[Effective Java] Item 2: Consider a builder when faced with many constructor parameters

Item 1里讲到了static factory method，它有时候可以用来替代constructor功能。但是static factory method和constructor有个共同的局限：不能很好地扩展到大量可选参数。这时候需要用到另一种方法：Builder Pattern。

用传统constructor的局限

如果我们用constructor来构造很多参数的类时，代码冗长不易管理。比如：

// constructor pattern - does not scale well!
public class MyClass {
    private final int para1; // required
    private final int para2; // required
    private final int para3; // optional
    private final int para4; // optional
    private final int para5; // optional
    private final int para6; // optional
}

public MyClass(int para1, int para2) {
    this(para1, para2, 0);
}
public MyClass(int para1, int para2, int para3) {
    this(para1, para2, int para3, 0);
}
public MyClass(int para1, int para2, int para3, int para4) {
    this(para1, para2, int para3, para4, 0);
}
public MyClass(int para1, int para2, int para3, int para4, int para5) {
    this(para1, para2, int para3, para4, para5, 0);
}
public MyClass(int para1, int para2, int para3, int para4, int para5, int para6) {
    this.para1 = para1;
    this.para2 = para2;
    this.para3 = para3;
    this.para4 = para4;
    this.para5 = para5;
    this.para6 = para6;
}

在使用时如果有些参数不想使用默认值，就要把所有参数都传入constructor，比如：
MyClass myClass = new MyClass(240, 8, 100, 0, 35, 27);

改用Builder Pattern

// Builder Pattern
public class MyClass {
    private final int para1;
    private final int para2;
    private final int para3;
    private final int para4;
    private final int para5;
    private final int para6;

    public static class Builder {
        // required parameters
        private final int para1;
        private final int para2;

        // optional parameters - initialized to default values
        private int para3 = 0;
        private int para4 = 0;
        private int para5 = 0;
        private int para6 = 0;

        public Builder() {
            this.para1 = para1;
            this.para2 = para2;
        }

        public Builder para3(int val) {
            para3 = val;
            return this;
        }
        public Builder para4(int val) {
            para4 = val;
            return this;
        }
        public Builder para5(int val) {
            para5 = val;
            return this;
        }

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

    private MyClass() {
        para1 = builder.para1;
        para2 = builder.para2;
        para3 = builder.para3;
        para4 = builder.para4;
        para5 = builder.para5;
        para6 = builder.para6;
    }
}

在client端的代码就会如下：

MyClass something = new MyClass.Buillder(240, 8)
                               .para3(100)
                               .para4(30)
                               .build();

Builder Pattern缺点

虽然Builder Pattern使用起来非常灵活省时，但是也有几个缺点：

1. 为了创建对象，必须先创建它的构造器。在十分注重性能的情况下，可能会成为问题。
2. 有点冗长，适合多参数时，比如4个参数或者更多。

Summary

如果class的constructor或者static factory有多个参数，设计这种类的时候，Builder Pattern是不错的选择。