Java Iterator Interface

The Iterator interface of the Java collections framework allows us to access elements of a collection. It has a subinterface ListIterator.

[The Listiterator interface extends the Java Iterator interface.]

All the Java collections include an iterator() method. This method returns an instance of iterator used to iterate over elements of collections.

Methods of Iterator

The Iterator interface provides 4 methods that can be used to perform various operations on elements of collections.

• hasNext() - returns true if there exists an element in the collection
• next() - returns the next element of the collection
• remove() - removes the last element returned by the next()
• forEachRemaining() - performs the specified action for each remaining element of the collection

Example: Implementation of Iterator

In the example below, we have implemented the hasNext(), next(), remove() and forEachRemining() methods of the Iterator interface in an ArrayList.

import java.util.ArrayList;
import java.util.Iterator;

class Main {
    public static void main(String[] args) {
        // Creating an ArrayList
        ArrayList<Integer> numbers = new ArrayList<>();
        numbers.add(1);
        numbers.add(3);
        numbers.add(2);
        System.out.println("ArrayList: " + numbers);

        // Creating an instance of Iterator
        Iterator<Integer> iterate = numbers.iterator();

        // Using the next() method
        int number = iterate.next();
        System.out.println("Accessed Element: " + number);

        // Using the remove() method
        iterate.remove();
        System.out.println("Removed Element: " + number);

        System.out.print("Updated ArrayList: ");

        // Using the hasNext() method
        while(iterate.hasNext()) {
            // Using the forEachRemaining() method
            iterate.forEachRemaining((value) -> System.out.print(value + ", "));
        }
    }
}

Output

ArrayList: [1, 3, 2]
Acessed Element: 1
Removed Element: 1
Updated ArrayList: 3, 2,

In the above example, notice the statement:

iterate.forEachRemaining((value) -> System.put.print(value + ", "));

Here, we have passed the lambda expression as an argument of the forEachRemaining() method.

Now the method will print all the remaining elements of the array list.