# Builder Design Pattern

There are several design patterns and they have their intent and applicability.

Like - Factory Method, Abstract Factory Method, Builder, Singleton, Prototype.

### Why Builder Design Pattern?

A Builder Pattern is a Creational Design Pattern that lets you to construct complex objects step by step.

*A* ***Builder Pattern*** *solves the issue with a large number of optional parameters and inconsistent states by providing a way to build the object step-by-step and provide a method that will actually return the final Object.*

### When We need Builder Design Pattern?

1.***Too Many arguments to pass from the client program*** to the Factory class.

2.***Some of the parameters might be optional.*** In Case of Factory pattern we have to create Separate Constructor in this case ( Constructor Overloading ) or we have to explicitly pass ***NULL*** to optional parameters.

3.***If the object is heavy and its creation is complex, then all that complexity will be part of Factory classes that may be confusing***

### Keep in Mind

With This Approach, That Is the ***Object State Will Be Inconsistent*** Unless All the Attributes Are Set Explicitly.

### Examples of Builder Pattern:

`java.lang.StringBuilder` and `java.lang.StringBuffer` has used the builder pattern for object building.

### Implementation:

School Class:

```java
public class School {
    private String Name;
    private String Address;
    private Long teachers; // Optional Parameter
    private Long students; // Optional Parameter
    private School(SchoolBuilder schoolBuilder){
        // private so that other class can not directly interact with school constructor.
        this.Name = schoolBuilder.Name;
        this.Address = schoolBuilder.Address;
        this.teachers = schoolBuilder.teachers;
        this.students = schoolBuilder.students;
    }

    public String getName() {
        return Name;
    }

    public String getAddress() {
        return Address;
    }

    public Long getTeachers() {
        return teachers;
    }

    public Long getStudents() {
        return students;
    }

    public static  class SchoolBuilder{
        private String Name;
        private String Address;
        private Long teachers; // Optional Parameter
        private Long students; // Optional Parameter

        public SchoolBuilder(String Name, String Address){
            // constructor for required parameters
            this.Name = Name;
            this.Address = Address;
        }

        public SchoolBuilder setTeachers(Long teachers) {
            this.teachers = teachers;
            return this;
        }

        public SchoolBuilder setStudents(Long students) {
            this.students = students;
            return this;
        }

        public School Build(){
            return new School(this);
        }
    }
}
```

Main Class:

```java
public class main {
    public static void main(String[] args) {
        // Make a new School Object.
        // School school =  new School("KHHS","Kolkata",2,3); --> Not possible this way.
        School newSchool = new School.SchoolBuilder("KHHS","Kolkata")
                .setStudents(8l)
                .setTeachers(12l)
                .Build();

        System.out.println(newSchool.getName()+" : "+ newSchool.getAddress()+" : "+newSchool.getStudents()+" : "+newSchool.getTeachers());
    }
}
```