Iterator vs Collection in Java

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.