What is bad?

package pl.codegym.task.task20.task2013; import java.io.Externalizable; import java.io.IOException; import java.io.ObjectInput; import java.io.ObjectOutput; import java.util.List; /* Externalizable dla klasy Person */ public class Solution { public static class Person implements Externalizable{ private String firstName; private String lastName; private int age; private Person mother; private Person father; private List<Person> children; public Person(String firstName, String lastName, int age) { this.firstName = firstName; this.lastName = lastName; this.age = age; } public Person() {} public void setMother(Person mother) { this.mother = mother; } public void setFather(Person father) { this.father = father; } public void setChildren(List<Person> children) { this.children = children; } @Override public void writeExternal(ObjectOutput out) throws IOException { out.writeObject(mother); out.writeObject(father); out.writeChars(firstName); out.writeChars(lastName); out.writeInt(age); for(Person child : children) out.writeObject(child); } @Override public void readExternal(ObjectInput in) throws IOException, ClassNotFoundException { firstName = in.readLine(); lastName = in.readLine(); father = (Person)in.readObject(); mother = (Person)in.readObject(); age = in.readInt(); while(in.available() < 0) children.add((Person) in.readObject()); } } public static void main(String[] args) { } }