考慮多個構造引數

一個類的構造引數很多，但是你不是每次構造都用上所有的引數的時候怎麼辦

傳統的使用重疊建構函式的寫法

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

    //定義了多個建構函式
    //順序都是往上呼叫的
    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;
    }

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

這種寫法很蠢，引數越多越不好，第二種解決方法：

採用javabean：

//就設定你要的引數
public class NutritionFacts {
    // Parameters initialized to default values (if any)
    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 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;
    }

    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);
    }
}

缺點：
在構造過程中javabean可能處於不一致的狀態
javabean模式阻止了把類做成不可變的可能（需要額外的努力來保證執行緒安全）

採用Builder模式

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;
    //一個內部類
    //以後維護的時候，新增新引數的時候也很方便
    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);
        }
    }
    //建構函式需要傳builder
    //可以使用單個builder構建多個物件
    private NutritionFacts(Builder builder) {
        servingSize = builder.servingSize;
        servings = builder.servings;
        calories = builder.calories;
        fat = builder.fat;
        sodium = builder.sodium;
        carbohydrate = builder.carbohydrate;
    }

    public static void main(String[] args) {
        //具有安全性和可讀性
        //build的時候才會驗證引數是否正確
        NutritionFacts cocaCola = new NutritionFacts.Builder(240, 8)
                .calories(100).sodium(35).carbohydrate(27).build();
    }
}