Design Pattern: Template Method Pattern

Summary

Pattern Name	Template Method Pattern
Pattern Type	Behavioral Pattern
Scope	Class
Tagline	Let subclasses redefine certain steps of algorithm
Use cases	When most of the steps will be the same for all subclasses and we want to rewrite a few steps in each subclass
Related Patterns	Factory Method Strategy
Difficulty Level	Easy
Implementations

Definition

Template method pattern when we want the subclasses to define some steps of the processing steps (or algorithm). The parent class can define some steps and subclasses can define some steps(which are required).

So the parent(abstract) class works as an outline template for the algorithm or lifecycle of the process.

Functionalities/steps which are completely different for subclasses are left to the subclasses for definition. The parent (abstract) class works are a skeleton here.

Use Cases

Here are a few use cases of the Template Method design pattern-

Implementation

Template Method pattern has 2 elements.

1. Abstract Class: this defines some functions, which are common for all subclasses, and declares other methods which are different for subclasses. Then subclasses can define those methods for the functionality specific to that subclass.
2. Concrete Classes: extends the abstract class. Defines the functions for the abstract functions.

Follow the steps below to implement Template Method pattern:

1. Create an abstract class.
2. In the abstract class, define functions that will be common for all the sub-classes.
3. In the abstract class, declare functions as abstract, for which functionalities are different for each subclass.
4. In the abstract class, create a function that uses all the functions and performs the processing.
5. Create concrete classes and extend the abstract class.
6. Implement the required abstract methods in the concrete class.

Examples

Example #1: Transport Builder

Let’s implement Template Method for a transport builder.

Transport Template Method

abstract class Transport

    abstract createBody()

    abstract addEngine()

    abstract addWheel()

    // Required only for Plane
    abstract addWing()

    method addSeat()
        // Add seats to the vehicle

        // Adding seats are same for all transports so same functions for all

        output "Adding seats"
    end method

    method build()
        createBody()

        addEngine()

        addWheel()

        addWing()

        addSeat()
    end method

end method

Transport Classes

// Car class

class Car extends Transport

    method createBody()
        output "Creating Car Body"
    end method

    method addEngine()
        output "Adding Engine to Car"
    end method

    method addWheel()
        output "Adding 4 Wheels to Car"
    end method

    method addWing()
        // Not required for Car
    end method

end class


// Bike class

class Bike extends Transport {
    
    method createBody() 
        output "Creating Bike Body"
    end method

    method addEngine()
        output "Adding Engine to Bike"
    end method

    method addWheel()
        output "Adding 2 Wheels to Bike"
    end method

    method addWing()
        // Not required for Bike
    end method

end class


// Plane class

class Plane  extends Transport 

    method createBody()
        output "Creating Plane Body"
    end method

    method addEngine()
        output "Adding Engine to Plane"
    end method

    method addWheel()
        output "Adding 3 Wheels to Plane"
    end method

    method addWing()
        output "Adding Wings Plane"
    end method

end class


// Train class

class Train extends Transport

    method createBody()
        output "Creating Train Body"
    end method

    method addEngine()
        output "Adding Engine to Train"
    end method

    method addWheel()
        output "Adding Wheels to Train"
    end method

    method addWing()
        // Not required for Train
    end method

end class

Context

// Context

class Context

    var state: State

    method setState(stateParam: State)
        state = stateParam
    end method

    method getState(): State
        return state
    end method

    method performActionOne()
        state.actionOne()
    end method


    method performActionTwo()
        state.actionTwo()
    end method

end class

Demo

var car: Transport = new Car()
car.build()

var plane: Transport = new Plane()
plane.build()

Output

Creating Car Body
Adding Engine to Car
Adding 4 Wheels to Car
Adding seats


Creating Plane Body
Adding Engine to Plane
Adding 3 Wheels to Plane
Adding Wings Plane
Adding seats

Code Implementations

Use the following links to check State pattern implementation in specific programming languages.