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 Widgers

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

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.
   • Validation seeks to ensure that the data entered is valid for further processing by the program handling it. The four validation methods are Checking and limiting the Length of the data ,Checking the Range of the data ,Checking the Type of the data and Checking the Format of the data

5. Validation: Length and Range

   • 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.
   • Range validation is used when inputs have to fall between certain values. If a form asks for the user's age, then it makes sense to check that they do not input a negative number, or a decimal. Defensive design in this case is writing the software code to check that these conditions are met.

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