Costruzione di un cerchio- esercizio per casa

import javax.swing.*;
import java.awt.*;
import java.awt.event.*;
public class Cerchio extends JPanel implements MouseListener {
private int x1;
private int y1;
private int x2;
private int y2;
private int raggio2=0;
private double raggio=0;
private int conta =0;
private int numero=2;
int i=0;
public Cerchio(){
addMouseListener(this);
}
public void mouseClicked(MouseEvent e){}
public void mouseEntered(MouseEvent e){}
public void mouseExited(MouseEvent e){}
public void mousePressed(MouseEvent e){
int x= e.getX();
int y=e.getY();
conta++;
if(conta==1){
x1=x;
y1=y;
}
if(conta==2){
x2=x;
y2=y;
int differenza1= x1-x2;
int differenza2= y1-y2;
raggio=Math.sqrt(Math.pow(differenza1,2)+(differenza2));
raggio2 =(int)raggio;
repaint();
}
}
public void mouseReleased(MouseEvent e){}
public void paintComponent(Graphics g){
g.drawOval(x1,y1,raggio2,raggio2);
}
}


import javax.swing.*;
import java.awt.*;
import java.awt.event.*;
public class TestaCerchio {
public static void main(String[]args){
JFrame f1 = new JFrame("Disegna un cerchio");
f1.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
f1.setSize(400,400);
Cerchio c1 = new Cerchio();
f1.add(c1,BorderLayout.CENTER);
f1.setVisible(true);
}
}