靜態工廠和夠構造器有一個共同的局限性：遇到大量的參數時無法很好的擴展。

先說說構造器。其實field不多時重疊構造器(telescoping constructor)是個不錯的方法，易于編寫也易于調用，這種方式在參數數量較少時也很常見。但問題是參數很多(可能越來越多)時，比如(現在已經很難找到對多個參數進行重疊構造的代碼了，于是在這里直接引用一下書中的代碼)：

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;    }}

我想創建實例的時候，代碼會變成這個樣子：

public static void main(String[] args) {    NutritionFacts cocaCola = new NutritionFacts(240, 8, 100, 0, 35, 27);}

為了讓創建實例的過程變得更加清晰，于是我們有另一中選擇——JavaBean模式。這個再熟悉不過了，如下：

public class NutritionFacts {    // Parameters initialized to default values (if any)    private int servingSize = -1;     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 val) {        servingSize = val;    }    public void setServings(int val) {        servings = val;    }    public void setCalories(int val) {        calories = val;    }    public void setFat(int val) {        fat = val;    }    public void setSodium(int val) {        sodium = val;    }    public void setCarbohydrate(int val) {        carbohydrate = val;    }}

用起來雖然比只有一行的構造器多幾個步驟，但非常清晰：

public static void main(String[] args) {    NutritionFacts cocaCola = new NutritionFacts();    cocaCola.setServingSize(240);    cocaCola.setServings(8);    cocaCola.setCalories(100);    cocaCola.setSodium(35);    cocaCola.setCarbohydrate(27);}

相比只通過一個構造器創建實例，JavaBean模式的實例的構造過程被分成了好幾個過程。我們完全有可能在屬性不完整的情況下使用這個實例。按書中原文就是:

A JavaBean may be in a inconsistent state partway through its construction.

當然，我們也可以做一些操作使其在未完成的情況下無法使用，但我不想因此讓一個類變得越來越復雜。另外，我根本無法用JavaBean模式完全排除了將類設計成不可變(Immutable)的可能性。

此時，我們選擇使用Builder來解決這一問題。示例如下：

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;    }}

調用起來非常清晰(模擬了具名參數)，而且非常簡單：

public static void main(String[] args) {    NutritionFacts cocaCola = new NutritionFacts.Builder(240, 8)                .calories(100).sodium(35).carbohydrate(27).build();}

當然，Builder也有缺點。

1. 創建實例前都要創建一個Builder實例。
2. Builder模式編寫起來較為冗長。

但是，當構建一個實例需要很多步驟(或者很多讓人混淆的參數)的時候，Builder模式是個不錯的選擇。