When would you use the Builder Pattern?

Question

What are some *common*, *real world examples* of using the Builder Pattern? What does it buy you? Why not just use a Factory Pattern?

Answer 1

In short: using the Builder Pattern is a good idea when using the factory pattern would result in factory methods with many parameters that are hard to remember and maintain.

Consider the Person class with a required name and an optional email address:

Person

1. name
2. email

If you choose to use Factory pattern you will end up writing the following constructors:

Person(String name) { ... }

Person(String name, String email) { ... }

After a while you realize you will need a new attribute: phone number. Your code or other's code already started using the constructors, so refactoring would be expensive, so you want to stay backward compatible. It leaves you no other choice, but overloading your constructor:

Person(String name) { ... }

Person(String name, String email) { ... }

Person(String name, String email, String homePhone) { ... }

With each new attribute you will end up adding a new constructor unless you can refactor the code calling this class. After adding more than a handful of parameters the code will become less readable and it will be harder to remember the order of parameters and the constructor to be called.

Now, consider the following Builder implementation:

public class Person {

private String name;

private String email;

private String phone;

public static class Builder {

private final String name;

private String email;

private String phone;

public Builder(String name) {

this.name = name;

}

public Builder setName(String email) {

this.email = email;

return this;

}

public Builder setPhone(String phone) {

this.phone = phone;

return this;

}

public Person build() {

return new Person(this);

}

}

private Person(Builder builder) {

name = builder.name;

email = builder.email;

phone = builder.phone;

}

}

With using Builder pattern you can do the below initially:

Person person = Person.Builder("Name")

.setEmail("[email protected]")

.build();

After adding the phone attribute, the calling code still stay readable:

Person person = Person.Builder("Name")

.setEmail("[email protected]")

.setPhone("123")

.build();