src/spec/doc/_design-pattern-template-method.adoc

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= Template Method Pattern


The https://en.wikipedia.org/wiki/Template_method_pattern[Template Method Pattern] abstracts
away the details of several algorithms.
The generic part of an algorithm is contained within a base class.
Particular implementation details are captured within child classes.
The generic pattern of classes involved looks like this:

[plantuml, TemplateMethodClasses, png]
....
skinparam nodesep 100
class AbstractClass {
    +algorithm1()
    +algorithm2()
    {abstract} +primitiveOperationA()
    {abstract} +primitiveOperationB()
}
class ConcreteClass1 {
    +primitiveOperationA()
    +primitiveOperationB()
}
class ConcreteClass2 {
    +primitiveOperationA()
    +primitiveOperationB()
}
object templateMethodUser
hide fields
templateMethodUser ......r......> "<<use>>" AbstractClass
AbstractClass <|-- ConcreteClass1
AbstractClass <|-- ConcreteClass2
....

== Example with traditional classes

In this example, the base `Accumulator` class captures the essence of the accumulation algorithm.
The child classes `Sum` and `Product` provide particular customised ways to use the generic accumulation algorithm.

[source,groovy]
----
include::../test/DesignPatternsTest.groovy[tags=template_method_example,indent=0]
----

== Example with simplifying strategies

In this particular case, you could use Groovy's inject method to achieve a similar result using Closures:

[source,groovy]
----
include::../test/DesignPatternsTest.groovy[tags=template_method_example2,indent=0]
----

Thanks to duck-typing, this would also work with other objects which support an add (`plus()` in Groovy) method, e.g.:

[source,groovy]
----
include::../test/DesignPatternsTest.groovy[tags=template_method_example3,indent=0]
----

We could also do the multiplication case as follows (re-writing as a one-liner):

[source,groovy]
----
include::../test/DesignPatternsTest.groovy[tags=template_method_example4,indent=0]
----
Using closures this way looks like the <<_strategy_pattern,Strategy Pattern>>, but if we realise
that Groovy's `inject` method is the generic part of the algorithm for our template method,
then the Closures become the customised parts of the template method pattern.

For Groovy 3+, we can use lambda syntax as an alternative to the closure syntax:

[source,groovy]
----
include::../test/DesignPatternsTest.groovy[tags=template_method_example5,indent=0]
----

Here the stream api's `reduce` method is the generic part of the algorithm for our template method,
and the lambdas are the customised parts of the template method pattern.