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The Introduce Assertion refactoring recommends that each method (and potentially each constructor) checks its preconditions using assertions.
This is a particular form of defensive programming. Some argue that if you have sufficient tests in place, you don't need to apply defensive programming. For the small extra performance penalty, it seems like a small price to pay for extra resilience in our system.
Example
Suppose we have the following method:
If we call it with valid parameters, everything is fine. If we call it with null we will receive a NullPointerException during the method's execution. This can sometimes be difficult to track down.
Applying this refactoring gives us the following code:
This is better because we become aware of any problems straight away.