Adding elements to ArrayList in java

Question

import java.util.ArrayList;

public class ArrayTest {

    private static ArrayList<String> list = new ArrayList<String>();


    public static void printList () {
        System.out.println("Printing the list");
        for (int i = 0; i < list.size(); i++)
        {
            if (list.get(i) != null)
                System.out.println(i + " => " + list.get(i));
        }   
    }

    public static void add(Integer index, String val) {
        while(list.size() <= index)
            list.add(null);
        list.add(index, val);   
    }   

    public static void main(String[] args) {
        ArrayTest.add(8, "cover");
        ArrayTest.printList();
        ArrayTest.add(6, "and");
        ArrayTest.printList();
    }   

}

produces the following output

Printing the list
8 => cover
Printing the list
6 => and
9 => cover

The add(Integer index, String val) function adds an appropriate number of nulls to the list as a check to avoid any IndexOutOfBoundsException exceptions. Can someone explain why does adding "and" to the list push "cover" a position further in the list ?

Read the documentation, the ArrayList.add(int,element) inserts an element at the specific index, shifting subsequent elements back. The ArrayList.set(int, element) sets the element at the index without shifting. — recursion.ninja
– recursion.ninja, Commented Apr 14, 2013 at 18:15

Miguel Prz · Accepted Answer · 2013-04-14 18:15:08Z

3

Because the add method you used inserts an element at specified index. It doesn't replace the existing element at this position, it add a new, so add the index position from that towards the end.

If you need to replace the value, use the set method.

answered Apr 14, 2013 at 18:15

Miguel Prz

13.8k32 silver badges42 bronze badges

Sign up to request clarification or add additional context in comments.

Comments

kryger · Accepted Answer · 2013-04-14 18:21:41Z

2

Because this is the actual specification of the List.add(int index, E element) method:

Inserts the specified element at the specified position in this list (optional operation). Shifts the element currently at that position (if any) and any subsequent elements to the right (adds one to their indices).

As to "why": this is done to avoid overwriting the element on the specified position.

edited Apr 14, 2013 at 18:21

answered Apr 14, 2013 at 18:16

kryger

13.2k8 gold badges47 silver badges68 bronze badges

Comments

Rahul Raj · Accepted Answer · 2018-04-10 09:06:57Z

0

In Java 9, there's an easy way with less number of lines without needing to initialize or add method. Use collection factory methods:

List<String> list = List.of("first", "second", "third");

answered Apr 10, 2018 at 9:06

Rahul Raj

3,4698 gold badges42 silver badges67 bronze badges

Collectives™ on Stack Overflow

Adding elements to ArrayList in java

3 Answers 3

Comments

Comments

Comments

Hot Network Questions

Collectives™ on Stack Overflow

3 Answers 3

Comments

Comments

Comments

Related