Pattern · Core

Iterator

Walk through a collection's elements one by one WITHOUT exposing how the collection stores them — and support multiple independent walks at once.

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A cursor over the collection
createsreads
«interface»
Collection
+ iterator(): Iterator
«interface»
Iterator
+ hasNext()
+ next()
BookShelf
+ iterator()
ShelfCursor
- index
+ hasNext()
+ next()
A cursor over the collection
 1interface Collection { Iterator iterator(); }
 2interface Iterator { boolean hasNext(); Object next(); }
 3class BookShelf implements Collection {
 4    Iterator iterator(){ return new ShelfCursor(this); }
 5}
 6class ShelfCursor implements Iterator {
 7    int index = 0;
 8    boolean hasNext(){ return index < shelf.size(); }
 9    Object next(){ return shelf.getAt(index++); }
10}
State
Collectioniterator() factory
IteratorhasNext() / next()
✦

Two interfaces, no storage in sight. A Collection can hand out an Iterator; an Iterator is just hasNext() and next(). Nothing here says "array" or "tree" — that's the whole point.