What is the difference between the template method and the strategy patterns?
Can someone please explain to me what is the difference between the template method pattern and the strategy pattern is?
As far as I can tell they are 99% the same - the only difference being that the template method pattern has an abstract class as the base class whereas the strategy class uses an interface that is implemented by each concrete strategy class.
However, as far as the client is concerned they are consumed in exactly the same way - is this correct?
Answer
The template pattern is used when a particular operation has some invariant behavior(s) that can be defined in terms of other varying primitive behaviors. The abstract class defines the invariant behavior(s), while the implementing classes defined the dependent methods.
In a strategy, the behavior implementations are independent -- each implementing class defines the behavior and there is no code shared between them. Both are behavioral patterns and, as such, are consumed in much the same way by clients. Typically strategies have a single public method -- the execute() method, whereas templates may define a set of public methods as well as a set of supporting private primitives that subclasses must implement.
The two patterns could easily be used together. You might have a strategy pattern where several implementations belong to a family of strategies implemented using a template pattern.