The Builder Pattern is a creational design pattern that allows you to construct complex objects step by step.

It separates the construction of a complex object from its representation,

allowing the same construction process to create different representations.

public class BankAccount {
private String name;
private String accountNumber;
private String email;
private boolean newsletter;

//The constructor that takes a builder from which it will create object
    //the access to this is only provided to builder
private BankAccount(BankAccountBuilder builder) {
this.name = builder.name;
this.accountNumber = builder.accountNumber;
this.email = builder.email;
this.newsletter = builder.newsletter;
    }

public static class BankAccountBuilder {
private String name;
private String accountNumber;
private String email;
private boolean newsletter;

//All Mandatory parameters goes with this constructor
public BankAccountBuilder(String name, String accountNumber) {
this.name = name;
this.accountNumber = accountNumber;
        }

//setters for optional parameters which returns this same builder
        //to support fluent design
public BankAccountBuilder withEmail(String email) {
this.email = email;
return this;
        }

public BankAccountBuilder wantNewsletter(boolean newsletter) {
this.newsletter = newsletter;
return this;
        }

//the actual build method that prepares and returns a BankAccount object
public BankAccount build() {
return new BankAccount(this);
        }
    }

//getters
public String getName() {
return name;
    }

public String getAccountNumber() {
return accountNumber;
    }

public String getEmail() {
return email;
    }

public boolean isNewsletter() {
return newsletter;
    }
}

public class BuilderPatternDriver {
 public static void main(String[] args) {
 BankAccount newAccount = new BankAccount
 .BankAccountBuilder("Jon", "22738022275")

                .withEmail("jon@example.com")

                .wantNewsletter(true)

                .build();


 System.out.println("Name: " + newAccount.getName());
 System.out.println("AccountNumber:" + newAccount.getAccountNumber());
 System.out.println("Email: " + newAccount.getEmail());
 System.out.println("Want News letter?: " + newAccount.isNewsletter());

    }

}

Name: Jon

AccountNumber:22738022275

Email: null

Want News letter?: false

@Data

class Pizza {
 private String size;
 private String crustType;
 private boolean cheese;
 private List<String> toppings;


 private Pizza(Builder builder) {
 this.size = builder.size;
 this.crustType = builder.crustType;
 this.cheese = builder.cheese;
 this.toppings = builder.toppings;

    }


 // Getters

    // You can add setters if needed



    // Static inner Builder class
 static class Builder {
 private String size;
 private String crustType;
 private boolean cheese;
 private List<String> toppings = new ArrayList<>();


 public Builder size(String size) {
 this.size = size;
 return this;

        }


 public Builder crustType(String crustType) {
 this.crustType = crustType;
 return this;

        }


 public Builder cheese(boolean cheese) {
 this.cheese = cheese;
 return this;

        }


 public Builder addTopping(String topping) {
 this.toppings.add(topping);
 return this;

        }


 public Pizza build() {
 return new Pizza(this);

        }

    }

}



public class Builder2 {
 public static void main(String[] args) {
     //        Pizza pizza1 = new Pizza(); // direct creation is not allowed
 Pizza pizza = new Pizza.Builder()

                .size("Large")

                .crustType("Thin crust")

                .cheese(true)

                .addTopping("Pepperoni")

                .addTopping("Mushrooms")

                .build();


 System.out.println("Pizza details:");
 System.out.println("Size: " + pizza.getSize());
 System.out.println("Crust Type: " + pizza.getCrustType());
 System.out.println("Cheese: " + pizza.isCheese());
 System.out.println("Toppings: " + pizza.getToppings());

    }

}

Pizza details:

Size: Large                         

Crust Type: Thin crust

Cheese: true

Toppings: [Pepperoni, Mushrooms]

  class User {

  
 // Required fields
 private int id;
 private String name;

  
 // Optional fields
 private String email;
 private String phone;
 private int age;

  
 // Private constructor
 private User(Builder builder) {
 this.id = builder.id;
 this.name = builder.name;
 this.email = builder.email;
 this.phone = builder.phone;
 this.age = builder.age;

    }

  
 // Static Builder Class
 public static class Builder {

  
 // Required fields
 private int id;
 private String name;

  
 // Optional fields
 private String email;
 private String phone;
 private int age;

  
 // Constructor for required fields
 public Builder(int id, String name) {
 this.id = id;
 this.name = name;

        }

  
 public Builder email(String email) {
 this.email = email;
 return this;

        }

  
 public Builder phone(String phone) {
 this.phone = phone;
 return this;

        }

  
 public Builder age(int age) {
 this.age = age;
 return this;

        }

  
 public User build() {
 return new User(this);

        }

    }

  
 @Override
 public String toString() {
 return "User{" +
 "id=" + id +
 ", name='" + name + '\'' +
 ", email='" + email + '\'' +
 ", phone='" + phone + '\'' +
 ", age=" + age +
 '}';

    }

}

  

  public class BuilderPattern {

  
 public static void main(String[] args) {

  
 User user = new User.Builder(1, "Saravanan")

                .email("sara@gmail.com")

                .phone("999999999")

                .age(30)

                .build();

  
 System.out.println(user);

    }

}

Memory Trick

Builder Pattern = "Construct complex object step-by-step"

Interview One-Liner

Builder Pattern provides a flexible way to create complex objects step-by-step using method chaining while avoiding telescoping constructors.