Iterator and Collection, both has helped and comforted the programmers at many a times. But their usage and application has a very wide difference.
1. Iterator
- Declaration
public interface Iterator Type Parameters: E - the type of elements returned by this iterator
- Iterators are used in Collection framework in Java to retrieve elements one by one.
- Method Summary
| Modifier and Type | Method | Description |
|---|---|---|
| default void | forEachRemaining(Consumer<? super E> action) | Performs the given action for each remaining element until all elements have been processed or the action throws an exception. |
| boolean | hasNext() | Returns true if the iteration has more elements. |
| E | next() | Returns the next element in the iteration. |
| default void | remove() | Removes from the underlying collection the last element returned by this iterator (optional operation). |
2. Collection
- Declaration:
public interface Collection<E> extends Iterable<E> Type Parameters: E - the type of elements returned by this iterator
- A Collection is a group of individual objects represented as a single unit. Java provides Collection Framework which defines several classes and interfaces to represent a group of objects as a single unit.
- Method Summary
| Modifier and Type | Method | Description |
|---|---|---|
| boolean | add(E e) | Ensures that this collection contains the specified element (optional operation). |
| boolean | addAll(Collection<? extends E> c) | Adds all of the elements in the specified collection to this collection (optional operation). |
| void | clear() | Removes all of the elements from this collection (optional operation). |
| boolean | contains(Object o) | Returns true if this collection contains the specified element. |
| boolean | containsAll(Collection<?> c) | Returns true if this collection contains all of the elements in the specified collection. |
| boolean | equals(Object o) | Compares the specified object with this collection for equality. |
| int | hashCode() | Returns the hash code value for this collection. |
| boolean | isEmpty() | Returns true if this collection contains no elements. |
| Iterator<E> | iterator() | Returns an iterator over the elements in this collection. |
| default Stream<E> | parallelStream() | Returns a possibly parallel Stream with this collection as its source. |
| boolean | remove(Object o) | Removes a single instance of the specified element from this collection, if it is present (optional operation). |
| boolean | removeAll(Collection<?> c) | Removes all of this collection’s elements that are also contained in the specified collection (optional operation). |
| default boolean | removeIf(Predicate<? super E> filter) | Removes all of the elements of this collection that satisfy the given predicate. |
| boolean | retainAll(Collection<?> c) | Retains only the elements in this collection that are contained in the specified collection (optional operation). |
| int | size() | Returns the number of elements in this collection. |
| default Spliterator<E> | spliterator() | Creates a Spliterator over the elements in this collection. |
| default Stream<E> | stream() | Returns a sequential Stream with this collection as its source. |
| Object[] | toArray() | Returns an array containing all of the elements in this collection. |
| T[] | toArray(T[] a) | Returns an array containing all of the elements in this collection; the runtime type of the returned array is that of the specified array. |
Iterator Vs. Collection
- Iterator can only move to next() element or remove() an element.
However Collection can add(), iterate, remove() or clear() the elements of the collection. - Iterator provides a better speed than Collections, as the Iterator interface has limited number of operations.
- java.sql.SQLException extends Iterable. Hence it allows the caller to safely iterate over causes of SQLException.
Using a collection, in this case, would be expensive because, in a chain of n exceptions, use of a collection in the SQLException interface would potentially require the construction of O(n^2) elements.
However, use of Iterable provides O(n) access to the exception chain.
