In general, very complex conditions are difficult to write and read, and increase the chance of defects.

Firstly, a condition can often be simplified by changing other parts of the code to initialize variables more consistently. For example, is there a semantic difference between id being null and having zero-length? If not, choosing one sentinel value and using it consistently simplifies most uses of that variable.

Secondly, extracting part of a condition into a Boolean-valued method can simplify the condition and also allow code reuse, with all its benefits.

Thirdly, assigning each subcondition of the condition to a local variable, and then using the variables in the condition instead can simplify the condition.

The following example shows a complex condition found in a real program used by millions of people. The condition is so confusing that even the programmer who wrote it is not sure if he got it right (see the TODO comment).

The condition can be simplified by extracting parts of the condition into Boolean-valued methods. These methods are then used in the condition.
  • R. C. Martin, Clean Code: A Handbook of Agile Software Craftsmanship, §17.G28. Prentice Hall, 2008.
  • S. McConnell, Code Complete: A Practical Handbook of Software Construction. Microsoft Press, 2004.