swift2.0 四则计算器 问题请教

在摸索学习swfit 2.0中，这个四则计算器不能出结果，恳请大神们看看问题所在！！

代码如下：
import UIKit
class ViewController: UIViewController {
@IBOutlet weak var textField1: UITextField!
@IBOutlet weak var textField2: UITextField!
@IBOutlet weak var resultLabel: UILabel!
override func viewDidLoad() {
super.viewDidLoad()
// Do any additional setup after loading the view, typically from a nib.
}
override func didReceiveMemoryWarning() {
super.didReceiveMemoryWarning()
// Dispose of any resources that can be recreated.
}
@IBAction func add(sender: AnyObject){
let number1 = textField1.text
let number2 = textField2.text
resultLabel.text = number1! + number2!
}
@IBAction func sub(sender: AnyObject) {
let number1 = Double(textField1.text!)
let number2 = Double(textField2.text!)
let sum = number1! - number2!
resultLabel.text = String(sum)
}
@IBAction func mult(sender: AnyObject) {
let number1 = Double(textField1.text!)
let number2 = Double(textField2.text!)
let sum = number1! * number2!
resultLabel.text = String(sum)
}
@IBAction func div(sender: AnyObject) {
let number1 = Double(textField1.text!)
let number2 = Double(textField2.text!)
let sum = number1! / number2!
resultLabel.text = String(sum)
}
}