比如要创建这么一个对象：

    创建对象的方式：
    1.可伸缩构造方法，但是参数过多会变得难以阅读和维护，读者不知道这些值是什么意思，并且必须仔细地去数参数才能找到答案。一长串相同类型的参数可能会导致一些细微的 bug。如果客户端不小心写反了两个这样的参数，编译器并不会报错，但是程序在运行时会出现错误行为。

    1. // Telescoping constructor pattern - does not scale well!
    2. public class NutritionFacts {
    3.     private final int servingSize; // (mL) required
    4.     private final int servings; // (per container) required
    5.     private final int calories; // (per serving) optional
    6.     private final int fat; // (g/serving) optional
    7.     private final int sodium; // (mg/serving) optional
    8.     private final int carbohydrate; // (g/serving) optional
    10.     public NutritionFacts(int servingSize, int servings) {
    11.         this(servingSize, servings, 0);
    12.     }
    14.     public NutritionFacts(int servingSize, int servings,
    15.                           int calories) {
    16.         this(servingSize, servings, calories, 0);
    17.     }
    19.     public NutritionFacts(int servingSize, int servings,
    20.                           int calories, int fat) {
    21.         this(servingSize, servings, calories, fat, 0);
    22.     }
    24.     public NutritionFacts(int servingSize, int servings,
    25.                           int calories, int fat, int sodium) {
    26.         this(servingSize, servings, calories, fat, sodium, 0);
    27.     }
    29.     public NutritionFacts(int servingSize, int servings,
    30.                           int calories, int fat, int sodium, int carbohydrate) {
    31.         this.servingSize = servingSize;
    32.         this.servings = servings;
    33.         this.calories = calories;
    34.         this.fat = fat;
    35.         this.sodium = sodium;
    36.         this.carbohydrate = carbohydrate;
    37.     }
    38. }

    2.通过java bean模式，在构造过程中 JavaBean 可能处于不一致的状态

    1. public class NutritionFacts {
    2.     // Parameters initialized to default values (if any)
    3.     private int servingSize = -1; // Required; no default value
    4.     private int servings = -1; // Required; no default value
    5.     private int calories = 0;
    6.     private int fat = 0;
    7.     private int sodium = 0;
    8.     private int carbohydrate = 0;
    9.     public NutritionFacts() { }
    10.     // Setters
    11.     public void setServingSize(int val) { servingSize = val; }
    12.     public void setServings(int val) { servings = val; }
    13.     public void setCalories(int val) { calories = val; }
    14.     public void setFat(int val) { fat = val; }
    15.     public void setSodium(int val) { sodium = val; }
    16.     public void setCarbohydrate(int val) { carbohydrate = val; }
    17. }
    20. NutritionFacts cocaCola = new NutritionFacts();
    21. cocaCola.setServingSize(240);
    22. cocaCola.setServings(8);
    23. cocaCola.setCalories(100);
    24. cocaCola.setSodium(35);
    25. cocaCola.setCarbohydrate(27);

    3.通过建造者模式，即能保证像重叠构造器的安全性，也能保证像 JavaBeans 模式那么好的可读性

    1. public class NutritionFacts {
    2.     private final int servingSize;
    3.     private final int servings;
    4.     private final int calories;
    5.     private final int fat;
    6.     private final int sodium;
    7.     private final int carbohydrate;
    9.     public static class Builder {
    10.         // Required parameters
    11.         private final int servingSize;
    12.         private final int servings;
    13.         // Optional parameters - initialized to default values
    14.         private int calories = 0;
    15.         private int fat = 0;
    16.         private int sodium = 0;
    17.         private int carbohydrate = 0;
    19.         public Builder(int servingSize, int servings) {
    20.             this.servingSize = servingSize;
    21.             this.servings = servings;
    22.         }
    24.         public Builder calories(int val) {
    25.             calories = val;
    26.             return this;
    27.         }
    29.         public Builder fat(int val) {
    30.             fat = val;
    31.             return this;
    32.         }
    34.         public Builder sodium(int val) {
    35.             sodium = val;
    36.             return this;
    37.         }
    39.         public Builder carbohydrate(int val) {
    40.             carbohydrate = val;
    41.             return this;
    42.         }
    44.         public NutritionFacts build() {
    45.             return new NutritionFacts(this);
    46.         }
    47.     }
    49.     private NutritionFacts(Builder builder) {
    50.         servingSize = builder.servingSize;
    51.         servings = builder.servings;
    52.         calories = builder.calories;
    53.         fat = builder.fat;
    54.         sodium = builder.sodium;
    55.         carbohydrate = builder.carbohydrate;
    56.     }
    57. }
    60. //使用
    61. NutritionFacts cocaCola = new NutritionFacts.Builder(240, 8)
    62.     .calories(100).sodium(35).carbohydrate(27).build();