This tutorial explains how to implement java.lang.Iterable<T> on a class to enable for-each loop based iteration through the collection of objects stored in that class. Tutorial starts off with explaining how Iterable and for-each loop are related, then explains how to implement Iterable<T>, and finally shows a Java code example showing Iterable interface implementation and implementing class’ use in a for-each loop.

Iterable<T> and the for-each loop
Java 5 introduced for-each loop which took away the error-prone aspect of looping, specifically the need to manage loop-counter variable and end-of-loop conditions. All one now needs to do to iterate over a collection using the for-each loop is to write something like this –

for(MyClass myClassObject: list){
  //code to use myClassObject
}

Where list is an instance of java.util.List<MyClass>.

Most of the important in-built collection types now support iteration using the enhanced for-each loop. This is by virtue of their implementing the interface Iterable<T>.

In fact, any class which implements Iterable<T>, can be used in a for-each loop to iterate over the objects of type T which it holds or encapsulates. Extending this logic to the small code snippet we saw above – MyCollection which implements Iterable<MyClass>, can be used in a for-each loop to iterate through MyClass objects stored in it.

Having understood the relationship between implementing Iterable interface and use of the implementing class in for-each loop, let us now understand how to go about implementing Iterable.

How to implement Iterable<T> interface
Any class implementing Iterable<T> needs to follow three simple steps –

1. Implement Iterable<T> interface.
2. Override Iterable’s iterator() method.
3. Return an instance of Iterator<T> from the iterator() method.

So, if you have an API/Class containing a collection of String type of elements, and you want clients of this API to be able to access theString objects using a for-each loop, then your three steps of implementing Iterable<String> would go like this –

1. Implement Iterable<String>.
2. Override Iterable’s iterator() method.
3. Return an instance of Iterator<String> from the iterator() method.

Simple, right! There is a small piece of logic missing though!!How do you get hold of an Iterator<String> instance pointing to your stored collection?

The general practice in this case is to return the in-built Iterator instance of the collection class you use to store the iterable objects in your API. So, if you use a List<String> to store the String objects to be iterated, then you return Iterator<String> returned by List.iterator() method as the output of overridden Iterable.iterator() method.

Let us see a Java code example to see how Iterable<T> implementation can be done.

Java code example showing Iterable<T> implementation
Lets take a simple case of aggregation to show an Iterable<T> implementation in action. For our example scenario we have 2 types – Department and Employee. A Department instance holds multiple Employee instances in a employee list, or List<Employee>.

We will make Department class implement the Iterable<Employee> interface. Doing so would would allow us to iterate through employees of a department using the for-each loop just by getting hold of a Department instance. Let us see the code in action now, which will be followed by detailed explanation of the code.

//Employee.java(POJO)
package com.javabrahman.corejava;
public class Employee {
  private String name;
  private Integer age;

  public Employee(String name, Integer age) {
    this.name = name;
    this.age = age;
  }

  //setters and getters for name & age go here

  //standard override of equals() & hashcode() methods goes here
}
//IterableDepartment.java which implements Iterable<Employee>
package com.javabrahman.corejava;

import java.util.List;
import java.util.Iterator;

public class IterableDepartment implements Iterable<Employee> {

  List<Employee> employeeList;

  public IterableDepartment(List<Employee> employeeList){
    this.employeeList=employeeList;
  }

 @Override
  public Iterator<Employee> iterator() {
    return employeeList.iterator();
  }
}
//Client class IterableDeptClient.java
//Iterates through IterableDepartment's employees using for-each loop
package com.javabrahman.corejava;

import java.util.Arrays;
import java.util.List;

public class IterableDeptClient {
  public static void main(String args[]){
    List<Employee> employeeList
        = Arrays.asList(new Employee("Tom Jones", 45),
        new Employee("Harry Jones", 42),
        new Employee("Ethan Hardy", 65),
        new Employee("Nancy Smith", 22),
        new Employee("Deborah Sprightly", 29));

    IterableDepartment iterableDepartment=new IterableDepartment(employeeList);

    for(Employee emp: iterableDepartment){
      System.out.println(emp.getName());
    }
  }
}

• Employee.java is the POJO class in this example. It has only 2 attributes name & age.
• IterableDepartment class contains a List attribute named employeeList which is initialized using IterableDepartment’s only public constructor.
• IterableDeptClient first creates an employee list consisting of 5 employees, and then passes this employee list to the constructor of the new IterableDepartment instance it creates.
• Then it iterates through the Employee objects in the IterableDepartment instance using a for-each loop.
• In each iteration of the for-each loop, name of the employee encountered is printed. As expected, the for-each loop correctly iterates through the 5 Employee objects stored in the IterableDepartment instance, and prints their names.

Summary
In the above tutorial we understood how Iterable interface can be implemented on a class holding a collection of objects. We then saw with a Java code example showing how such a collection can be iterated through using the enhanced for-each loop.

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