建造者模式的简易实现（在同一个类中用静态内部类实现）

1. 实体情况：

@ToString
public class Person {
    private Long id;
    private String personName;
    private int age;

    public Person(Long id, String personName, int age) {
        this.id = id;
        this.personName = personName;
        this.age = age;
    }

    public static Person.PersonBuilder builder() {
        return new Person.PersonBuilder();
    }

    public static class PersonBuilder {
        private Long id;
        private String personName;
        private int age;

        public Person.PersonBuilder id(Long id) {
            this.id = id;
            return this;
        }

        public Person.PersonBuilder personName(String personName) {
            this.personName = personName;
            return this;
        }

        public Person.PersonBuilder age(int age) {
            this.age = age;
            return this;
        }

        public Person build() {
            return new Person(this.id, this.personName, this.age);
        }
    }
}

2. 测试代码：

    public static void main(String[] args) {
//        Person person = new Person.PersonBuilder()
//                .id(1L)
//                .personName("zhangsan")
//                .age(20)
//                .build();

        Person person = Person.builder()
                .id(1L)
                .personName("zhangsan")
                .age(20)
                .build();

        System.out.println(person);
    }