可以使用静态内部类的方式来解决Builder模式受保护构造函数的问题。

示例代码：

public class Coffee {
    private final String type;
    private final String size;
    private final int sugar;
    private final boolean milk;

    private Coffee(Builder builder) {
        this.type = builder.type;
        this.size = builder.size;
        this.sugar = builder.sugar;
        this.milk = builder.milk;
    }

    public String getType() {
        return type;
    }

    public String getSize() {
        return size;
    }

    public int getSugar() {
        return sugar;
    }

    public boolean hasMilk() {
        return milk;
    }

    public static class Builder {
        private final String type;
        private final String size;
        private int sugar = 0;
        private boolean milk = false;

        public Builder(String type, String size) {
            this.type = type;
            this.size = size;
        }

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

        public Builder addMilk(boolean milk) {
            this.milk = milk;
            return this;
        }

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

这里我们定义一个外部的Coffee类和一个内部的Builder类，Coffee类的构造函数为私有的，通过Builder构建Coffee实例，Builder类的构造函数为公开的，用户可以通过Builder构建Coffee实例，但是不能直接new一个Coffee实例，保证Coffee实例的不可变性。