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= Strategy Pattern

The http://en.wikipedia.org/wiki/Strategy_pattern[Strategy Pattern] allows you to abstract away particular algorithms from their usage. This allows you to easily swap the algorithm being used without having to change the calling code. The general form of the pattern is:

[plantuml, StrategyClasses, png]
....
!pragma layout smetana
hide fields
hide <<Hidden>> circle
skinparam ClassBorderColor<<Hidden>> Transparent
skinparam ClassBackgroundColor<<Hidden>> Transparent
skinparam ClassStereotypeFontColor<<Hidden>> Transparent
skinparam ClassFontSize<<Hidden>> 24
skinparam ClassFontStyle<<Hidden>> bold
skinparam shadowing<<Hidden>> false
class Context {
    +Strategy getStrategy()
}
class Strategy {
    +algorithmMethod()
}
class ConcreteStrategy1 {
    +algorithmMethod()
}
class ConcreteStrategy2 {
    +algorithmMethod()
}
class "..." as ConcreteHidden
class ConcreteHidden <<Hidden>> {
}
class ConcreteStrategyN {
    +algorithmMethod()
}
Context o---r--- Strategy
Strategy <|-- ConcreteStrategy1
Strategy <|-- ConcreteStrategy2
Strategy <|-[hidden]- ConcreteHidden
Strategy <|-- ConcreteStrategyN
....

In Groovy, because of its ability to treat code as a first class object using anonymous methods (which we loosely call __Closures__), the need for the strategy pattern is greatly reduced. You can simply place algorithms inside Closures.

== Example using traditional class hierarchy

First let's look at the traditional way of encapsulating the Strategy Pattern.

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

Here we have defined an interface `Calc` which our concrete strategy classes will implement (we could also have used an abstract class).
We then defined two algorithms for doing simple multiplication: `CalcByMult` the normal way, and ++CalcByManyAdds++ using only addition (don't try this one using negative numbers - yes we could fix this but it would just make the example longer).
We then use normal http://en.wikipedia.org/wiki/Polymorphism_in_object-oriented_programming[polymorphism] to invoke the algorithms.

== Example using closures

Here is the Groovier way to achieve the same thing using Closures:

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

== Example using lambdas

For Groovy 3+, we can leverage lambda syntax:

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

Or we can use the built-in JDK `BiFunction` class:

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