Behavioral · DWG NO. 023

Template Method

Fix the overall steps of an algorithm in a base class, and let subclasses fill in just the steps that vary.

TypeBehavioral
ScopeClass hierarchy
ComplexityLow
Common inData import pipelines, test frameworks, report generation
01 · The problem

The overall steps repeat, but one or two steps differ each time

Importing CSV, JSON, and XML files all follow the same shape — open file, parse, validate rows, save to database — but each format parses differently. Copy-pasting the whole pipeline per format duplicates the steps that are actually identical.

02 · Structure

The base class owns the sequence

A base class defines a method that calls a fixed sequence of steps, some implemented directly, others left abstract. Subclasses override only the steps that differ; the order and the shared steps live in one place.

DataImporterimport()CsvImporterparse() onlyJsonImporterparse() only
03 · Reference implementation

Shared import pipeline, format-specific parsing

`import()` is defined once, in the base class. Each format subclass only implements `parse()`.

data-importer.js 
class DataImporter {
  import(raw) {
    console.log("opening source");
    const rows = this.parse(raw);      // step that varies — implemented by subclass
    const valid = rows.filter((r) => this.validate(r));
    console.log(`saving ${valid.length} rows`);
    return valid;
  }
  validate(row) { return row != null; } // shared default, subclasses may override
  parse(raw) { throw new Error("parse() must be implemented"); }
}

class CsvImporter extends DataImporter {
  parse(raw) { return raw.split("\n").map((line) => line.split(",")); }
}

class JsonImporter extends DataImporter {
  parse(raw) { return JSON.parse(raw); }
}

new CsvImporter().import("a,1\nb,2");
new JsonImporter().import('[{"a":1},{"b":2}]');
04 · When to use it

Trade-offs