The Interface Segregation Principle (ISP) - CLIENTS SHOULD NOT BE FORCED TO DEPEND UPON INTERFACES THAT THEY DO NOT USE
*This is the definition straight out of the Object Mentor article book.
Translation: Avoid “fat” interfaces or interface pollution. In other words, do not make your classes implement interfaces that have methods/properties the class does not need. This is a code smell and should be an indicator that the interface should be split into two interfaces and each applied only as needed.
The example in the article (pgs 2-7) explains it all.