What is the difference between iterator and iterable and how to use them

Question

I am new in Java and I m really confused with iterator and iterable  Can anyone explain to me and give some examples

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I know this is an old question  but for anybody reading this who is stuck with the same question and who may be overwhelmed with all the terminology  here s a good  simple analogy to help you understand this distinction between iterables and iterators  Think of a public library  Old school  With paper books   Yes  that kind of library  A shelf full of books would be like an iterable   You can see the long line of books in the shelf   You may not know how many  but you can see that it is a long collection of books  The librarian would be like the iterator   He can point to a specific book at any moment in time   He can insert remove modify read the book at that location where he s pointing   He points  in sequence  to each book at a time every time you yell out  quot next  quot  to him   So  you normally would ask him   quot has Next  quot   and he ll say  quot yes quot   to which you say  quot next  quot  and he ll point to the next book   He also knows when he s reached the end of the shelf  so that when you ask   quot has Next  quot  he ll say  quot no quot   I know it s a bit silly  but I hope this helps

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If a collection is iterable  then it can be iterated using an iterator  and consequently can be used in a for each loop   The iterator is the actual object that will iterate through the collection

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Basically speaking  both of them are very closely related to each other   Consider Iterator to be an interface which helps us in traversing through a collection with the help of some undefined methods like hasNext    next   and remove    On the flip side  Iterable is another interface  which  if implemented by a class forces the class to be Iterable and is a target for For-Each construct  It has only one method named iterator   which comes from Iterator interface itself   When a collection is iterable  then it can be iterated using an iterator   For understanding visit these   ITERABLE   http   grepcode com file repository grepcode com java root jdk openjdk 6-b14 java lang Iterable java  ITERATOR http   grepcode com file repository grepcode com java root jdk openjdk 6-b14 java util Iterator java

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In addition to ColinD and Seeker answers   In simple terms  Iterable and Iterator are both interfaces provided in Java s Collection Framework    Iterable  A class has to implement the Iterable interface if it wants to have a for-each loop to iterate over its collection  However  the for-each loop can only be used to cycle through the collection in the forward direction and you won t be able to modify the elements in this collection  But  if all you want is to read the elements data  then it s very simple and thanks to Java lambda expression it s often one liner  For example   iterableElements forEach  x - gt  System out println x       Iterator  This interface enables you to iterate over a collection  obtaining and removing its elements  Each of the collection classes provides a iterator   method that returns an iterator to the start of the collection  The advantage of this interface over iterable is that with this interface you can add  modify or remove elements in a collection  But  accessing elements needs a little more code than iterable  For example   for  Iterator i   c iterator    i hasNext               Element e   i next         Get the element        System out println e        access or modify the element       Sources    Java Doc Iterable  Java Doc Iterator

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The most important consideration is whether the item in question should be able to be traversed more than once  This is because you can always rewind an Iterable by calling iterator   again  but there is no way to rewind an Iterator

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Consider an example having 10 apples  When it implements Iterable  it is like putting each apple in boxes from 1 to 10 and return an iterator which can be used to navigate   By implementing iterator  we can get any apple  apple in next boxes etc   So implementing iterable gives an iterator to navigate its elements although to navigate  iterator needs to be implemented

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Iterable were introduced to use in for each loop in java  public interface Collection lt E gt  extends Iterable lt E gt      Iterator is class that manages iteration over an Iterable  It maintains a state of where we are in the current iteration  and knows what the next element is and how to get it

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An Iterable is a simple representation of a series of elements that can be iterated over  It does not have any iteration state such as a  current element   Instead  it has one method that produces an Iterator   An Iterator is the object with iteration state  It lets you check if it has more elements using hasNext   and move to the next element  if any  using next     Typically  an Iterable should be able to produce any number of valid Iterators

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Question Difference between Iterable and Iterator   Ans   iterable  It is related to forEach loop  iterator  Is is related to Collection  The target element of the forEach loop shouble be iterable   We can use Iterator to get the object one by one from the Collection  Iterable present in java  ang package Iterator present in java util package  Contains only one method iterator    Contains three method hasNext    next    remove    Introduced in 1 5 version  Introduced in 1 2 version

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As explained here  The    Iterable    was introduced to be able to use in the foreach loop  A class implementing the Iterable interface can be iterated over    Iterator is class that manages iteration over an Iterable  It maintains a state of where we are in the current iteration  and knows what the next element is and how to get it

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I will answer the question especially about ArrayList as an example in order to help you understand better     Iterable interface forces its subclasses to implement abstract method  iterator        public interface Iterable           abstract Iterator lt T gt  iterator      Returns an  Iterator  not iterator  over elements of type T              Iterator interface forces its subclasses to implement abstract method  hasNext    and  next        public interface Iterator            abstract boolean hasNext      Returns  true  if the iteration has more elements    abstract E next               Returns the next element in the iteration              ArrayList implements List  List extends Collection and Collection extends Iterable   That is  you could see the relationship like       Iterable  lt - Collection  lt - List  lt - ArrayList        And Iterable  Collection and List just declare abstract method  iterator    and ArrayList alone implements it    I am going to show ArrayList source code with  iterator    method as follows for more detailed information     iterator    method returns an object of class  Itr  which implements  Iterator     public class ArrayList lt E gt      implements List lt E gt                 public Iterator lt E gt  iterator                   return new Itr            private class Itr implements Iterator lt E gt                             public boolean hasNext                   return cursor    size                         SuppressWarnings  unchecked             public E next                   checkForComodification                  int i   cursor                if  i  gt   size                    throw new NoSuchElementException                  Object   elementData   ArrayList this elementData                if  i  gt   elementData length                    throw new ConcurrentModificationException                  cursor   i   1                return  E  elementData lastRet   i                                       Some other methods or classes will iterate elements of collections like ArrayList through making use of Iterator  Itr        Here is a simple example    public static void main String   args         List lt String gt  list   new ArrayList lt  gt         list add  a        list add  b        list add  c        list add  d        list add  e        list add  f         Iterator lt String gt  iterator   list iterator        while  iterator hasNext              String string   iterator next            System out println string             Now  is it clear

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An implementation of Iterable is one that provides an Iterator of itself   public interface Iterable lt T gt        Iterator lt T gt  iterator        An iterator is a simple way of allowing some to loop through a collection of data without assignment privileges  though with ability to remove    public interface Iterator lt E gt        boolean hasNext        E next        void remove        See Javadoc

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Implementing Iterable interface allows an object to be the target of the  foreach  statement   class SomeClass implements Iterable lt String gt      class Main      public void method            SomeClass someClass   new SomeClass                    for String s   someClass           do something               Iterator is an interface  which has implementation for iterate over elements  Iterable is an interface which provides Iterator

[java] What is the difference between iterator and iterable and how to use them?

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