Preventing runtime errors from clicking cancel

Question

          String input;
          double number1;
          double number2;
          double number3;
          double number4;
                  input = JOptionPane.showInputDialog("Enter a number:  ");
                     number1 = Double.parseDouble(input);
                     input = JOptionPane.showInputDialog("Enter a number:  ");
                     number2 = Double.parseDouble(input);
                     input = JOptionPane.showInputDialog("Enter a number:  ");
                     number3 = Double.parseDouble(input);
                     input = JOptionPane.showInputDialog("Enter a number:  ");
                     number4 = Double.parseDouble(input);
                     JOptionPane.showMessageDialog(null, number1 + "\n" +
                                                                  number2 + "\n" +
                                                                  number3 + "\n" +
                                                                  number4);
     How do i set it so if the user hits "Cancel" on ANY of the input dialog messages, the console does not display a runtime error message?

Answer 1

          String input;
          double number1 = 0;
          double number2 = 0;
          double number3 = 0;
          double number4 = 0;
                  input = JOptionPane.showInputDialog("Enter a number:  ");
                  // Prevents a runtime error for clicking "Cancel"
                  if( input != null ) {
                     number1 = Double.parseDouble(input);
                     input = JOptionPane.showInputDialog("Enter a number:  ");
                     number2 = Double.parseDouble(input);
                     input = JOptionPane.showInputDialog("Enter a number:  ");
                     number3 = Double.parseDouble(input);
                     input = JOptionPane.showInputDialog("Enter a number:  ");
                     number4 = Double.parseDouble(input);
                     JOptionPane.showMessageDialog(null, number1 + "\n" +
                                                                  number2 + "\n" +
                                                                  number3 + "\n" +
                                                                  number4);
}I tried that, but it only works for the first input dialog box.