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

I will answer the question especially about ArrayList as an example in order to help you understand better..

1. Iterable interface forces its subclasses to implement abstract method 'iterator()'.

public interface Iterable {
  ...
  abstract Iterator<T> iterator(); //Returns an 'Iterator'(not iterator) over elements of type T.
  ...
}

2. 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.
  ...
}

3. ArrayList implements List, List extends Collection and Collection extends Iterable.. That is, you could see the relationship like

   'Iterable <- Collection <- List <- ArrayList'

. And Iterable, Collection and List just declare abstract method 'iterator()' and ArrayList alone implements it.

4. 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<E> ... implements List<E>, ...
{
  ...
  public Iterator<E> iterator() {
              return new Itr();
  }


  private class Itr implements Iterator<E> {
          ...

          public boolean hasNext() {
              return cursor != size;
          }
          @SuppressWarnings("unchecked")
          public E next() {
              checkForComodification();
              int i = cursor;
              if (i >= size)
                  throw new NoSuchElementException();
              Object[] elementData = ArrayList.this.elementData;
              if (i >= elementData.length)
                  throw new ConcurrentModificationException();
              cursor = i + 1;
              return (E) elementData[lastRet = i];
          }
          ...
  }
}

5. 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<String> list = new ArrayList<>();
    list.add("a");
    list.add("b");
    list.add("c");
    list.add("d");
    list.add("e");
    list.add("f");

    Iterator<String> iterator = list.iterator();
    while (iterator.hasNext()) {
        String string = iterator.next();
        System.out.println(string);
    }
}

Now, is it clear? :)

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.

An implementation of Iterable is one that provides an Iterator of itself:

public interface Iterable<T>
{
    Iterator<T> 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<E>
{
    boolean hasNext();
    E next();
    void remove();
}

See Javadoc.