Clean Architecture & Design Patterns in Practice · Lesson

Template Method and State Patterns

Define algorithm skeletons with Template Method and allow objects to alter behavior with State.

Template Method and State Patterns is a free Clean Architecture & Design Patterns in Practice lesson on CoddyKit — lesson 3 of 4. You can read the complete lesson below for free — then practise it hands-on in the browser with a built-in code editor and a 24/7 AI tutor. It is part of the Clean Architecture & Design Patterns in Practice learning path, one of 4 lessons in the course, and your progress syncs across the web and the CoddyKit app.

Behavioral Patterns Focus

Welcome back! In this lesson, we'll dive into two powerful behavioral design patterns: the Template Method and the State patterns.

These patterns help manage algorithms and object behavior in flexible ways, making your code easier to extend and maintain.

What is Template Method?

The Template Method pattern defines the skeleton of an algorithm in an operation, deferring some steps to subclasses.

It lets subclasses redefine certain steps of an algorithm without changing the algorithm's overall structure.

Algorithm Skeleton

Imagine a recipe with fixed steps, but some ingredients or cooking times can vary. The core "how-to" is set, but details are flexible.

An abstract class defines the overall algorithm with a final "template method."
This method calls a series of primitive operations (abstract methods) that subclasses must implement.
It can also include concrete methods (shared steps) and hook methods (optional steps).

Template Method in Action

Let's consider making beverages. Both coffee and tea require boiling water and pouring into a cup. But brewing and adding condiments differ.

The Template Method allows us to define the common steps once, while allowing specific beverage classes to customize their unique steps.

Beverage Maker Example

Here's how we can implement a generic BeverageMaker using the Template Method. Notice the prepareBeverage() method is final, fixing the algorithm.

public abstract class BeverageMaker {

  // The template method - defines the algorithm's skeleton
  public final void prepareBeverage() {
    boilWater();
    brew();
    pourInCup();
    addCondiments();
  }

  // Common steps
  private void boilWater() {
    System.out.println("Boiling water");
  }

  private void pourInCup() {
    System.out.println("Pouring into cup");
  }

  // Abstract steps - must be implemented by subclasses
  protected abstract void brew();
  protected abstract void addCondiments();
}

public class CoffeeMaker extends BeverageMaker {
  @Override
  protected void brew() {
    System.out.println("Dripping coffee through filter");
  }

  @Override
  protected void addCondiments() {
    System.out.println("Adding sugar and milk");
  }
}

public class TeaMaker extends BeverageMaker {
  @Override
  protected void brew() {
    System.out.println("Steeping the tea bag");
  }

  @Override
  protected void addCondiments() {
    System.out.println("Adding lemon");
  }
}

public class Main {
  public static void main(String[] args) {
    System.out.println("--- Making Coffee ---");
    BeverageMaker coffee = new CoffeeMaker();
    coffee.prepareBeverage();

    System.out.println("\n--- Making Tea ---");
    BeverageMaker tea = new TeaMaker();
    tea.prepareBeverage();
  }
}

What is the State Pattern?

The State pattern allows an object to alter its behavior when its internal state changes. It appears as if the object has changed its class.

Instead of using many if/else or switch statements, you encapsulate each state's behavior into a separate class.

Context and States

Think of a traffic light. Its behavior (what light is active) changes based on its current state (Red, Yellow, Green).

The Context class holds a reference to a State object and delegates state-specific behavior to it.
The State Interface declares methods for state-specific behaviors.
Concrete State classes implement the State Interface, providing behavior for a particular state.

Traffic Light States

A TrafficLight object doesn't have complex logic itself. Instead, it holds a reference to its current LightState (e.g., RedLightState, GreenLightState).

When an event occurs (like a timer tick), the TrafficLight delegates the action to its current LightState object, which then handles the transition to the next state.

Traffic Light Simulation

Here’s a simple traffic light simulation using the State pattern. The TrafficLight context changes its internal LightState object, which dictates its behavior.

// State Interface
interface LightState {
  void handleRequest(TrafficLight light);
  String getStateName();
}

// Concrete State: Red Light
class RedLightState implements LightState {
  @Override
  public void handleRequest(TrafficLight light) {
    System.out.println("Red light: STOP!");
    light.setState(new GreenLightState()); // Transition to Green
  }

  @Override
  public String getStateName() {
    return "Red";
  }
}

// Concrete State: Green Light
class GreenLightState implements LightState {
  @Override
  public void handleRequest(TrafficLight light) {
    System.out.println("Green light: GO!");
    light.setState(new YellowLightState()); // Transition to Yellow
  }

  @Override
  public String getStateName() {
    return "Green";
  }
}

// Concrete State: Yellow Light
class YellowLightState implements LightState {
  @Override
  public void handleRequest(TrafficLight light) {
    System.out.println("Yellow light: CAUTION!");
    light.setState(new RedLightState()); // Transition to Red
  }

  @Override
  public String getStateName() {
    return "Yellow";
  }
}

// Context
class TrafficLight {
  private LightState currentState;

  public TrafficLight() {
    // Initial state
    this.currentState = new RedLightState();
  }

  public void setState(LightState state) {
    this.currentState = state;
  }

  public void change() {
    System.out.print("Current state: " + currentState.getStateName() + " -> ");
    currentState.handleRequest(this);
  }
}

public class Main {
  public static void main(String[] args) {
    TrafficLight light = new TrafficLight();

    // Simulate light changes
    light.change(); // Red -> Green
    light.change(); // Green -> Yellow
    light.change(); // Yellow -> Red
    light.change(); // Red -> Green
  }
}

Pattern Identification

You are designing a document processing system where different document types (PDF, DOCX, TXT) share a common conversion process to HTML, but each has unique steps for parsing its content.

Which design pattern would best suit defining the overall conversion process while allowing specific parsing steps to vary?

Summary of Patterns

Great job! You've learned about two powerful behavioral patterns:

The Template Method pattern allows you to define a fixed algorithm structure while letting subclasses implement specific steps.
The State Pattern enables an object to change its behavior based on its internal state, encapsulating state-specific logic into separate classes.

These patterns boost flexibility and maintainability in your designs!

Frequently asked questions

Is the “Template Method and State Patterns” lesson free?

Yes — the full text of “Template Method and State Patterns” is free to read here on the web, and the Clean Architecture & Design Patterns in Practice course includes 4 lessons in total. To practise it interactively (a built-in code editor and a 24/7 AI tutor) and unlock the rest of the Clean Architecture & Design Patterns in Practice course, upgrade to CoddyKit PRO.

What will I learn in “Template Method and State Patterns”?

Define algorithm skeletons with Template Method and allow objects to alter behavior with State. You practise Clean Architecture & Design Patterns in Practice with hands-on code you run directly in the browser, and a 24/7 AI tutor answers your questions as you work through the lesson.

Do I need any experience to start Clean Architecture & Design Patterns in Practice?

No prior experience is required. Clean Architecture & Design Patterns in Practice on CoddyKit is structured for beginners through advanced learners; this is — lesson 3 of 4, so you can start here or from the beginning and move at your own pace.

How long does the “Template Method and State Patterns” lesson take?

Most CoddyKit lessons take about 5–10 minutes. Each one is bite-sized and interactive, so you make steady progress and pick up exactly where you left off across the web and the app.

Can I write and run code in this Clean Architecture & Design Patterns in Practice lesson?

Yes. Every Clean Architecture & Design Patterns in Practice lesson includes a built-in code editor, so you write and run real code right in your browser and get instant AI feedback — no local setup required.

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