建造者设计模式

建造者模式

　　建造者模式也是非常常见的一种设计模式，我们经常看到有很多的框架都为我们提供了形如XXXBuilder的类型，我们一般也是使用这些类来创建我们需要的对象。

比如，我们在JavaSE中就学习过的StringBuiler类：

public static void main(String[] args) {
    StringBuilder builder = new StringBuilder();   //创建一个StringBuilder来逐步构建一个字符串
    builder.append(666);   //拼接一个数字
    builder.append("老铁");   //拼接一个字符串
       builder.insert(2, '?');  //在第三个位置插入一个字符
    System.out.println(builder.toString());   //差不多成形了，最后转换为字符串
}

实际上我们是通过建造者来不断配置参数或是内容，当我们配置完所有内容后，最后再进行对象的构建。

相比直接去new一个新的对象，建造者模式的重心更加关注在如何完成每一步的配置，同时如果一个类的构造方法参数过多，我们通过建造者模式来创建这个对象，会更加优雅。

比如我们现在有一个学生类：

class Student {
    private int id;
    private int age;
    private String name;
    private String college;
    private String profession;
    private List<String> hobby;
}

可以看到这个学生类的属性是非常多的，所以构造方法不是一般的长，如果我们现在直接通过new的方式去创建：

public static void main(String[] args) {
    Student student = new Student(1, 18,  "小明", "计算机学院", "学生", Arrays.asList("抽烟", "喝酒","烫头"));
}

可以看到，我们光是填参数就麻烦，我们还得一个一个对应着去填，一不小心可能就把参数填到错误的位置了。

所以，我们现在可以使用建造者模式来进行对象的创建：

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class Student {
    private int id;
    private int age;
    private String name;
    private String college;
    private String profession;
    private List<String> hobby;
 
    // 设置为私有，只给内部类用
    private Student(int id, int age, String name, String college, String profession, List<String> hobby) {
        this.id = id;
        this.age = age;
        this.name = name;
        this.college = college;
        this.profession = profession;
        this.hobby = hobby;
    }
 
    /**
     * 获取建造者
     *
     * @return 建造者
     */
    public static StudentBuild build() {
        return new StudentBuild();
    }
 
    public int getId() {
        return id;
    }
 
    public void setId(int id) {
        this.id = id;
    }
 
    public int getAge() {
        return age;
    }
 
    public void setAge(int age) {
        this.age = age;
    }
 
    public String getName() {
        return name;
    }
 
    public void setName(String name) {
        this.name = name;
    }
 
    public String getCollege() {
        return college;
    }
 
    public void setCollege(String college) {
        this.college = college;
    }
 
    public String getProfession() {
        return profession;
    }
 
    public void setProfession(String profession) {
        this.profession = profession;
    }
 
    public List<String> getHobby() {
        return hobby;
    }
 
    public void setHobby(List<String> hobby) {
        this.hobby = hobby;
    }
 
    @Override
    public String toString() {
        return "Student{" +
                "id=" + id +
                ", age=" + age +
                ", name='" + name + '\'' +
                ", college='" + college + '\'' +
                ", profession='" + profession + '\'' +
                ", hobby=" + hobby +
                '}';
    }
 
    public static class StudentBuild {
        //Builder也需要将所有的参数都进行暂时保存，所以Student怎么定义的这里就怎么定义
        private int id;
        private int age;
        private String name;
        private String college;
        private String profession;
        private List<String> hobby;
 
        // 这里返回this是为了链式调用
        public StudentBuild id(int id) {
            this.id = id;
            return this;
        }
 
        public StudentBuild age(int age) {
            this.age = age;
            return this;
        }
 
        public StudentBuild name(String name) {
            this.name = name;
            return this;
        }
 
        public StudentBuild college(String college) {
            this.college = college;
            return this;
        }
 
        public StudentBuild profession(String profession) {
            this.profession = profession;
            return this;
        }
 
        public StudentBuild hobby(String... hobby) {
            this.hobby = Arrays.asList(hobby);
            return this;
        }
 
        public Student build() {
            return new Student(id, age, name, college, profession, hobby);
        }
    }
}

现在，我们就可以使用建造者来为我们生成对象了：

public static void main(String[] args) {
    Student student = Student.build()
            .id(1)
            .age(23)
            .name("张三")
            .college("信息工程学院")
            .profession("学生")
            .hobby("抽烟", "喝酒", "烫头")
            .build();
    //Student{id=1, age=23, name='张三', college='信息工程学院', profession='学生', hobby=[抽烟, 喝酒, 烫头]}
    System.out.println(student);
}