Java 泛型-SelfType | Better

SelfType 指的是方法所运行的类

Java 中并没有这个概念，在方法重载协变返回值可能会使用到。

protected Object clone()

子类实现clone（）方法时，需要强制转换cast类型

@Override
public Person clone(){
    return (Persion)super.clone();
}

在Builder模式中如果子类扩展父类Builder,会发现编译不过

public class PersonBuilder {

    private String name;

    public PersonBuilder withName(String name) {
        this.name = name;
        return this;
    }

    public Person build() {
        return new Person(name);
    }

}

public class EmployeeBuilder {

    public EmployeeBuilder withName(String name) {
        return (EmployeeBuilder) super.withName(name);
    }

}

Employee doe = new EmployeeBuilder()
    // 返回的是 PersonBuilder
    .withName("John Doe")
    // 方法无法找到
    .withHiringDate(LocalDateTime.now()) // compile error :(
    .build();

解决方式是，重载对应的方法修改返回类型，利用类型转换返回子类类型

public EmployeeBuilder withName(String name) {
    return (EmployeeBuilder) super.withName(name);
}

另外一种解决方式是利用自身泛型类型PersonBuilder<T extends PersonBuilder>

public static abstract class PersonBuilder<T extends PersonBuilder<T>> {
        private String name;

        public T withName(String name) {
            this.name = name;
            return selfType();
        }

        abstract T selfType();


        public Person build() {
            Person person = new Person();
            person.setName(name);
            return person;
        }
}

    public static class EmployeeBuilder extends PersonBuilder<Employee.EmployeeBuilder> {
        Date hiringData;

        public EmployeeBuilder withHiringDate(Date date) {
            hiringData = date;
            return this;
        }

        @Override
        EmployeeBuilder selfType() {
            return new EmployeeBuilder();
        }
    }

调用上就是正常

new Employee.EmployeeBuilder()
           .withName("Haha")
           .withHiringDate(new Date())
           .build();

自身泛型避免了类型转换，不需要使用cast