Get Previous and Next Index using Java ListIterator

The previous index and next index in an ArrayList can be obtained using the methods previousIndex() and nextIndex() respectively of the ListIterator interface. previousIndex() can also return -1 if it at the beginning of the list.

Example:

Input: list = [2, 3, 6, 8]
listiterator is at the beginning find previous index.
 
Output: -1

Input: list = [2, 3, 6, 8]
listiterator is at the beginning find next index.

Steps to use previousIndex() and nextIndex():

• Create an empty ArrayList
• Add elements to ArrayList.
• Create a listiterator using listIterator() method.

Listiterator<Integer>iterator = arrList.listIterator();

• Now get the required index using the below commands

Syntax

iterator.previousIndex();

Returns: the index of the element that would be returned by a subsequent call to previous, or -1 if the list iterator is at the beginning of the list

Syntax

iterator.nextIndex();

Returns: the index of the element that would be returned by a subsequent call to next, or list size if the list iterator is at the end of the list

Example

Java

// Java program to get Previous and
// next index using ListIterator

import java.io.*;
import java.util.ArrayList;
import java.util.ListIterator;

class PreviousAndNextIndex {

    public static void main(String[] args)
    {
        // create empty ArrayList
        ArrayList<Integer> arrList
            = new ArrayList<Integer>();

        // add elements to the ArrayList
        arrList.add(5);
        arrList.add(10);
        arrList.add(15);
        arrList.add(20);
        arrList.add(25);
        arrList.add(30);

        // print the initial list
        System.out.println("Initial arraylist =>"
                           + arrList);

        // initializing ListIterator
        ListIterator<Integer> iterator
            = arrList.listIterator();

        // initially iterator is the beginning so
        // previousIndex() will return -1
        System.out.println("previous index =>"
                           + iterator.previousIndex());

        // from -1 moving iterator to the 1st index
        iterator.next();
        iterator.next();

        // now iterator is at 1st index
        // so nextIterator() will return 2
        System.out.println("Next index =>"
                           + iterator.nextIndex());
    }
}

Output:

Initial arraylist =>[5, 10, 15, 20, 25, 30]
previous index =>-1
Next index =>2

Time Complexity: O(n)

Auxiliary Space: O(1)

As constant extra space is used.