Data Structures

Iterable

In Java, Iterable is an interface that allows an object to be the target of the for-each loop (also known as the enhanced for loop). Any class that implements the Iterable interface promises to provide an Iterator, which is the object that actually steps through the collection’s elements one by one.

The Core Concept

Think of Iterable as a contract that says, “I can be looped over.” When you write a for-each loop like this:

for (Element e : myIterableObject)
{
    // do something with e
}

The Java compiler automatically uses the Iterable interface to get an Iterator from myIterableObject. It then uses that Iterator to go through each element until it runs out.

Iterable vs. Iterator

These two interfaces often confuse students, but their roles are distinct:

• Iterable: This is a collection-level interface. A class like ArrayList or LinkedList is Iterable. It has a single method, iterator(), which returns an Iterator object. It’s the “factory” that produces the Iterator.
• Iterator: This is the object that does the actual work of iterating. It has three main methods:
  • hasNext(): Returns true if there are more elements to iterate over.
  • next(): Returns the next element in the sequence.
  • remove(): (Optional) Removes the last element returned by next().

So, the Iterable object is the container, and the Iterator is the “pointer” that moves through the container.

Why is it Useful?

The Iterable interface is a powerful part of Java’s Collections Framework because it provides a consistent, simple way to loop over different types of collections. Whether you have a List, a Set, or a custom collection, as long as it’s Iterable, you can use the same for-each loop syntax. This simplifies your code and makes it more readable, abstracting away the underlying implementation details of how the elements are stored.