Iterable and Iterator Interfaces

Overview

The Iterable and Iterator interfaces provide a standardized way to iterate over collections of objects.

• Iterable: An object that can return an Iterator (e.g., lists, stacks, queues).
• Iterator: An object that tracks the current position in a collection and enables traversal.

Interface Definitions

Iterable Interface

public interface Iterable<Item> {
    // Returns an iterator over elements of type `Item`
    public Iterator<Item> iterator();
}

Iterator Interface (Nested)

public interface Iterator<Item> {
    // Checks if there are more elements to iterate over
    public boolean hasNext();
    
    // Returns the next element in the iteration
    public Item next();
    
    // Removes the last element returned by the iterator (optional operation)
    public void remove();
}

Example Usage

Long Form (Explicit Iterator)

Iterator<String> i = stack.iterator();
while (i.hasNext()) {
    String s = i.next();
    StdOut.println(s);
}

Shorthand (Enhanced For-Each Loop)

for (String s : stack) {
    StdOut.println(s);
}

Both code snippets achieve the same result, but the enhanced for loop abstracts the Iterator logic.

Key Notes

1. remove() Method:
   • Optional: Not all iterators implement remove(). If unsupported, it throws an UnsupportedOperationException.
   • Behavior: Removes the last element returned by next(). Must be called after a next().
2. Implementing Iterable:
   • Classes implementing Iterable must provide an iterator() method that returns a valid Iterator.
   • Enables compatibility with the enhanced for loop syntax.
3. Fail-Fast vs. Fail-Safe:
   • Iterators may throw a ConcurrentModificationException if the collection is modified during iteration (fail-fast behavior).
4. Generic Types:
   • Replace Item with the actual data type (e.g., String, Integer) for type safety.

Best Practices

• Prefer the enhanced for loop for readability when no explicit remove() is needed.
• Avoid modifying the underlying collection while iterating unless the iterator supports it.