# Strategy Pattern

version 2.5.20

The 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:

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.

## 1. Example

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

    interface Calc {
        def execute(n, m)
    }

    class CalcByMult implements Calc {
        def execute(n, m) { n * m }
    }

    class CalcByManyAdds implements Calc {
        def execute(n, m) {
            def result = 0
            n.times{
                result += m
            }

            result
        }
    }

    def sampleData = [
        [3, 4, 12],
        [5, -5, -25]
    ]

    Calc[] multiplicationStrategies = [
        new CalcByMult(),
        new CalcByManyAdds()
    ]

    sampleData.each{ data ->
        multiplicationStrategies.each { calc ->
            assert data[2] == calc.execute(data[0], data[1])
        }
    }

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 polymorphism to invoke the algorithms.

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

    def multiplicationStrategies = [
        { n, m -> n * m },
        { n, m -> def result = 0; n.times{ result += m }; result }
    ]

    def sampleData = [
        [3, 4, 12],
        [5, -5, -25]
    ]

    sampleData.each{ data ->
        multiplicationStrategies.each { calc ->
            assert data[2] == calc(data[0], data[1])
        }
    }