Java - Builder Design Pattern - Example

Builder is a creational pattern that provides an efficient way to build objects. Compared with the traditional way of using multiple constructors, using the Builder pattern can ensure the readability and stability of the code. This mode is particularly useful when creating complex objects.

Use Builder pattern in the following situations:

• When the algorithm for creating a complex object should be independent of the object's components and how they are assembled
• When the construction process must allow the constructed object to have different representation

Advantages

1. It provides a clear disseverment between the construction and representation of an object.

2. It provides better control over the construction process.

3. It fortifies transmuting the internal representation of objects.

Implementation 

Let's see an example and learn how to implement a builder pattern. Consider a POJO of Employee below.

public class Employee {
    // All final attributes
    private final String firstName; // required
    private final String lastName; // required
    private final int age; // optional
    private final String phone; // optional
    private final String address; // optional
    private Employee(EmployeeBuilder builder) {
        this.firstName = builder.firstName;
        this.lastName = builder.lastName;
        this.age = builder.age;
        this.phone = builder.phone;
        this.address = builder.address;
    }
    // All getter, and NO setter to provide immutability
    public String getFirstName() {
        return firstName;
    }
    public String getLastName() {
        return lastName;
    }
    public int getAge() {
        return age;
    }
    public String getPhone() {
        return phone;
    }
    public String getAddress() {
        return address;
    }
    @Override
    public String toString() {
        return "Employee: " + this.firstName + ", " + 
        this.lastName + ", " + this.age + ", " + this.phone + ", "
                + this.address;
    }
    public static class EmployeeBuilder {
        private final String firstName;
        private final String lastName;
        private int age;
        private String phone;
        private String address;
        public EmployeeBuilder(String firstName, String lastName) {
            this.firstName = firstName;
            this.lastName = lastName;
        }
        public EmployeeBuilder age(int age) {
            this.age = age;
            return this;
        }
        public EmployeeBuilder phone(String phone) {
            this.phone = phone;
            return this;
        }
        public EmployeeBuilder address(String address) {
            this.address = address;
            return this;
        }
        // Return the finally constructed Employee object
        public Employee build() {
            Employee employee = new Employee(this);
            validateEmplopyeeObject(employee);
            return employee;
        }
        private void validateEmplopyeeObject(Employee employee) {
            // Do some basic validations to check
            // if employee object does not break any assumption of system
        }
    }
}

Main class

public class BuilderPatternDemo {
  public static void main(String[] args) {
    Employee employee1 = new Employee.
           EmployeeBuilder("Sibin", "Scott").
              age(30).
                phone("9898989898")
                 .address("YYYYYYYYYYY").
                    build();
    System.out.println(employee1);

    // No address
    Employee employee2 = new Employee.
            EmployeeBuilder("July", "Reacher").
              age(40).
                phone("2132134242").
                 build();
    System.out.println(employee2);
    // No age
    // No phone
    // No address
    Employee employee3 = new Employee.
         EmployeeBuilder("He", "Man").
           build();
    System.out.println(employee3);
  }
}

Console Output:

Employee: Sibin, Scott, 30, 9898989898, YYYYYYYYYYY
Employee: July, Reacher, 40, 2132134242, null
Employee: He, Man, 0, null, null