Java Programming

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 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.