Who did it ?

GoF (Gang of Four)

Erich Gamma (@ErichGamma)

• Switzerland (Zürich)
• Ambassador of JUnit

Richard Helm

• Australia

Ralph Johnson

• USA

John Vlissides

• USA

What does the Builder Pattern solve ?

It eradicates the Telelescoping constructor antipattern !

(Telelescoping constructor antipattern)

public Person(String firstName, String lastName) {}

(Telelescoping constructor antipattern)

public Person(String firstName, String lastName,
	Gender gender
) {}

(Telelescoping constructor antipattern)

public Person(String firstName, String lastName,
	Gender gender,
	int age
) {}

(Telelescoping constructor antipattern)

public Person(String firstName, String lastName,
	Gender gender,
	int age,
	String job
) {}

(Telelescoping constructor antipattern)

public Person(String firstName, String lastName,
	Gender gender,
	int age,
	String job,
	String hobbys
) {}

(Telelescoping constructor antipattern)

public Person(String firstName, String lastName,
	Gender gender,
	int age,
	String job,
	String hobbys,
	String favouriteMovie
) {}

(Telelescoping constructor antipattern)

new Person(String, String, Gender, int, String, String, String);

new Person("John", null, null, -1, null, null, null); // DOH !

new Person("Will", "Smith", Gender.MALE, int -1,
	"cooking", "men in black", "actor") // OUPS !!!

Builder pattern builds a complex object using simple objects and using a step by step approach. This type of design pattern comes under creational pattern as this pattern provides one of the best ways to create an object.

Source: tutorialspoint.com

With builder pattern

PersonBuilder builder = new PersonBuilder();
Person bob = builder.firstName("Bob")
              	.lastName("Builder")
              	.age(33)
              	.job("Man I love building stuff!")
              	.build();

Variants

Address address = new Builder().url("www.google.com").port(80).build();

What happens if the user of this class tries to create an Address without an url ?

Josh Bloch adaptation

Address address = new Builder("www.google.com").port(80).build();
// Url is passed in the constructor parameter and is required
// -> problem solved!

Source: Effective Java iirc

Sample in JAVA

// Source: https://gist.githubusercontent.com/vojtechruz/1ea4f51a016002bbdd0d493b049140dc/raw/a9c2888f3c36f3dd291d4cc48c4eae83f5b1f055/InnerBuilderExample.java
public class Person {

	private final String firstName;
	private final String lastName;
	private final String description;
	private final int age;

	private Person(Builder builder) {
		firstName = builder.firstName;
		lastName = builder.lastName;
		description = builder.description;
		age = builder.age;
	}

	public static final class Builder {
		private String firstName;
		private String lastName;
		private String description;
		private int age;

		public Builder() {
		}

		public Builder firstName(String val) {
			firstName = val;
			return this;
		}

		public Builder lastName(String val) {
			lastName = val;
			return this;
		}

		public Builder description(String val) {
			description = val;
			return this;
		}

		public Builder age(int val) {
			age = val;
			return this;
		}

		public Person build() {
			return new Person(this);
		}
	}
}

Pro / Cons

• + Immutable objects
• + Easier to read
• + Easier to instanciate
• + Great for DSL languages (we can also order it !) -> select().where().and().or().join()
• - Verbose implementation (for the developper)
• - Harder to maintain (If you forget to add a method to define a new field :/ )

Questions ?