Iterable and Iterator Interface

Iterable interface is used to represent a collection of elements that can be iterated over, while the Iterator interface provides a way to iterate over the elements of the collection.

1. Iterable<T> Interface

   Implementing a iterable interface means that now objects of this class can be iterated over using a loop.

   Note - Iterable interface uses interator interface under the hood.

   public interface Iterable<T> {
       Iterator<T> iterator();
   }
   

2. Iterator<T> Interface

   public interface Iterator<T> {
       boolean hasNext(); // checks if elements are left
       T next(); // gives next element
       default void remove(); // optional
   }
   

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Implementing Iterable

import java.util.Iterator;

public class CollectionTesting implements Iterable<Integer> {
    private Integer arr[];

    public CollectionTesting(Integer[] arr) {
        this.arr = arr;
    }

    @Override
    public Iterator<Integer> iterator() {
        class ArrayIterator implements Iterator<Integer> {
            private int indx = 0;

            @Override
            public boolean hasNext() {
                return indx < arr.length;
            }

            @Override
            public Integer next() {
                return arr[indx++];
            }
        }
        return new ArrayIterator();
    }

    public static void main(String[] args) {
        Integer[] numbers = {1, 2, 3, 4, 5};
        CollectionTesting collectionTesting = new CollectionTesting(numbers);

        for (Integer number : collectionTesting){ // call iterator() which return ArrayIterator object 
            System.out.println(number);
            
        // Andar ye hora hai while upr foreach loop me magic tha 
        Iterator<Integer> iterator = collectionTesting.iterator();
        while (iterator.hasNext()) {
            System.out.println(iterator.next());
        }

    }
    }
}