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:
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 profession(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")
.profession("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);
}