Item 02 - Consider a builder when faced with many constructor parameters

From Effective Java 2/e by Joshua Bloch

Advantages

1. Possible to apply invariant
2. Possible to accept multiple varargs parameters
3. Possible to make multiple objects with one builder object

Useful for class which

1. contains many fields
2. needs static factory methods
3. contains many optional fields

Telescoping Constructor Pattern, Java Bean Pattern, Builder Pattern

Telescoping Constructor Pattern

public class FoodTelescopingDemo {

   private final int id;
   private final String name;
   private final int calories;
   private final int servingSize;
   private final int fat;
   private final String description;

   public FoodTelescopingDemo(int id, String name) {
      this(id, name, 0, 0, 0, "default description");
   }

   public FoodTelescopingDemo(int id, String name, int calories) {
      this(id, name, calories, 0, 0, "default description");
   }

   public FoodTelescopingDemo(int id, String name, int calories, int servingSize) {
      this(id, name, calories, servingSize, 0, "default description");
   }

   public FoodTelescopingDemo(int id, String name, int calories, int servingSize, int fat) {
      this(id, name, calories, servingSize, fat, "default description");
   }

   /**
   * Main Constructor - actually builds the object
   */
   public FoodTelescopingDemo(int id, String name, int calories, int servingSize, int fat,
      String description) {
      this.id = id;
      this.name = name;
      this.calories = calories;
      this.servingSize = servingSize;
      this.fat = fat;
      this.description = description;
   }

   public int getId() {
      return id;
   }

   public String getName() {
      return name;
   }

   public int getCalories() {
      return calories;
   }

   public int getServingSize() {
      return servingSize;
   }

   public int getFat() {
      return fat;
   }

   public String getDescription() {
      return description;
   }

}

Builder Pattern

public class StreetMap {
    private final Point origin;
    private final Point destination;

    private final Color waterColor;
    private final Color landColor;
    private final Color highTrafficColor;
    private final Color mediumTrafficColor;
    private final Color lowTrafficColor;

    public static class Builder {
        // Required parameters
        private final Point origin;
        private final Point destination;

        // Optional parameters - initialize with default values
        private Color waterColor         = Color.BLUE;
        private Color landColor          = new Color(30, 30, 30);
        private Color highTrafficColor   = Color.RED;
        private Color mediumTrafficColor = Color.YELLOW;
        private Color lowTrafficColor    = Color.GREEN;

        public Builder(Point origin, Point destination) {
            this.origin      = origin;
            this.destination = destination;
        }

        public Builder waterColor(Color color) {
            waterColor = color;
            return this;
        }

        public Builder landColor(Color color) {
            landColor = color;
            return this;
        }

        public Builder highTrafficColor(Color color) {
            highTrafficColor = color;
            return this;
        }

        public Builder mediumTrafficColor(Color color) {
            mediumTrafficColor = color;
            return this;
        }

        public Builder lowTrafficColor(Color color) {
            lowTrafficColor = color;
            return this;
        }

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

    }

    private StreetMap(Builder builder) {
        // Required parameters
        origin      = builder.origin;
        destination = builder.destination;

        // Optional parameters
        waterColor         = builder.waterColor;
        landColor          = builder.landColor;
        highTrafficColor   = builder.highTrafficColor;
        mediumTrafficColor = builder.mediumTrafficColor;
        lowTrafficColor    = builder.lowTrafficColor;
    }

    public static void main(String args[]) {
        StreetMap map = new StreetMap.Builder(new Point(50, 50), new Point(100,
                100)).landColor(Color.GRAY).waterColor(Color.BLUE.brighter())
                .build();
    }
}

Posted by The Finest Artist