private void jButton1ActionPerformed(java.awt.event.ActionEvent evt)
{ b = true;
  c = false;
  t = 0;
  x_1 = Float.parseFloat(jTextField1.getText())-d1/2;
  y_1 = Float.parseFloat(jTextField2.getText())-d1/2;
  a   = Float.parseFloat(jTextField3.getText());
  v0  = Float.parseFloat(jTextField4.getText());
  v0x =  v0*Math.cos(a*Math.PI/180);
  v0y = -v0*Math.sin(a*Math.PI/180);
  timer = new Timer();
  timer.schedule(new Step(), 0,50);  
}