Template Method

Defines the skeleton of an algorithm in a base class, deferring specific steps to subclasses.

The Problem

Multiple classes implement similar algorithms with the same structure but different steps. Duplicated scaffolding code exists in every class.

How It Works

›Base class defines the algorithm's skeleton — the overall sequence of steps
›Subclasses override specific steps without changing the overall structure
›final on the template method prevents subclasses from altering the sequence
›Hook methods are optional overrides that provide extension points
›Hollywood Principle: 'Don't call us, we'll call you' — the base class drives

When to Use

✓Multiple classes share the same algorithm structure with only some steps differing

✓Framework code that calls user-defined steps at specific points

✓Eliminating code duplication across similar classes

✓Data processing pipelines: fetch → parse → format → store

Real-World Examples

🍎

iOS

UIViewController lifecycle (viewDidLoad, viewWillAppear, viewDidAppear), UIView draw(_:)

🤖

Android

Fragment lifecycle (onCreate, onCreateView, onResume), Activity onCreate flow

🌐

Web

React component lifecycle, Express route handler flow, Jest test lifecycle (beforeEach / afterEach)

Code Examples

Swift

class ReportGenerator {
    // Template method — final so subclasses can't change the sequence
    final func generate() -> String {
        let data    = fetchData()
        let parsed  = parseData(data)
        let report  = formatReport(parsed)
        return report
    }

    func fetchData() -> String { "raw:server" }
    func parseData(_ raw: String) -> [String] { [raw] }
    func formatReport(_ items: [String]) -> String { items.joined(separator: "\n") }
}

class JSONReport: ReportGenerator {
    override func parseData(_ raw: String) -> [String] { ["JSON:\(raw)"] }
    override func formatReport(_ items: [String]) -> String {
        "{\(items.joined(separator: ","))}"
    }
}

class CSVReport: ReportGenerator {
    override func formatReport(_ items: [String]) -> String {
        items.joined(separator: ",")
    }
}

print(JSONReport().generate())  // {JSON:raw:server}
print(CSVReport().generate())   // raw:server