🏛Template Method Pattern

Define the skeleton of an algorithm in an operation — defer some steps to subclasses.

📌 Key IdeaBase class defines `final` template method calling primitive operations (abstract or hooks). Invariant sequence stays closed; steps open for subclasses. Common with Factory Method on same hierarchy.

🖼

Visual

🏛 `final` orchestrates hooks

  abstract class DataExporter {
      final void export() {        // template method
          open();
          writeRows();             // subclass
          close();
      }
  }

Hollywood principle — base calls subclasses at defined points.

✅

When to Use

Several variants share same control flow but differ in steps (ETL export, UI wizards).
You want to lock ordering of operations but allow customisation.
Framework hooks (Servlet.service template, JUnit lifecycle).

🚫

When NOT to Use

Algorithms diverge completely — Strategy composition clearer.
Deep inheritance trees become fragile — favour composition.

🚨

Anti-Pattern (What NOT to Do)

Duplicate export pipelinesBadjava

void exportCsv(Data d) { open(); writeCsv(d); close(); }
void exportJson(Data d) { open(); writeJson(d); close(); }

open/close duplicated; swap order risk.

✅

The Right Way

Template fixes sequence — subclasses fill hooksGoodjava

abstract class ExportJob {
    final void run(Data d) {
        open();
        writeBody(d);   // subclass
        close();
    }
    protected abstract void writeBody(Data d);
}

Hooks like beforeWrite() default empty — optional override.

⚖️

Pros & Cons

👍 Pros

Code reuse of invariant structure.
Hooks (doHook() default empty) optional extension points.

👎 Cons

Inheritance coupling — subclass tied to base design.
Harder to test pieces without subclassing.

🔀

Common Confusions Cleared Up

Template Method vs Strategy

Template Method uses inheritance — skeleton in superclass. Strategy uses composition — context delegates to injected strategy object.

🎤

Interview Q&A

🔮

Likely Follow-up Questions

How does this pair with Factory Method in same class hierarchy?

🧾

Quick Cheat Sheet

IntentFixed algorithm outline; vary steps.

Mechanismfinal template + hooks / abstract prims.

vs StrategyInheritance vs composed strategy object.

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