Example: Car Class

public class Car {
    private int doors;
    private int wheels;
    private int seats;

    public Car(int doors, int wheels, int seats) {
        this.doors = doors;
        this.wheels = wheels;
        this.seats = seats;
    }

    // getters, other methods, etc.
}

Let’s make some cars!

Car c1 = new Car(2, 4, 4);
Car c2 = new Car(4, 4, 5);
Car c3 = new Car(4, 4, 7);

Let’s make some cars!

// A 2-door car with four wheels and four seats.
Car c1 = new Car(2, 4, 4);
Car c2 = new Car(4, 4, 5);
Car c3 = new Car(4, 4, 7);

Let’s make some cars!

// A 2-door car with four wheels and four seats.
Car c1 = new Car(2, 4, 4);
// A 4-door car with four wheels and five seats.
Car c2 = new Car(4, 4, 5);
// A 4-door car with four wheels and seven seats.
Car c3 = new Car(4, 4, 7);

Let’s make some cars!

// A 2-door car with four wheels and four seats.
Car c1 = new Car(2, 4, 4);
// A 4-door car with four wheels and five seats.
Car c2 = new Car(4, 4, 5);
// A 4-door car with four wheels and seven seats.
Car c3 = new Car(4, 4, 7);
// Most cars have 4 wheels, maybe make another
// constructor with a default value?

Let’s make some cars!

// A 2-door car with four wheels and four seats.
Car c1 = new Car(2, 4, 4);
// A 4-door car with four wheels and five seats.
Car c2 = new Car(4, 4, 5);
// A 4-door car with four wheels and seven seats.
Car c3 = new Car(4, 4, 7);
// Most cars have 4 wheels, maybe make another
// constructor with a default value?
// We can also make dumb mistakes.
// A 4-door car with seven wheels and four seats?
// Arguments of the same type are easy to confuse.
Car c4 = new Car(4, 7, 4);

Problems with Constructors

• Too many arguments - easily confused
• Optional arguments - overload constructors with defaults?
• Too many constructors

Use setters instead?

Car car = new Car();
car.setWheels(4);
car.setSeats(2);

Use setters instead?

Car car = new Car();
car.setWheels(4);
car.setSeats(2);

• We forgot to set the doors!
• Do we have reasonable default values?
• What if we didn’t finish configuring the object?
• Does it even make sense to change the number of wheels after the Car is constructed?

The Builder Pattern

• Use the Builder Pattern to encapsulate the construction of a product and allow it to be constructed in steps.

CarBuilder

public class CarBuilder {
    private int doors;
    private int wheels = 4;
    private int seats;
    public void setDoors(int doors) {
        this.doors = doors;
    }
    public void setWheels(int wheels) {
        this.wheels = wheels;
    }
    public void setSeats(int seats) {
        this.seats = seats;
    }
    public Car getCar() {
        return new Car(doors, wheels, seats);
    }
}

Let’s Make a Car!

CarBuilder cb = new CarBuilder();
cb.setDoors(2);
cb.setSeats(4);
Car car = cb.getCar();

• We can build up a complex object with a step by step approach.
• We could add error checking before actually constructing the Car object to make sure we’ve properly configured all the fields.
• We could use a builder to create an immutable object.

Fluent Interface

• The Builder pattern is often implemented with a fluent interface, where each method in the builder returns a reference to the builder object itself so we can easily chain the methods together.

  Car car = new CarBuilder().setDoors(2)
                            .setSeats(4)
                            .getCar();

• This coding idiom of returning this and method chaining is independent of the Builder pattern, but crops up often enough that it’s worth mentioning here.

CarBuilder with Fluent Interface

public class CarBuilder {
    private int doors;
    private int wheels = 4;
    private int seats;
    public CarBuilder setDoors(int doors) {
        this.doors = doors;
        return this;
    }
    public CarBuilder setWheels(int wheels) {
        this.wheels = wheels;
        return this;
    }
    public CarBuilder setSeats(int seats) {
        this.seats = seats;
        return this;
    }
    public Car getCar() {
        return new Car(doors, wheels, seats);
    }
}

Code

Code with extra examples here