CODEWITHSUNDEEP
java
Bharat Khatri
Bharat Khatri

Reputation: 1407

Adding elements to ArrayList in java

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 ?

Upvotes: 0

Views: 5460

Answers (3)

Rahul Raj
Rahul Raj

Reputation: 3439

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");

Upvotes: 0

kryger
kryger

Reputation: 13181

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.

Upvotes: 2

Miguel Prz
Miguel Prz

Reputation: 13792

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.

Upvotes: 3

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