Creational_Builder Pattern

🤷🏻‍♀️ What would happen if we did not have builder?

• Imagine we have a trip class
• attributes such as place, how many nights, how many days, hotel, start date…
• when we create a trip,

• we have some requirements

• ⚠️ how many days should be how many nights + 1
• ⚠️ for a one-day trip, we do not want to include how many nights, how many days, hotel

• 💊 maybe we can use several constructors?

• 👍🏻 When creation of an instance is complicated
• condition among attributes,
• several constructors needed
• we can use a builder pattern to facilitate the creation of an instance

✅ Diagram

👎🏻 Before builder

• tour plan has many attributes

public class TourPlan {
    private String title;
    private int nights;
    private int days;
    private LocalDate startDate;
    private String whereToStay;
    private List<DetailPlan> plans;
}

• In main class

//for several day trip, need to set several attributes
TourPlan longTripPlan = new TourPlan("Spain", 3, 4, "hotel"...);
longTripPlan.setTitle("Spain");
longTripPlan.setNights(3);
longTripPlan.setDays(4); //day should always be night + 1
// 👎🏻 how can I set conditions among attributes
// when creating instance?

//for one day trip, I only want to set name
// 👎🏻 need several constructors with different parameters
TourPlan onedayPlan = new TourPlan("beach");

👍🏻 After implementing builder pattern

✔️ Builder class

• methods return the builder class itself,
• so later method chaining is possible
• look at App

public interface TourPlanBuilder {
    //methods return itself, 내가 내 자신을 return함으로써 내 자신의 메소드 또 부르기 가능
    TourPlanBuilder title(String title);
    TourPlanBuilder nightsAndDays(int nights, int days);
    TourPlanBuilder startDate(LocalDate localDate);
    TourPlanBuilder whereToStay(String whereToStay);
    TourPlanBuilder addPlan(int day, String plan);

    //last, return Plan
    TourPlan getPlan(); //and finally return the plan

}

✔️ Concrete Builder class

public class DefaultTourBuilder implements TourPlanBuilder{
    private String title;
    private int nights;
    private int days;
    private LocalDate startDate;
    private String whereToStay;
    private List<DetailPlan> plans;

    @Override
    public TourPlanBuilder title(String title) {
        this.title = title;
        return this;
    }
    
    //override all methods

    @Override
    public TourPlan getPlan() {
        return new TourPlan(title, nights, days, startDate, whereToStay, plans); //return Plan
    }
}

✔️ Main class

public class App {
    public static void main(String[] args) {
        TourPlanBuilder builder = new DefaultTourBuilder();

        // 👍🏻 without creating many constructors, can call methods I want from builder class
        TourPlan shortPlan = builder.title("beach")
                .startDate(LocalDate.of(2020, 10, 10))
                .getPlan();

        TourPlan longPlan = builder.title("Spain")
                .nightsAndDays(4, 5)
                .whereToStay("hotel")
                .startDate(LocalDate.of(2020, 10, 10))
                .addPlan(0, "madrid")
                .addPlan(1, "barcelona")
                .addPlan(2, "sevilla")
                .addPlan(3, "toledo")
                .getPlan();
    }

✔️ Director class

• if instance is created in the same way several times,
• create director class
• can hide how to make the instance

public class Director {
    private TourPlanBuilder tourPlanBuilder;

    public Director(TourPlanBuilder tourPlanBuilder) {
        this.tourPlanBuilder = tourPlanBuilder;
    }

    //when I want to set everything for the plan
    public TourPlan SpainPlan(){
        return tourPlanBuilder.title("Spain")
                .nightsAndDays(4, 5)
                .whereToStay("hotel")
                .startDate(LocalDate.of(2020, 10, 10))
                .addPlan(0, "madrid")
                .addPlan(1, "barcelona")
                .addPlan(2, "sevilla")
                .addPlan(3, "toledo")
                .getPlan();
    }

    //when I want to only set title and startDate for plan
    public TourPlan shortPlan(){
        return tourPlanBuilder.title("beach")
                .startDate(LocalDate.of(2020, 10, 10))
                .getPlan();
    }
}

✔️ Main class

public class App {
    public static void main(String[] args) {
        Director director = new Director(builder);
        TourPlan shortPlan1 = director.shortPlan();
        TourPlan longPlan2 = director.SpainPlan();
    }

➕ Minimum code

• in TourPlanBuilder interface add method newInstance()

  public interface TourPlanBuilder {
    TourPlanBuilder newInstance();
  }
  

• do not have to re-write all the attributes in DefaultTourBuilder

public class DefaultTourBuilder implements TourPlanBuilder{
    private TourPlan tourplan; //now do not need all fields

    @Override
    public TourPlanBuilder newInstance() { //but need to initiate new instance
        this.tourplan = new TourPlan();
        return this;
    }

    @Override
    public TourPlanBuilder title(String title) {
        this.tourplan.setTitle(title);
        return this;
    }
}

👍🏻

• can create difficult instances easily
• can put orders in creating instances
• can put conditions in creating instances
• do not need so many constructors for all different cases

🛠️

• StringBuilder(not synchronized)

• can use append() to keep adding String

• Stream.builder()

• can use add()

• annotation @Builder

• creates builder for that class

• UriComponentsBuilder
• has methods such as scheme(), host()