Iterator Pattern : A Simple Explanation

Situation: You’re Dealing with a List of Users

You’ve got a List<User> from the database.
You want to go through this list and:

Send emails
Log activity
Filter some out

Easy, right? So you write:

for (int i = 0; i < users.size(); i++) {
    User u = users.get(i);
    // do something
}

Then Things Get Ugly

Suddenly, your collection isn’t always a List anymore:

Sometimes it’s a Set<User>
Sometimes it’s a Queue<User>
Sometimes a custom data structure (like Tree<User> or a weird in-house object)

Now your code is littered with:

if (collection instanceof List) {
    // loop one way
} else if (collection instanceof Set) {
    // different loop
} else if ...

You’re stuck in type-checking hell just to loop through elements.

What’s the Real Problem?

You’re writing looping logic that’s tied to how data is stored
You can’t swap data structures without rewriting your logic
Your business logic is now tightly coupled to the internal structure of the collection

It’s fragile, repetitive, and breaks the Single Responsibility Principle

How Iterator Pattern Saves You

Iterator says:

“Stop caring about how the collection is built. I’ll give you a uniform way to walk through it - safely.”

So now:

Every collection gives you an Iterator
That iterator has .hasNext() and .next() methods
You don’t care if the data is a List, Set, or custom tree, the loop looks the same

You focus on what you want to do, not how to get to each element

class NameCollection {
    private final String[] names = {"Name1", "Name2", "Name3", "Name4"};

    public MyIterator iterator() {
        return new NameIterator();
    }

    private class NameIterator implements MyIterator {
        private int index = 0;

        public boolean hasNext() {
            return index < names.length;
        }

        public String next() {
            return names[index++];
        }
    }
}

When to Use Iterator?

When you want to traverse collections without knowing their internals
When you want a common interface to loop through any collection
When building custom collections but still want them loopable