Для чего написано: while (it.hasNext()

package com.company;
import java.util.ArrayList;
import java.util.Iterator;
public class Main
{
public static void main(String[] args)
{
person p = new person();
person a = new person();
person b = new person();
person z = new person();
person q = new person();
UI ui = new UI();
ui.set(p);
ui.set(a);
ui.set(b);
ui.set(z);
ui.set(q);
ArrayList<person> people = new ArrayList<>();
people.add(p);
people.add(a);
people.add(b);
people.add(z);
people.add(q);
Iterator<person> it = people.iterator();
while (it.hasNext())
{
ui.show(it.next());
}
System.out.println("Люди старше 25 лет: ");
ArrayList<person> adult = new ArrayList<>();
 
if (p.age > 25 ) {adult.add(p); }
if (a.age > 25 ) {adult.add(a); }
if (b.age > 25 ) {adult.add(b); }
if (z.age > 25 ) {adult.add(z); }
if (q.age > 25 ) {adult.add(q); }
 
Iterator<person> iter = adult.iterator();
while (iter.hasNext())
{
ui.show(iter.next());
}
}
}