List iterator Java

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 numbers = new ArrayList[]; numbers.add[1]; numbers.add[3]; numbers.add[2]; System.out.println["ArrayList: " + numbers]; // Creating an instance of ListIterator ListIterator 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 numbers = new ArrayList[]; numbers.add[1]; numbers.add[3]; numbers.add[2]; System.out.println["ArrayList: " + numbers]; // Creating an instance of ListIterator ListIterator 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.