Implementing Iterable interface in Java 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, and finally shows a Java code example showing Iterable interface implementation and implementing class’ use in a for-each loop.

Iterable 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 –

Typical for-each construct usage

for(MyClass myClassObject: list){

//code to use myClassObject

}

Where list is an instance of java.util.List.

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.

In fact, any class which implements Iterable, 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, 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 interface

Any class implementing Iterable needs to follow three simple steps –

1. Implement Iterable interface.
2. Override Iterable’s iterator() method.
3. Return an instance of Iterator 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 would go like this –

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

Simple, right! There is a small piece of logic missing though!!How do you get hold of an Iterator 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 to store the String objects to be iterated, then you return Iterator returned by List.iterator() method as the output of overridden Iterable.iterator() method.

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

Java code example showing Iterable implementation

Lets take a simple case of aggregation to show an Iterable 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.

We will make Department class implement the Iterable 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.

Java code example showing Iterable implementation

//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

package com.javabrahman.corejava;

import java.util.List;

import java.util.Iterator;

public class IterableDepartment implements Iterable {

List employeeList;

public IterableDepartment(List employeeList){

this.employeeList=employeeList;

}

@Override

public Iterator 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

}

}

OUTPUT of the above code

Tom Jones

Harry Jones

Ethan Hardy

Nancy Smith

Deborah Spright

Explanation of the code

• 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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