> For AI agents: the complete documentation index is available at https://nhanbe.id.vn/llms.txt, the full documentation bundle is available at https://nhanbe.id.vn/llms-full.txt.

# Iterator

## Overview

Provide a way to access elements of a collection sequentially without exposing its representation.

## When to use

- You want consistent traversal across different structures.
- You need multiple traversal strategies.

## Java example

```java
class Range implements Iterable<Integer> {
    private final int start, end;
    Range(int start, int end) { this.start = start; this.end = end; }

    public Iterator<Integer> iterator() {
        return new Iterator<>() {
            private int current = start;
            public boolean hasNext() { return current <= end; }
            public Integer next() { return current++; }
        };
    }
}
```

## TypeScript example

```ts
class Range implements Iterable<number> {
  constructor(private start: number, private end: number) {}
  *[Symbol.iterator](): Iterator<number> {
    for (let i = this.start; i <= this.end; i++) yield i;
  }
}
```

## Pros and cons

Pros:

- Hides internal structure.
- Enables multiple traversal algorithms.

Cons:

- Extra objects for simple collections.

## Common pitfalls

- Exposing mutable internals through iterator.
- Ignoring concurrent modification semantics.

## Related patterns

- [Composite](https://nhanbe.id.vn/design-patterns/structural/composite.md)
- [Visitor](https://nhanbe.id.vn/design-patterns/behavioral/visitor.md)