Builder pattern-code understanding

table of Contents

introduction:

1. Builder mode

2. Code

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introduction:

What scenario is the builder mode suitable for? : Some basic components will not change, but their combination often changes.

Reference: https://www.runoob.com/design-pattern/builder-pattern.html

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1. Builder mode

Intent: To separate a complex construction from its representation, so that the same construction process can create different representations.

Main solution: The main solution is that in software systems, sometimes faced with the creation of " a complex object ", which is usually composed of sub-objects of various parts with a certain algorithm; due to changes in requirements, various parts of this complex object often face There are drastic changes, but the algorithm that combines them is relatively stable.

When to use: Some basic components will not change, but the combination often changes.

How to solve it: Separate the change from the unchanging.

Key code: Builder: Create and provide examples, Director: Manage dependencies of constructed examples.

Application examples:  1. Going to KFC, burgers, cola, french fries, fried chicken wings, etc. are unchanged, and their combination changes frequently, resulting in a so-called "set meal". 2. StringBuilder in JAVA.

Advantages:  1. The builder is independent and easy to expand. 2. It is easy to control detailed risks.

Disadvantages:  1. The products must have common points and the scope is limited. 2. If the internal changes are complicated, there will be many construction types.

Usage scenarios:  1. The object to be generated has a complicated internal structure. 2. The internal properties of the objects that need to be generated depend on each other.

Note: The difference with the factory mode is that the builder mode pays more attention to the order of parts assembly.

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2. Code

As shown in the figure, it shows the interface diagram of a fast food restaurant that provides a "package" that provides a variety of food combinations. The purpose is to be able to return the corresponding "meal" according to customer requests.

Scenario: The basic components of food may not change much, but the packages provided may have different combinations.

• First design an interface that represents food items, and then different types of food implement this interface;
• Food has packaging, so design an interface that represents the packaging, and the specific packaging implements this interface to form an implementation class;

Specific code reference article.

Core class:

1) Meal class with Item object:

Meal.java

import java.util.ArrayList;
import java.util.List;
 
public class Meal {
   private List<Item> items = new ArrayList<Item>();    
 
   public void addItem(Item item){
      items.add(item);
   }
 
   public float getCost(){
      float cost = 0.0f;
      for (Item item : items) {
         cost += item.price();
      }        
      return cost;
   }
 
   public void showItems(){
      for (Item item : items) {
         System.out.print("Item : "+item.name());
         System.out.print(", Packing : "+item.packing().pack());
         System.out.println(", Price : "+item.price());
      }        
   }    
}

2) MealBuilder class, the actual builder class is responsible for creating Meal objects, that is, returning different combinations.

MealBuilder.java

public class MealBuilder {
 
   public Meal prepareVegMeal (){
      Meal meal = new Meal();
      meal.addItem(new VegBurger());
      meal.addItem(new Coke());
      return meal;
   }   
 
   public Meal prepareNonVegMeal (){
      Meal meal = new Meal();
      meal.addItem(new ChickenBurger());
      meal.addItem(new Pepsi());
      return meal;
   }
}

3) Use MealBuilder to generate Meal objects and test:

public class BuilderPatternDemo {
   public static void main(String[] args) {
      MealBuilder mealBuilder = new MealBuilder();
 
      Meal vegMeal = mealBuilder.prepareVegMeal();
      System.out.println("Veg Meal");
      vegMeal.showItems();
      System.out.println("Total Cost: " +vegMeal.getCost());
 
      Meal nonVegMeal = mealBuilder.prepareNonVegMeal();
      System.out.println("\n\nNon-Veg Meal");
      nonVegMeal.showItems();
      System.out.println("Total Cost: " +nonVegMeal.getCost());
   }
}

执行程序，输出结果：

Veg Meal
Item : Veg Burger, Packing : Wrapper, Price : 25.0
Item : Coke, Packing : Bottle, Price : 30.0
Total Cost: 55.0


Non-Veg Meal
Item : Chicken Burger, Packing : Wrapper, Price : 50.5
Item : Pepsi, Packing : Bottle, Price : 35.0
Total Cost: 85.5

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3. Summary

In one sentence: Provide an object that returns different combinations of several components.