[パーフェクトjava]アイテム2コンストラクション関数に多くのパラメータがある場合は、コンストラクタを考慮します.

Builderモードビルダーモード

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();

      protected abstract T self();
   }

   Pizza(Builder<?> builder) {
      toppings = builder.toppings.clone();
   }
}

public class NyPizza extends Pizza {
   public enum Size { SMALL, MEDIUM, LARGE }
   private final Size size;

   public static class Builder extends Pizza.Builder<Builder> {
      private final Size size;

      public Builder(Size size) {
         this.size = Objects.requireNonNull(size);
      }

      @Override public NyPizza build() {
         return new NyPizza(this);
      }

      @Override protected Builder self() { return this; }
   }

   private NyPizza(Builder builder) {
      super(builder);
      size = builder.size;
   }
}

public class Calzone extends Pizza {
   private final boolean sauceInside;

   public static class Builder extends Pizza.Builder<Builder> {
      private boolean sauceInside = false;

      public Builder sauceInside() {
         sauceInside = true;
         return this;
      }

      @Override public Calzone build() {
         return new Calzone(this);
      }

      @Override protected Builder self() { return this; }
   }

   private Calzone(Builder builder) {
      super(builder);
      sauceInside = builder.sauceInside;
   }
}

NyPizza pizza =  new NyPizza.Builder(SMALL)
      .addTopping(SAUSAGE).addTopping(ONION).build();
Calzone calzone = new Calzone.Builder()
      .addTopping(HAM).sauceInside().build();

コンストラクタモードの利点