Java ListIterator Interface

Java ListIterator Interface

In this tutorial, we will learn about the Java ListIterator interface with the help of an example.

The ListIterator interface of the Java collections framework provides the functionality to access elements of a list.

It is bidirectional. This means it allows us to iterate elements of a list in both the direction.

It extends the Iterator interface.

The ListIterator interface extends the Java Iterator interface.

The List interface provides a listIterator() method that returns an instance of the ListIterator interface.


Methods of ListIterator

The ListIterator interface provides methods that can be used to perform various operations on the elements of a list.

  • hasNext() - returns true if there exists an element in the list
  • next() - returns the next element of the list
  • nextIndex() returns the index of the element that the next() method will return
  • previous() - returns the previous element of the list
  • previousIndex() - returns the index of the element that the previous() method will return
  • remove() - removes the element returned by either next() or previous()
  • set() - replaces the element returned by either next() or previous() with the specified element

Example 1: Implementation of ListIterator

In the example below, we have implemented the next(), nextIndex() and hasNext() methods of the ListIterator interface in an array list.

import java.util.ArrayList;
import java.util.ListIterator;

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 ListIterator
        ListIterator<Integer> iterate = numbers.listIterator();

        // Using the next() method
        int number1 = iterate.next();
        System.out.println("Next Element: " + number1);

        // Using the nextIndex()
        int index1 = iterate.nextIndex();
        System.out.println("Position of Next Element: " + index1);

        // Using the hasNext() method
        System.out.println("Is there any next element? " + iterate.hasNext());
    }
}

Output

ArrayList: [1, 3, 2]
Next Element: 1
Position of Next Element: 1
Is there any next element? true

Example 2: Implementation of ListIterator

In the example below, we have implemented the previous() and previousIndex() methods of the ListIterator interface in an array list.

import java.util.ArrayList;
import java.util.ListIterator;

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 ListIterator
        ListIterator<Integer> iterate = numbers.listIterator();
        iterate.next();
        iterate.next();

        // Using the previous() method
        int number1 = iterate.previous();
        System.out.println("Previous Element: " + number1);

        // Using the previousIndex()
        int index1 = iterate.previousIndex();
        System.out.println("Position of the Previous element: " + index1);
    }
}

Output

ArrayList: [1, 3, 2]
Previous Element: 3
Position of the Previous Element: 0

In the above example, initially, the instance of the Iterator was before 1. Since there was no element before 1 so calling the previous() method will throw an exception.

We then used the next() methods 2 times. Now the Iterator instance will be between 3 and 2.

Hence, the previous() method returns 3.