous version which would cause confusion. The change is tested.

Sub AppStart (Form1 As Form, Args() As String)
    MainForm = Form1
    MainForm.RootPane.LoadLayout("Main")
    MainForm.Show
    cvs.Initialize(pnlGraph)
   
    Dim fromCenter As List: fromCenter.Initialize
    fromCenter.Add(Array(-0.4, 0.0))
    fromCenter.Add(Array(-0.2, 0.0))
    fromCenter.Add(Array(0.0, 0.0))
    fromCenter.Add(Array(0.2, 0.0))
    fromCenter.Add(Array(0.4, 0.0))
    drawRotatedShape (fromCenter, 250, 250, 2, 200, 30)
End Sub

Private Sub drawRotatedShape(FromCenter As List, CenterX As Float, CenterY As Float, Thickness As Float, Radius As Float, Angle As Float)
    Dim pntList As List: pntList.Initialize
    For Each pt() As Object In FromCenter
        pntList.Add(Array(CenterX + pt(0).As(Float) * Radius, CenterY + pt(1).As(Float) * Radius))
    Next
    cvs.DrawPathRotated(listToPath(pntList, Thickness), xui.Color_Black, True, Thickness, Angle, CenterX, CenterY)
End Sub

Private Sub listToPath(pntList As List, thickness As Float) As B4XPath
    'This method works for all shapes - it determines whether the shape is a line shape and then adjusts the path
    Dim aPath As B4XPath
    Dim epsilon As Float = .001
    Dim firstPt(2), lastPt(2), prevPt(2) As Object
    Dim isHorizontalLine = True, isVerticalLine = True, isOtherLine = True As Boolean
    For i = 0 To pntList.Size - 1
        Dim pt() As Object = pntList.Get(i)
        Select i
            Case 0
                firstPt(0) = pt(0)
                firstPt(1) = pt(1)
            Case pntList.Size - 1
                lastPt(0) = pt(0)
                lastPt(1) = pt(1)
        End Select
        If i = 0 Then
            aPath.Initialize(pt(0), pt(1))
        Else
            aPath.LineTo(pt(0), pt(1))
            If Abs(prevPt(0) - pt(0)) > epsilon Then isVerticalLine = False
            If Abs(prevPt(1) - pt(1)) > epsilon Then isHorizontalLine = False
        End If
        prevPt(0) = pt(0)
        prevPt(1) = pt(1)
    Next
    If isVerticalLine Then
        aPath.LineTo(lastPt(0) + thickness, lastPt(1))
        aPath.LineTo(firstPt(0) + thickness, firstPt(1))
    Else If isHorizontalLine Then
        aPath.LineTo(lastPt(0), lastPt(1) + thickness)
        aPath.LineTo(firstPt(0), firstPt(1) + thickness)
    Else
        Dim deltaX As Float = lastPt(0) - firstPt(0)
        Dim deltaY As Float = lastPt(1) - firstPt(1)
        If Abs(deltaX) < epsilon Then
            isOtherLine = False
        Else If Abs(deltaY) < epsilon Then
            isOtherLine = False
        Else
            Dim overallSlope As Float = deltaY / deltaX
            Log(overallSlope)
            prevPt(0) = firstPt(0)
            prevPt(1) = firstPt(1)
            For i = 1 To pntList.Size - 1
                Dim pt() As Object = pntList.Get(i)
                Dim thisSlope As Float = (pt(1) - prevPt(1)) / (pt(0) - prevPt(0))
                If Abs(overallSlope - thisSlope) > epsilon Then
                    isOtherLine = False
                    Exit
                End If
                prevPt(0) = pt(0)
                prevPt(1) = pt(1)
            Next
        End If
        If isOtherLine Then
            Dim offsetX, offsetY As Float
            Dim angle As Float = cPI / 2 + ATan(overallSlope)
            offsetX = thickness * Cos(angle)
            offsetY = thickness * Sin(angle)
            aPath.LineTo(lastPt(0) + offsetX, lastPt(1) + offsetY)
            aPath.LineTo(firstPt(0) + offsetX, firstPt(1) + offsetY)
        End If
    End If
    Return aPath
End Sub