Builder pattern snippet from Effective Java

gist.java

// Builder pattern for class hierarchies
public abstract class Pizza {
    public enum Topping {
        HAM,
        MUSHROOM,
        ONION,
        PEPPER,
        SAUSAGE
    }
    final Set < Topping > toppings;
    abstract static class Builder < T extends Builder < T >> {
        EnumSet < Topping > toppings = EnumSet.noneOf(Topping.class);
        public T addTopping(Topping topping) {
            toppings.add(Objects.requireNonNull(topping));
            return self();
        }
        abstract Pizza build();
        // Subclasses must override this method to return "this"
        protected abstract T self();
    }
    Pizza(Builder < ? > builder) {
        toppings = builder.toppings.clone(); // See Item 50
    }
}