Template Method Pattern

May 31, 2026

The Template Method Pattern is a design pattern where a base class defines the fixed skeleton of an operation and calls out to specific steps, or "hooks", that subclasses override to customise behaviour (Gamma, 1995). This locks the overall control flow and lifecycle into the base class while letting subclasses supply only the parts that vary.

For example, the base class owns the export() skeleton, marks writeBody() as a step subclasses must implement, and leaves afterWrite() as an optional hook:

abstract class DataExporter {
  // Template method — the fixed skeleton, never overridden
  export(): void {
    this.open();
    this.writeBody();    // varies per subclass
    this.afterWrite();   // optional hook
    this.close();
  }

  protected abstract writeBody(): void;  // subclasses MUST implement
  protected afterWrite(): void {}         // hook — no-op by default

  private open(): void { /* ...shared setup... */ }
  private close(): void { /* ...shared teardown... */ }
}

class CsvExporter extends DataExporter {
  protected writeBody(): void {
    // write rows as CSV
  }
}

References

Erich Gamma, editor. Design Patterns: Elements of Reusable Object-Oriented Software. Addison-Wesley Professional Computing Series. Addison-Wesley, Reading, Mass, 1995. ISBN 978-0-201-63361-0. ↩