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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.

Get Previous and Next Index using Java ListIterator

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