设计模式:模板方法模式

Template Method is a behavioral design pattern. Template Method design pattern is used to create a method stub and deferring some of the steps of implementation to the subclasses.

Template method defines the steps to execute an algorithm and it can provide default implementation that might be common for all or some of the subclasses.

Let’s understand this pattern with an example, suppose we want to provide an algorithm to build a house. The steps need to be performed to build a house are – building foundation, building pillars, building walls and windows. The important point is that the we can’t change the order of execution because we can’t build windows before building the foundation. So in this case we can create a template method that will use different methods to build the house.

Now building the foundation for a house is same for all type of houses, whether its a wooden house or a glass house. So we can provide base implementation for this, if subclasses want to override this method, they can but mostly it’s common for all the types of houses.

To make sure that subclasses don’t override the template method, we should make it final.

Since we want some of the methods to be implemented by subclasses, we have to make our base class as abstract class.

HouseTemplate.java

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package com.journaldev.design.template;

public abstract class  {

	
	public final void buildHouse(){
		buildFoundation();
		buildPillars();
		buildWalls();
		buildWindows();
		System.out.println("House is built.");
	}

	//default implementation
	private void buildWindows() {
		System.out.println("Building Glass Windows");
	}

	//methods to be implemented by subclasses
	public abstract void buildWalls();
	public abstract void buildPillars();

	private void buildFoundation() {
		System.out.println("Building foundation with cement,iron rods and sand");
	}
}

buildHouse() is the template method and defines the order of execution for performing several steps.

We can have different type of houses, such as Wooden House and Glass House.

WoodenHouse.java

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package com.journaldev.design.template;

public class WoodenHouse extends  {

	
	public void buildWalls() {
		System.out.println("Building Wooden Walls");
	}

	
	public void buildPillars() {
		System.out.println("Building Pillars with Wood coating");
	}

}

We could have overridden other methods also, but for simplicity I am not doing that.

GlassHouse.java

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package com.journaldev.design.template;

public class GlassHouse extends  {

	
	public void buildWalls() {
		System.out.println("Building Glass Walls");
	}

	
	public void buildPillars() {
		System.out.println("Building Pillars with glass coating");
	}

}

Let’s test our template method pattern example with a test program.

HousingClient.java

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package com.journaldev.design.template;

public class HousingClient {

	public static void main(String[] args) {
		
		HouseTemplate houseType = new WoodenHouse();
		
		//using template method
		houseType.buildHouse();
		System.out.println("************");
		
		houseType = new GlassHouse();
		
		houseType.buildHouse();
	}

}

Notice that client is invoking the template method of base class and depending of implementation of different steps, it’s using some of the methods from base class and some of them from subclass.

Output of the above program is:

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Building foundation with cement,iron rods and sand
Building Pillars with Wood coating
Building Wooden Walls
Building Glass Windows
House is built.
************
Building foundation with cement,iron rods and sand
Building Pillars with glass coating
Building Glass Walls
Building Glass Windows
House is built.