How do I use the if else statement to check if the user has inputted a valid entry in relation to array list index?

Im trying to create a modify method that allows the user to see their entry that was previously inputted and be able to re input that entry thus replacing the entry. However, when I try to code the if else statement that checks if their input is a valid index the program claims that no matter what you input its incorrect. please advise.

public static void Modify() 
{
readelements(); 
System.out.println("Enter the desired # of the entry you would like to modify"); 
int Mod = Cons.nextInt(); 
if (Mod != Notes.size())
{
System.out.println("You've selected an invalid entry!\n");
}
else {
System.out.println(Notes.get(Mod));
Notes.remove(Mod); 
System.out.println("Please copy and paste your entry for re-entry");
String NewMod = Cons.next(); 
Notes.add(Mod, NewMod); 
}
Execute.Instruct();
}

When you have a n-sized array, valid indices are 0..n-1. Same applies to Lists.

So for instance, if your list has 4 elements, valid indices are 0, 1, 2, and 3.

Your input validation condition should become

if (Mod < 0 || Mod >= Notes.size()) {
  // complain
} else {
  // index is good
}