Iterator Pattern

What

Provides a way to access elements of a collection sequentially without exposing its underlying structure.

When

You need to traverse a data structure without exposing its internal implementation. You want to access elements of a collection without exposing its structure.

class MyCollection {
    private String[] items = {"a", "b", "c"};
 
    public String[] getItems() {
        return items; // Exposes internal structure
    }
}
 
class Client {
    public static void main(String[] args) {
        MyCollection c = new MyCollection();
        for (String item : c.getItems()) {
            System.out.println(item);
        }
    }
}
 

How

• Define an Iterator interface with methods like hasNext() and next().
• Concrete iterator implements this interface.
• The collection provides a method to return an iterator.

import java.util.Iterator;
import java.util.List;
 
class MyCollection implements Iterable<String> {
    private List<String> items = List.of("a", "b", "c");
    public Iterator<String> iterator() {
        return items.iterator();
    }
}
 

Examples

• Looping over a list, tree, or custom collection.
• File system navigation.

Why

Advantages

• Uniform interface for different types of collections.
• Supports multiple simultaneous traversals.
• Encapsulates traversal logic.

Disadvantages

• Can be less efficient if not implemented carefully.
• May expose internal data if not designed securely.