How can we implement a custom iterable in Java?

An Iterable interface is defined in java.lang package and introduced with Java 5 version. An object that implements this interface allows it to be the target of the "for-each" statement. This for-each loop is used for iterating over arrays and collections. An Iterable interface can also be implemented to create custom behavior.

Syntax

public interface Iterable<T>

Example

import static java.lang.String.format;
import java.util.*;
// Person class
class Person {
   private String firstName, lastName;
   private int age;
   public Person(){ }
   public Person(String firstName, String lastName, int age) {
      this.firstName = firstName;
      this.lastName = lastName;
      this.age = age;
   }
   public String getFirstName() {
      return firstName;
   }
   public String getLastName() {
      return lastName;
   }
   public int getAge() {
      return age;
   }
   public void setFirstName(String firstName) {
      this.firstName = firstName;
   }
   public void setLastName(String lastName) {
      this.lastName = lastName;
   }
   public void setAge(int age) {
      this.age = age;
   }
   @Override
   public String toString() {
      return format("First Name:\t%s\tLast Name:\t%s\tAge:\t%d", firstName, lastName, age);
   }
}
// PersonArrayList class
public class PersonArrayList implements Iterable<Person> {
   private List<Person> persons;
   private static final int MIN_AGE = 10;
   public PersonArrayList() {
      persons = new ArrayList<Person>(MIN_AGE);
   }
   public PersonArrayList(int age) {
      persons = new ArrayList<Person>(age);
   }
   public void addPerson(Person p) {
      persons.add(p);
   }
   public void removePerson(Person p) {
      persons.remove(p);
   }
   public int age() {
      return persons.size();
   }
   @Override
   public Iterator<Person> iterator() {
      return persons.iterator();
   }
   public static void main(String[] args) {
      Person p1 = new Person("Adithya", "Sai", 20);
      Person p2 = new Person("Jai","Dev", 30);
      PersonArrayList pList = new PersonArrayList();
      pList.addPerson(p1);
      pList.addPerson(p2);
      for (Person person : pList) {
         System.out.println(person);
      }
   }
}

Output

First Name:  Adithya   Last Name:   Sai   Age:   20
First Name:  Jai       Last Name:   Dev   Age:   30