# Purpose

The Iterator pattern provides a way to access the elements of an aggregate object sequentially without exposing its underlying representation.

# Problem

Need to traverse collections (arrays, lists, trees) uniformly regardless of their internal structure.

# Solution

Use an iterator object that encapsulates the traversal logic. This separates the traversal logic from the business logic.

# Structure

# Participants

• Iterator: Interface for traversal.
• ConcreteIterator: Keeps track of the current position.
• Aggregate: Interface to create an iterator.
• ConcreteAggregate: Returns the proper iterator instance.
• Client: Uses the iterator for traversal.

# When to Use It

• To hide the internal representation of a collection.

• To support multiple traversals of aggregate objects.

# Example: Digital Recipe Book

Navigate recipe steps sequentially (forward/backward) without showing how they are stored.

# Java Code

interface Iterator { boolean hasNext(); Object next(); }

class RecipeIterator implements Iterator {
    private String[] steps;
    private int pos = 0;
    public boolean hasNext() { return pos < steps.length; }
    public Object next() { return steps[pos++]; }
}

class Recipe implements Aggregate {
    public Iterator createIterator() { return new RecipeIterator(steps); }
}

# Mapping Participants

• RecipeIterator: ConcreteIterator.
• Recipe: ConcreteAggregate.
• CookbookApp: Client.

# Conclusions

Provides great flexibility to modify the internal data structure without impacting the client code which traverses it.

# Related Patterns