用构建器构造多参数实例

如果一个类构造器的参数大于四个就要考虑使用构造器来构建类了。它可以使参数更加灵活，扩展性更好并且可以减少一些参数位置写错的情况。

可采用Builder模式的一种形式，不直接生成想要的对象，而是让客户端利用所有必要的参数调用构造器（或静态工厂）
得到一个builder对象。然后客户端在builder对象上调用类似setter的方法，来设置每个相关的可选参数。最后，客户端调用无参的build方法来生成不可变的对象。这个builder是它构建的类的静态成员类。

示例：

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 {
        private final int servingSize;
        private final int servings;

        private int calories = 0;
        private int fat = 0;
        private int sodium = 0;
        private int carbohydrate = 0;

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

        public Builder calories(int calories) {
            this.calories = calories;
            return this;
        }

        public Builder fat(int fat) {
            this.fat = fat;
            return this;
        }

        public Builder sodium(int sodium) {
            this.sodium = sodium;
            return this;
        }

        public Builder carbohydrate(int carbohydrate) {
            this.carbohydrate = carbohydrate;
            return this;
        }

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

    }

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

    @Override
    public String toString() {
        return "NutritionFacts{" +
                "servingSize=" + servingSize +
                ", servings=" + servings +
                ", calories=" + calories +
                ", fat=" + fat +
                ", sodium=" + sodium +
                ", carbohydrate=" + carbohydrate +
                '}';
    }
}

测试：

public class test {
    public static void main(String[] args) {

        NutritionFacts.Builder builder = new NutritionFacts.Builder(1, 2);

        NutritionFacts nutritionFacts = builder.calories(3)
                .carbohydrate(4)
                .fat(5)
                .sodium(6)
                .build();

        System.out.println(nutritionFacts.toString());

    }
}

如果类的构造器或者静态工厂中具有多个参数，设计这种类时，Builder模式就是种不错的选择，特别是当大多数参数都是可选的时候。与使用传统的重叠构造器的模式相比，使用Builder模式的客户端代码更易于阅读和编写，构建器也比JavaBeans更加安全。

参考：《Effective Java》