Java JTextField空字符串错误

我不断收到空字符串错误，如下所示

Exception in thread "AWT-EventQueue-0" java.lang.NumberFormatException: For input string: ""

我不确定为什么会一直收到此错误，因为在JTextField中不键入数字就不会尝试计算。下面是我用于“计算”按钮的代码，这很简单。

private void ButtonCalculateActionPerformed(java.awt.event.ActionEvent evt) {                                                
         Height = Integer.parseInt(this.TextFieldHeight.getText());
         Weight = Integer.parseInt(this.TextFieldWeight.getText());
         BMI = Integer.parseInt(this.TextFieldBMI.getText());
         answer = Height + Weight;
        this.TextFieldBMI.setText("The answer is: " + answer);
    }                                               

这些是定义的变量

//variables
    static int Height;
    static int Weight;
    static int BMI;
    static int answer;