Builder pattern

1. Demand for housing projects

• Need to build a house: this process is piling, building walls and capping
• There are all kinds of houses, such as ordinary houses, high-rise buildings, villas. Although the process of all kinds of houses is the same, the requirements are not the same
• Please write a program to complete the requirements

2. Solve the housing demand in traditional ways

• Train of thought analysis (Figure interpretation)

  

• Code demonstration:

  public abstract class AbstractHouse {
      //Laying foundation
      public abstract void buildBasic();
      //Build a wall
      public abstract void buildWalls();
      //Capping
      public abstract void roofed();
      public void build() {
          buildBasic();
          buildWalls();
          roofed();
      }
  }
  public class CommonHouse extends AbstractHouse {
      @Override
      public void buildBasic() {
          System.out.println(" Laying foundations for ordinary houses ");
      }
      @Override
      public void buildWalls() {
          System.out.println(" Ordinary house wall ");
      }
      @Override
      public void roofed() {
          System.out.println(" Ordinary house capping ");
      }
  }
  public class Client {
      public static void main(String[] args) {
          CommonHouse commonHouse = new CommonHouse();
          commonHouse.build();
      }
  }
  

3. Analysis of traditional ways to solve the problem of building demand

• The advantages are easy to understand, simple and easy to operate.
• The designed program structure is too simple, there is no cache layer object, and the program expansion and maintenance are not good In other words, this design scheme encapsulates the product (i.e. house) and the process of creating the product (i.e. house building process), and the coupling is enhanced.
• Solution: decouple product and product construction process = > builder mode

4. Basic introduction of builder mode

Basic introduction

1. Builder Pattern, also known as Builder Pattern, is an object construction pattern. It can abstract the construction process of complex objects (abstract categories), so that different implementation methods of this abstract process can construct objects with different representations (attributes)
2. Builder mode is to create a complex object step by step. It allows users to build complex objects only by specifying their types and contents. Users do not need to know the specific internal construction details.

Four roles of builder model

• Product role: a specific product object
• Builder (Abstract builder): create an interface / abstract class specified by each part of a Product object
• ConcreteBuilder: implement interfaces, build and assemble various components.
• Director: build an object that uses the Builder interface. It is mainly used to create a complex object. It has two main functions: one is to isolate the production process of customers and objects; the other is to control the production process of product objects.

Schematic class diagram of builder mode

5. The builder mode solves the building demand

• Need to build a house: this process is piling, building walls and capping. Both ordinary houses and villas need to go through these processes. Next, we use builder pattern to complete them

• Train of thought analysis diagram (class diagram)

• code implementation

public class Client {
    public static void main(String[] args) {
        //Build an ordinary house
        CommonHouse commonHouse = new CommonHouse();
        //The commander ready to create the house
        HouseDirector houseDirector = new HouseDirector(commonHouse);
        //Finish building a house and return to the product (ordinary house)
        House house = houseDirector.constructHouse();
        //System.out.println("output process");
        System.out.println("--------------------------");
        //Build a tall building
        HighBuilding highBuilding = new HighBuilding();
        //Reset builder
        houseDirector.setHouseBuilder(highBuilding);
        //Finish building the house and return to the product (high-rise building)
        houseDirector.constructHouse();
    }
}
//Product - > Product
public class House {
    private String baise;
    private String wall;
    private String roofed;
    public String getBaise() {
        return baise;
    }
    public void setBaise(String baise) {
        this.baise = baise;
    }
    public String getWall() {
        return wall;
    }
    public void setWall(String wall) {
        this.wall = wall;
    }
    public String getRoofed() {
        return roofed;
    }
    public void setRoofed(String roofed) {
        this.roofed = roofed;
    }
}
// Abstract builder
public abstract class HouseBuilder {
    protected House house = new House();
    //Write the construction process and abstract method
    public abstract void buildBasic();
    public abstract void buildWalls();
    public abstract void roofed();
    //Build a house and return the product (house)
    public House buildHouse() {
        return house;
    }
}
//Commander, go here to specify the production process and return the product
public class HouseDirector {
    HouseBuilder houseBuilder = null;
    //Constructor passed into houseBuilder
    public HouseDirector(HouseBuilder houseBuilder) {
        this.houseBuilder = houseBuilder;
    }
    //Pass in houseBuilder through setter
    public void setHouseBuilder(HouseBuilder houseBuilder) {
        this.houseBuilder = houseBuilder;
    }
    //How to deal with the process of building a house and leave it to the commander
    public House constructHouse() {
        houseBuilder.buildBasic();
        houseBuilder.buildWalls();
        houseBuilder.roofed();
        return houseBuilder.buildHouse();
    }
}
public class HighBuilding extends HouseBuilder {
    @Override
    public void buildBasic() {
        System.out.println(" The foundation of the tall building is 100 meters ");
    }
    @Override
    public void buildWalls() {
        System.out.println(" Wall construction of tall buildings 20 cm ");
    }
    @Override
    public void roofed() {
        System.out.println(" Transparent roof of tall buildings ");
    }
}
public class CommonHouse extends HouseBuilder {
    @Override
    public void buildBasic() {
        // TODO Auto-generated method stub
        System.out.println(" The foundation of an ordinary house is 5m ");
    }
    @Override
    public void buildWalls() {
        // TODO Auto-generated method stub
        System.out.println(" Ordinary house walling 10 cm ");
    }
    @Override
    public void roofed() {
        // TODO Auto-generated method stub
        System.out.println(" Ordinary house roof ");
    }
}

6. Application and source code analysis of builder mode in JDK

• java. Builder pattern in lang.stringbuilder

  

  

• Analysis on the role of builder mode in source code

  • The Appendable interface defines multiple append methods (abstract methods), that is, Appendable is an abstract builder and defines abstract methods
  • AbstractStringBuilder implements the Appendable interface method. AbstractStringBuilder here is already the builder, but it cannot be instantiated
  • StringBuilder acts as both the commander and the specific builder. The implementation of the construction method is completed by AbstractStringBuilder, which inherits AbstractStringBuilder

7. Precautions and details of builder mode

1. The client (user program) does not need to know the details of the internal composition of the product, and decouples the product itself from the product creation process, so that the same creation process can create different product objects

2. Each specific builder is relatively independent and has nothing to do with other specific builders. Therefore, it is convenient to replace or add new specific builders. Users can get different product objects by using different specific builders

3. You can more finely control the product creation process. The creation steps of complex products are decomposed into different methods, which makes the creation process clearer and more convenient to use programs to control the creation process

4. Adding a new specific builder does not need to modify the code of the original class library. The commander class is programmed for the abstract builder class, which is convenient for system expansion and conforms to the "opening and closing principle"

5. The products created by the builder mode generally have more in common and their components are similar. If there are great differences between products, it is not suitable to use the builder mode, so its scope of use is limited.

6. If the internal changes of the product are complex, it may lead to the need to define many specific builder classes to realize this change, resulting in the system becoming very large. Therefore, in this case, it is necessary to consider whether to choose the builder mode

7. Abstract factory pattern VS builder pattern

   Abstract factory mode realizes the creation of product family. A product family is a series of products: product combinations with different classification dimensions. Adopting abstract factory mode does not need to care about the construction process, but only about what products are produced by what factory. The builder model requires the product to be built according to the specified blueprint. Its main purpose is to produce a new product by assembling spare parts