Builder模式

1 为什么要用Builder模式

Builder模式主要用于解决初始化类时（也就是new一个类的实例出来），类的构造函数种类过多且不易管理的问题。
有3个参数的类能构建出多少个构造函数：

    public class Student {
        private String name;
        private int age;
        private boolean sex;
    
        public Student() {
        }
    
        public Student(String name) {
            this.name = name;
        }
    
        public Student(int age) {
            this.age = age;
        }
    
        public Student(boolean sex) {
            this.sex = sex;
        }
    
        public Student(String name, int age) {
            this.name = name;
            this.age = age;
        }
    
        public Student(String name, boolean sex) {
            this.name = name;
            this.sex = sex;
        }
    
        public Student(int age, boolean sex) {
            this.age = age;
            this.sex = sex;
        }
    
        public Student(String name, int age, boolean sex) {
            this.name = name;
            this.age = age;
            this.sex = sex;
        }
    
        public Student(StudentBuilder studentBuilder) {
            this.name = studentBuilder.name;
            this.age = studentBuilder.age;
            this.sex = studentBuilder.sex;
        }
    }

一共有9种之多。如果这些构造函数都写到类里面，不但代码量大、代码不美观，而且调用起来也容易出错。

2 Builder模式的实现

有的需求是Student类的构造函数不要那么多，但是又要满足初始化Student类变量。
可以考虑设计一个内部类，这个内部类的参数跟Student类的参数一样，而Student类的构造函数的参数，我们就设定为这个内部类。
因此，只需要将这个内部类的变量初始化即可。内部类变量设定的时候，我们采用链式结构，这样可以通过setxx().setxx.()形式一直写下去。目前RxJava、OkHttp等框架均采用了这样的链式结构设计。

public class Student {

    private String name;
    private int age;
    private boolean sex;

    public Student() {
    }

    public Student(StudentBuilder studentBuilder) {
        this.name = studentBuilder.name;
        this.age = studentBuilder.age;
        this.sex = studentBuilder.sex;
    }

    public static StudentBuilder newInstance() {
        return new StudentBuilder();
    }

    public static class StudentBuilder {
        private String name;

        private int age;

        private boolean sex;

        public StudentBuilder setName(String name) {
            this.name = name;
            return this;
        }

        public StudentBuilder setAge(int age) {
            this.age = age;
            return this;
        }

        public StudentBuilder setSex(boolean sex) {
            this.sex = sex;
            return this;
        }

        public Student build() {
            return new Student(this);
        }
    }
    @Override
    public String toString() {
        return "Student{" +
                "name='" + name + '\'' +
                ", age=" + age +
                ", sex=" + sex +
                '}';
    }
    
    public Student setName(String name) {
        this.name = name;
        return this;
    }

    public Student setAge(int age) {
        this.age = age;
        return this;
    }

    public Student setSex(boolean sex) {
        this.sex = sex;
        return this;
    }
}

有3种使用方法可以实现Builder模式，代码如下：

Builder模式调用的过程清晰、代码简洁明了。

 public static void main(String[] args) {
       //第一种
       Student student1 = new Student.StudentBuilder().setName("张三").setAge(18).
                setSex(true).build();
       //第二种
       Student student2 = Student.newInstance().setName("李四").setAge(20).setSex(false).
                build();
       //第三种
       Student student3 = new Student().setAge(22).setName("王五").setSex(true);

        System.out.println("student1= "+student1.toString());
        System.out.println("student2= "+student2.toString());
        System.out.println("student3= "+student3.toString());
    }