漫谈设计模式之：Builder模式

简介

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。
我们看一下有三个参数的类，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！
如果这些构造函数都写到类里面，不仅代码量大，代码不美观，而且调用起来容易搞错。

解决方案

我们的想法是Student类的构造函数不要那么多，但是又要满足初始化Student类变量的需求。
我们可以设计一个内部类，这个内部类的参数跟Student类参数一样，而Student类的构造函数的参数，我们就设定为这个内部类。所以我们只需要将这个内部类的变量初始化完即可。
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.()这样的形式一直写下去。
目前RxJava、OkHttp等框架均采取了这样的链式结构设计。

代码

public class Student {

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

    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 +
                '}';
    }
}

调用：

Student student1 = new Student.StudentBuilder().setName("张三").setAge(18).setSex(true).build();
mTextView1.setText(student1.toString());
Student student2 = Student.newInstance().setName("李四").setAge(20).setSex(false).build();
mTextView2.setText(student2.toString());

git地址

https://github.com/ddnosh/AndroidQuick