Reuse

This rather vague term is just as important a concept as encapsulation and has a great deal to do with reliability and with the reduction of unwanted or unexpected behaviours (bugs).

Code which has been used many times and has proved reliable is less likely to exhibits bugs than ‘new’ code.

Code which is used in many places gets more use and more testing than code that is only used in one place. This is also a reliability enhancer.

Code that is written once and used many times is quicker and cheaper to produce than more individual code elements. This is often stated but it is unlikely that it is actually true. However the thought and the design process necessary to achieve this also adds to reliability.

In Object Oriented Programming re-use can effectively mean creating one object more than once in different programming scenarios to achieve different behaviour.