What is the difference between the template method

Question

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 1

Template Method:

1. It's based on inheritance
2. Defines skeleton of algorithm which can't be changed by sub classes. Only certain operations can be overridden in sub classes
3. Parent class completely controls the algorithm and differs only certain steps to concrete classes
4. Binding is done at compile time

Template_method structure:

Strategy:

1. It's based on delegation/composition
2. It changes guts of the object by modifying method behaviour
3. It's used to switch between family of algorithms
4. It changes the behaviour of the object at run time by completely replacing one algorithm with other algorithm at run time
5. Binding is done at run time

Strategy structure:

Have a look at Template method and Strategy articles for better understanding.

Related posts:

Template design pattern in JDK, could not find a method defining set of methods to be executed in order

Real World Example of the Strategy Pattern

Answer 2

In strategy pattern subclasses are running the show and they control the algorithm. Here code is duplicated across the subclasses. The knowledge of the algorithm and how to implement it is distributed over many classes.

In template pattern, base class has algorithm. It maximizes the reuse among the subclasses. Since algorithm lies in one place, base class protects it.

Answer 3

I think the Class-Diagrams of both pattern are showing the differences.

Strategy
Encapsulates an algorithm inside a class
Link to image

Template Method
Defer the exact steps of an algorithm to a subclass
Link to Image

Answer 4

No, they are not necessarily consumed in the same way. The "template method" pattern is a way of providing "guidance" to future implementers. You are telling them, "All Person objects must have a Social Security Number" (that's a trivial example but it gets the idea across correctly).

The strategy pattern allows multiple possible implementations to be switched in and out. It is not (usually) implemented through inheritance, but instead by letting the caller pass in the desired implementation. An example might be allowing a ShippingCalculator to be provided with one of several different ways of calculating taxes (a NoSalesTax implementation, and a PercentageBasedSalesTax implementation perhaps).

So, sometimes, the client will actually tell the object which strategy to use. As in

myShippingCalculator.CalculateTaxes(myCaliforniaSalesTaxImpl);

But the client would never do that for an object that was based on Template Method. In fact, the client might not even know an object is based on Template Method. Those abstract methods in the Template Method pattern might even be protected, in which case the client wouldn't even know they exist.