无聊代码一段

class Person implements Serializable {private static final long serialVersionUID = -9016986248073447608L;public static enum Gender {MALE, FEMALE}private static class InstanceHolder {private static class InstanceHolder2 {private static final Person instatnce = new Person("John", 31, Gender.MALE);}}public static Person getInstance() {return InstanceHolder.InstanceHolder2.instatnce; // 这里很奇怪的有木有}String name = null;Integer age = null;Gender gender = null;private Person() {System.out.println("none-arg constructor");}private Person(String name, Integer age, Gender gender) {System.out.println("arg constructor");this.name = name;this.age = age;this.gender = gender;}}