What is the ListIterator.add method in Java?

The ListIterator is an iterator which we can traverse the list in both directions (forward and backward), modify the list, and get the index of the iterator’s current position. Also, it doesn’t have the currentElement like Iterator.

The add method inserts the passed element into the list.

The element is inserted:

Before the element which will be returned on calling the next() method on the list iterator. (In other words, the new element will be inserted on the index that is returned by calling the nextIndex method).
After the element which will be returned on calling the previous() method on the list iterator. (In other words, the new element will be inserted after the index that is returned by calling the previousIndex method if it is not -1).

The new element is inserted before the implicit cursor. So the call to the next() method is unaffected. Also, the newly inserted element is returned if we call the previous() method.

Syntax

void add(E e)

This method takes the element to be inserted into the list as an argument.

This method does not return any value.

Code

The code below demonstrates how to use the add method:

import java.util.ListIterator;
import java.util.ArrayList;
  
class Add {
    public static void main(String[] args) {
        // Initialize array
        ArrayList<Integer> list = new ArrayList<>();
        list.add(10);
        list.add(20);
        list.add(30);
        System.out.println("The array list is  " + list);
        // ListIterator
        ListIterator<Integer> it = list.listIterator();
        System.out.println("Calling it.nextIndex() " + it.nextIndex());
        System.out.println("Calling it.previousIndex() " + it.previousIndex());
        // Add 11 using ListIterator
        it.add(11);
        System.out.println("\nAdding using it.add(11)");
        System.out.println("\nCalling it.nextIndex() " + it.nextIndex());
        System.out.println("Calling it.previousIndex() " + it.previousIndex());
        System.out.println("The array list is  " + list);
    }
}

Explanation

In lines 1 and 2: Import the ListIterator and ArrayList class.
From lines 7 to 11: Create a new ArrayList object with name list and add three elements 10,20,30 to it using the add methods.
In line 14: Use the listIterator method of the ArrayList object to get the ListIterator for the ArrayList.
In line 15: Use the nextIndex method of the listIterator object to get the next element index which will be returned on calling the next method. We will get 0 as result.
In line 16: Use the previousIndex method of the listIterator object to get the previous element index which will be returned on calling the previous method. We will get -1 because there is no element present before the cursor of the listIterator.
In line 18: Use the add method with 11 as an argument. This method will add 11 in the index of the nextIndex. In our case, nextIndex is 0 so the element will be inserted in index 0. Now the cursor will be pointing to index 0. If we call nextIndex again we will get 1 and if we call previousIndex we will get 0.

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