Defensive Design

Defensive Design

1. Code

   • minimum length
   • Maximum length
   • Invalid Duplication
   • required fields

2. Screen Widgets

   • A hard-coded menu is difficult if not impossible for the user to get wrong
     • A good compromise is to accept information through graphical widgets, rather than input text.
   • The calendar widget
     • the user can select a date
       • the user cannot select an invalid value.
   • The colour picker widget
     • this allows them to pick the colour by eye or to enter a code if they know exactly what they require
     • the user has the potential to get things wrong by entering an invalid value
   • Drop down list and spinners
     • presents the user with a number of items that they are allowed to select
     • allows a number to be selected by clicking on an up or down icon

3. Repeat Entry

   • Users can mistype, or enter data into the wrong field, or simply input gibberish.
   • 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.
   • password entry form
     • The user is presented with a set of rules that their password must follow
       • these rules should make it clear what kind of password will be accepted by the application.

4. The input box

   • The data input type that is most vulnerable to problems - deliberate or otherwise - is the input box and access to keyboard 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'.
     • Validation seeks to ensure that the data entered is valid for further processing by the program handling it.
   • The four validation methods
     • Checking and limiting the Length of the data
     • Checking the Range of the data
     • Checking the Type of the data
     • Checking the Format of the data

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