Strategy Pattern

In computer programming, the strategy pattern (also known as the policy pattern) is a particular software design pattern, whereby algorithms can be selected at runtime. Formally speaking, the strategy pattern defines a family of algorithms, encapsulates each one, and makes them interchangeable. Strategy lets the algorithm vary independently from clients that use it.

Called ConcreteStrategyAdd's execute()
Called ConcreteStrategySubtract's execute()
Called ConcreteStrategyMultiply's execute()

http://en.wikipedia.org/wiki/Strategy_pattern