Effective Java: Item 02 Builder Pattern

2013-06-11 676 words 4 minutes

Contents

Builder pattern is used to tackle problem where class contains a large number of optional parameters, besides builder pattern there are two other methods: Telescoping constructor pattern and JavaBeans Pattern, they all have some limits.

The telescoping constructor pattern works, but it is hard to write client code when there are many parameters, and harder still to read it
In JavaBeans Pattern: JavaBean may be in an inconsistent state partway through its construction
The JavaBeans Pattern precludes the possibility of making a class immutable
The Builder pattern simulates named optional parameters in Python
The Builder pattern is a good choice between when designing classes whose constructors or static factories would have more than a handful of parameters

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// Telescoping constructor pattern - does not scale well!
public class NutritionFacts {
  private final int servingSize;      // (mL)            required
    private final int servings;       // (per container) required
    private final int calories;       //                 optional
    private final int fat;            // (g)             optional
    private final int sodium;         // (mg)            optional
    private final int carbohydrate;   // (g)             optional

    public NutritionFacts(int servingSize, int servings) {
        this(servingSize, servings, 0);
    }

    public NutritionFacts(int servingSize, int servings, int calories) {
        this(servingSize, servings, calories, 0);
    }

    public NutritionFacts(int servingSize, int servings, int calories, int fat) {
        this(servingSize, servings, calories, fat, 0);
    }

    public NutritionFacts(int servingSize, int servings, int calories, int fat,
            int sodium) {
        this(servingSize, servings, calories, fat, sodium, 0);
    }

    public NutritionFacts(int servingSize, int servings, int calories, int fat,
            int sodium, int carbohydrate) {
        this.servingSize = servingSize;
        this.servings = servings;
        this.calories = calories;
        this.fat = fat;
        this.sodium = sodium;
        this.carbohydrate = carbohydrate;
    }

}

NutritionFacts cocaCola = new NutritionFacts(240, 8, 100, 0, 35, 27);

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// JavaBeans Pattern - allows inconsistency, mandates mutability
public class NutritionFacts {
	private int servingSize = -1; // Required; no default value
	private int servings = -1;
	private int calories = 0;
	private int fat = 0;
	private int sodium = 0;
	private int carbohydrate = 0;

	public NutritionFacts() {
	}

	// Setters
	public void setServingSize(int servingSize) { servingSize = servingSize; }
	public void setServings(int servings) { servings = servings; }
	public void setCalories(int calories) { calories = calories; }
	public void setFat(int fat) { fat = fat; }
	public void setSodium(int sodium) { sodium = sodium; }
	public void setCarbohydrate(int carbohydrate) { carbohydrate = carbohydrate; }

}

NutritionFacts cocaCola = new NutritionFacts();
cocaCola.setServingSize(240);
cocaCola.setServings(8);
cocaCola.setCalories(100);
cocaCola.setSodium(35);
cocaCola.setCarbohydrate(27);

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// Builder Pattern
public class NutritionFacts {
	private final int servingSize;
	private final int servings;
	private final int calories;
	private final int fat;
	private final int sodium;
	private final int carbohydrate;

	public static class Builder {
		// Required parameters
		private final int servingSize;
		private final int servings;

		// Optional parameters - initialized to default values
		private int calories     = 0;
		private int fat			 = 0;
		private int carbohydrate = 0;
		private int sodium       = 0;

		public Builder(int servingSize, int servings){
			this.servingSize = servingSize;
			this.servings = servings;
		}

		public Builder calories(int val)
			{ calories = val;     return this; }
		public Builder fat(int val)
			{ fat = val;          return this; }
		public Builder carbohydrate(int val)
			{ carbohydrate = val; return this; }
		public Builder sodium(int val)
			{ sodium = val;       return this; }

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

	private NutritionFacts(Builder builder){
			servingSize     = builder.servingSize;
			servings 		= builder.servings;
			calories		= builder.calories;
			fat 			= builder.fat;
			sodium			= builder.sodium;
			carbohydrate	= builder.carbohydrate;
	}
}

NutritionFacts cocaCola = new NutritionFacts.Builder(240,8).calories(100).sodium(35).carbohydrate(27).build();