StudyPane.java

import javafx.scene.Scene;
import javafx.scene.Node;
import javafx.scene.layout.Pane;
import javafx.scene.paint.Color;
import javafx.scene.shape.*;
import java.util.*;

import javafx.scene.control.*;


public class StudyPane extends Pane
{
    double s = 10;
    double radius1 = 1.5;
    double qx = 22;
    double qy = 8;
    Circle circ1 = new Circle(qx*s,qy*s,radius1*s);
    double radius2 = 1.0;
    double px = 10;
    double py = 13; 
    Circle circ2 = new Circle(px*s,py*s,radius2*s);
    double vx = 8;
    double vy = -4;
    Line vec;
    double t1, t2;
    double wd,ht;
    TextArea area;
    boolean flag = true;
    int ns;
    
    StudyPane()
    {
	wd = 300;
	ht = 200;

	setStyle("-fx-background-color: honeydew;");

	Rectangle rect = new Rectangle(0,0,wd,ht);
	rect.setFill(Color.WHITE);

	vec = new Line(px*s,py*s,(px+vx)*s,(py+vy)*s);


	circ1.setFill(Color.GREEN);
	circ2.setFill(Color.BLUE);

	getChildren().addAll(rect,vec,circ1,circ2);

        area = new TextArea();
	area.setPrefRowCount(6);
	area.setWrapText(true);
	area.setEditable(false);
	area.appendText("Ball study");

	setOnMousePressed( e ->
	{
	    double x = e.getX();
	    double y = e.getY();
	    ns = 0;
	    if (flag) {
		px = x/s;
		py = y/s;
		circ2.setCenterX(x);
		circ2.setCenterY(y);
		vx = 0;
		vy = 0;
		flag= false;
	    } 
	    else {
		vx = x/s - px;
		vy = y/s - py;
		flag = true;
	   }
	   vec.setStartX(px*s);
	   vec.setStartY(py*s);
	   vec.setEndX((px+vx)*s);
	   vec.setEndY((py+vy)*s);
	   solve();	 	    
	}
	);

    }

    public void solve() {
	t1 = t2 = -1;
	// relative velocity
	double rx = vx;
	double ry = vy;
	// relative position
	double dx = px - qx;
	double dy = py - qy;
	// test radius
	double r = radius1 + radius2;
	double A = rx*rx+ry*ry;
	double B = rx*dx+ry*dy;
	double C =  dx*dx + dy*dy - r*r;
	area.clear();
	area.appendText(String.format("A %g B %g C %g%n",A,B,C));
	double radical = B*B-A*C;
	area.appendText(String.format("radical B*B - A*C =  %g%n",radical));
	if (A<=0.0) {
		area.appendText(String.format("A==0, No motion%n"));
		return;
	}
	if (radical<0) {
		area.appendText(String.format("radical < 0, No solution%n"));
		return;
	}
	if (B>0) {
		area.appendText(String.format("B>0, balls moving away from one another%n"));
		return;
	}
	double R = Math.sqrt(radical);
	t1 = (-B-R)/A;
	t2 = (-B+R)/A;
	area.appendText(String.format("t1 = %g  t2 = %g%n",t1,t2)); 
    }

    public void move(int pos) {
	ns = pos;
	if (pos==0) {
		circ2.setCenterX(px*s);
		circ2.setCenterY(py*s);
	}
	if (pos==1 && t1>0) {
		circ2.setCenterX((px+vx*t1)*s);
		circ2.setCenterY((py+vy*t1)*s);
	}
	if (pos==2 && t2>0) {
		circ2.setCenterX((px+vx*t2)*s);
		circ2.setCenterY((py+vy*t2)*s);
	}

    }
}