Because I think that it depends on some parameters of cameras. And I don't know that Leap Motion supports APIs to transform from 3D coordinate to 2D coordinate.
I read here at section " Mapping to 2D" https://developer-archive.leapmotion.com/documentation/java/devguide/Leap_Coordinate_Mapping.html
And I tried as below:
int appWidth = 800;
int appHeight = 600;
InteractionBox iBox = controller.frame().interactionBox();
Pointable pointable = controller.frame().pointables().frontmost();
Vector leapPoint = pointable.stabilizedTipPosition();
Vector normalizedPoint = iBox.normalizePoint(leapPoint, true);
float appX = normalizedPoint.getX() * appWidth;
float appY = (1 - normalizedPoint.getY()) * appHeight;
However, it is not correct to transform.