Java项目中怎么对ArrayList进行嵌套

import java.util.ArrayList;import com.heima.bean.Person;public class Demo5_ArrayListArrayList {  public static void main(String[] args) {  ArrayList<ArrayList<Person>> school = new ArrayList<>();  ArrayList<Person> class1 = new ArrayList<>();  class1.add(new Person("张三", 23));  class1.add(new Person("李四", 24));  class1.add(new Person("王五", 25));  ArrayList<Person> class2 = new ArrayList<>();  class2.add(new Person("张三一", 23));  class2.add(new Person("李四一", 24));  class2.add(new Person("王五一", 25));  school.add(class1);  school.add(class2);  for (ArrayList<Person> outer : school) {   for (Person in : outer) {    System.out.println(in);   }  } }}