Hi

I am getting the error - For input string: "" for the below program. Does anyone have idea why I might be getting this error? Any help appreciated.

Thanks

import interfascia.*;

IFButton go,fact,restart;
GUIController g,t,r;
PFont font;
IFTextField textfield;
int f1=0,num,ans;
String a;

void setup()
{
  size(1350,700);
  g = new GUIController(this);
  t = new GUIController(this);
  go = new IFButton ("GO!", 600, 400, 100, 20);
  fact = new IFButton ("Factorial", 620, 400);
  go.addActionListener(this);
  fact.addActionListener(this);
  textfield=new IFTextField("",620,350);
  t.add(fact);
  t.add(textfield);
 
  //goLook = new IFLookAndFeel(this, IFLookAndFeel.DEFAULT);
  //goLook.baseColor = color(180);
  //goLook.highlightColor = color (80);
  //c.setLookAndFeel(goLook);
  //c.setHeight(50);
  g.add (go);
  r = new GUIController (this);
  restart = new IFButton ("Restart", 1170, 620, 100, 20);
  restart.addActionListener(this);
  r.add(restart);
  font=loadFont("ArialRoundedMTBold-60.vlw");
  textFont(font);
  t.setVisible(false);
  r.setVisible(false);
}

void draw()
{
  smooth();
  if(f1==0)
  {
    fill(0);
    //choice.setVisible(false);
    background(70,150,20);
    textSize(60);
    text("Let's learn about recursion",260,300);
    textSize(40);
    text("In Computer Science",480,350);
    //c.setVisible(false);
    //for(int i=3;i<93;i++)
  }
  if(f1==1)
  {
    background(70,150,20);
    stroke(0);
    strokeWeight(4);
    textSize(50);
    text("Enter a number from 1 to 4 to find its factorial",150,300);
  }
  if(f1==2)
  {
    if(num==1)
    {
      background(255);
      textSize(40);
      text("Factorial of 1 = 1",200,350);
    }
  }
   
}

void actionPerformed (GUIEvent e)
{
  if(e.getSource()==go)
  {
    //frameRate(5);
    f1=1;
    g.setVisible(false);
    t.setVisible(true);
    //c.setVisible(true);
    r.setVisible(true);
   
    draw();
  }
  if(e.getSource()==fact)
  {
    f1=2;
    a=textfield.getValue();
    //println(str);
    num=Integer.parseInt(a);
    t.setVisible(false);
    draw();
    //println(a);
  }
  /*if(e.getSource()==restart)
  {
    f1=0;
    num=0;
    ans=0;
    t.setVisible(false);
    r.setVisible(false);
    g.setVisible(true);
    textfield.setValue("");
    draw();
  }*/
}