Defensive Design

Defensive Design

1. User Interface

   • Menu Interface
     • A menu-driven user interface limits the user to being able to pick from a displayed list of choices.
   • The first port of call in defensive design is to decide on what kind of user interface to code for. There is a fine balancing act with giving the user access to all the information and abilities that they might need, while at the same time imposing limits on the things that they commonly get wrong.

2. Screen Widget

   • Example
     • Drop down list and spinners
     • Calender Widget
     • Colour picker widget
   • A 'graphical widget' is a small self-contained object on-screen to allow data selection to be made.

3. Repeat Entry

   • Text entry is the most flexible way of accepting inputs from the user, but also the most problematic. Users can mistype, or enter data into the wrong field, or simply input gibberish
   • Defensive design involves making it as easy as possible for the user to get things right on the first try.
   • Probably the simplest thing for the programmer to do to reduce errors is to inform the user, with clear and unambiguous instructions on-screen, of what they need to enter.

4. Input Box

   • The data input type that is most vulnerable to problems - deliberate or otherwise - is the input box and access to keyboard entry. For example the picture of a form below includes three input boxes for data entry.
   • As the user is now free to try and enter whatever they like. The program handling the input needs to include some defensive measures. These defensive features are called 'validation'.
   • As the user is now free to try and enter whatever they like. The program handling the input needs to include some defensive measures. These defensive features are called 'validation'.

5. Length/Range validation

   • Length validation is checking to see if the entered value is within the allowed number of characters. For example, password forms may reject an entry if it is less than three characters, as it would be too short for security. Empty fields (those with a length of zero) may also be rejected.
   • The truncation option is useful for message input boxes such as the one above. For example the user may have typed in a long message, but the program truncates it to say 200 characters.
   • A text prompt is useful to explain to the user what is needed within each input box. For instance the first two fields above is telling the user to enter a valid email address.

6. Type format validation

   • Type validation is checking that the input conforms to the allowed data types.
   • he data format is often important, inputting data records into a database often needs to check that the input is in the correct format.