【Effective JAVA】构造参数中多个变量可选时，用Bu

【Builder模式介绍】
不直接生产想要的对象，而是让客户端利用所有必要的参数调用构造器，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，客户端调用午餐的build方法来生成不可变的对象。这个builder就是它构建的类的静态成员类。
【示例】

public class BuilderPattern
{
    private final int servingSize;
    private final int servings;
    private final int calories;
    private final int fat;

    public static class Builder
    {
        //必填参数
        private final int servingSize;
        private final int servings;

        //可选参数，设置默认值
        private int calories = 0;
        private int fat = 0;

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

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

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

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

    private BuilderPattern(Builder builder)
    {
        this.servingSize = builder.servingSize;
        this.servings = builder.servings;
        this.calories = builder.calories;
        this.fat = builder.fat;
    }

    public void myPrint()
    {
        System.out.println("servingSize = "+servingSize+", servings = "+servings+", calories = " +
                +calories+", fat = "+fat);
    }

    public static void main(String[] args)
    {
        BuilderPattern bp = new Builder(240,8).calories(100).fat(35).build();
        bp.myPrint();
    }
}