Iterators in Java

Traverse collections using Iterator and ListIterator, implement the Iterable interface on custom classes, and understand the for-each contract.

Why Iterators Matter

1. What is an Iterator?

An `Iterator` is an object that traverses a collection one element at a time without exposing the underlying structure. It answers three questions: does a next element exist (`hasNext`), give me the next element (`next`), and optionally remove it (`remove`).

The Iteration Protocol

1. Iterator Interface

The `Iterator<T>` interface has two key methods: `hasNext()` — returns true if more elements exist, and `next()` — returns the next element and advances the cursor.

2. Built-in Iterables

All standard Java collections — `List`, `Set`, `Map.entrySet()`, arrays via `Arrays.asList()` — implement `Iterable` and support `iterator()` and for-each.

Removing Elements While Iterating

1. Safe Removal Using Iterator

Never use `list.remove()` inside a for-each loop — it throws `ConcurrentModificationException`. Use `iterator.remove()` instead.

ListIterator

1. Bidirectional Traversal & Modification

`ListIterator` extends `Iterator` with `hasPrevious()`, `previous()`, `set()`, and `add()`. It works only on `List`, not `Set` or `Map`.

Creating a Custom Iterable

1. Custom Iterator Object

Implement `Iterable<T>` on your class and return an `Iterator<T>` from `iterator()`. This lets your class work in for-each loops.

Lazy Evaluation

1. Infinite Sequence with Stream.iterate

`Stream.iterate()` and `Stream.generate()` create potentially infinite sequences — values are produced only when consumed.

Backend Use Cases

1. Batch Processing

Process large datasets in fixed-size chunks to avoid loading everything into memory at once.

Pitfalls & Best Practices

1. Recommended Guidelines

Practical rules for writing safe, idiomatic iteration code in Java.