ArrayList - how can I check if an index exists?

The method arrayList.size() returns the number of items in the list - so if the index is greater than or equal to the size(), it doesn't exist.

if(index >= myList.size() || index < 0){
  //index does not exists
}else{
 // index exists
}

This is what you need ...

public boolean indexExists(final List list, final int index) {
    return index >= 0 && index < list.size();
}

Why not use an plain old array? Indexed access to a List is a code smell I think.

Usually I just check if the index is less than the array size

if (index < list.size()) {
    ...
}

If you are also concerned of index being a negative value, use following

if (index >= 0 && index < list.size()) {
    ...
}

This is for Kotlin developers ending up here:

if (index in myList.indices) {
  // index is valid
}

Other solutions:

if (index in 0..myArray.lastIndex) {
  // index is valid
}

if (index >= 0 && index <= myList.lastIndex) {
  // index is valid
}

// Note: elements of the list should be non-null
if (myList.getOrNull(index) != null) {
  // index is valid
}

// Note: elements of the list should be non-null
myList.getOrNull(index)?.let { element ->
  // index is valid; use the element
}

Regarding your update (which probably should be another question). You should use an array of these objects instead an ArrayList, so you can simply check the value for null:

Object[] array = new Object[MAX_ENTRIES];
..
if ( array[ 8 ] == null ) {
   // not available
}
else {
   // do something
}

Best-Practice

If you don't have hundred of entries in your array you should consider organizing it as a class to get rid of the magic numbers 3,8 etc.

Control flow using exception is bad practice.

You can check the size of an ArrayList using the size() method. This will return the maximum index +1

You could check for the size of the array.

package sojava;
import java.util.ArrayList;

public class Main {
    public static Object get(ArrayList list, int index) {
        if (list.size() > index) { return list.get(index); }
        return null;
    }

    public static void main(String[] args) {
        ArrayList list = new ArrayList();
        list.add(""); list.add(""); list.add("");        
        System.out.println(get(list, 4));
        // prints 'null'
    }
}

a simple way to do this:

try {
  list.get( index ); 
} 
catch ( IndexOutOfBoundsException e ) {
  if(list.isEmpty() || index >= list.size()){
    // Adding new item to list.
  }
}

If your index is less than the size of your list then it does exist, possibly with null value. If index is bigger then you may call ensureCapacity() to be able to use that index.

If you want to check if a value at your index is null or not, call get()

Scanner input = new Scanner(System.in);
    System.out.println("Enter the index to check");
    int i  = input.nextInt();
    if (i > tasksList.size() || i < 0 ){
        System.out.println("invalid input");
    }else {
            //your code
        }