# Template Method Pattern

version 2.5.18

The 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 base classes. The generic pattern of classes involved looks like this:

## 1. Example

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

    abstract class Accumulator {
        protected initial
        abstract doAccumulate(total, v)
        def accumulate(values) {
            def total = initial
            values.each { v -> total = doAccumulate(total, v) }
            total
        }
    }

    class Sum extends Accumulator {
        def Sum() { initial = 0 }
        def doAccumulate(total, v) { total + v }
    }

    class Product extends Accumulator {
        def Product() { initial = 1 }
        def doAccumulate(total, v) { total * v }
    }

    println new Sum().accumulate([1,2,3,4])
    println new Product().accumulate([1,2,3,4])

The resulting output is:

    10
    24

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

    Closure addAll = { total, item -> total += item }
    def accumulated = [1, 2, 3, 4].inject(0, addAll)
    println accumulated    // => 10

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

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

    accumulated = [ "1", "2", "3", "4" ].inject("", addAll)
    println accumulated    // => "1234"

We could also do the multiplication case as follows:

    Closure multAll = { total, item -> total *= item }
    accumulated = [1, 2, 3, 4].inject(1, multAll)
    println accumulated    // => 24

Using closures this way looks more like the Strategy Pattern but if we realise that the built-in 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.