Using 2 dimensional Lists List<List<Object>>

Question

I'm having some trouble with my code using 2 dimensional lists.

Here's where I create the list and fill it in (not necessarily will the query return something):

List<List<Task>> Tareas_Servicio = new List<List<Task>>();
    For (User Us : Servicio){
        List <Task> ta = [Select Id, ActivityDate, Who.name, subject, status, OwnerId From task Where ActivityDate=:Date.today() and OwnerId=:us.id and status!='Completed'];
        Tareas_Servicio.add(ta);           
    }  

The issue is when there are no elements that match the query (which can be very likely) I get an error in the following code:

If (!Tareas_Comercial.isEmpty()){
            string subject = 'Tareas Incompletas Departamento Comercial<br/><br/>';
            for (integer k = 0;k < Tareas_Comercial.size();k++){
                string nombre = Tareas_Comercial[K][0].who.name;
                subject = subject + nombre + ': ' + Tareas_Comercial[k].size()+'<br/>';                
          }}

The error I receive is

System.ListException: List index out of bounds: 0

I'm assuming because the list is empty and therefore can't access any data there. That's why I added the if statement to check if the list was empty before trying to do anything with it but I keep receiving the error.

How can I avoid this error if the list is empty?

Answer 1

The inner list appears to be empty, not the outer list. That is, the array at [0] has no elements. A simple modification should work:

string subject = 'Tareas Incompletas Departamento Comercial<br/><br/>';
for (integer k = 0;k < Tareas_Comercial.size(); k++){
    if(Tareas_Comercial[k].size()>0) {
        string nombre = Tareas_Comercial[K][0].who.name;
        subject = subject + nombre + ': ' + Tareas_Comercial[k].size()+'<br/>';                
    }
}

The first if statement is not actually necessary, because if k is less than Tareas_Comercial.size(), the loop will not execute at all.

Answer 2

add it to the List only when ta is not empty. Here it adds an empty List at index [0] of list Tareas_Servicio.

List<List<Task>> Tareas_Servicio = new List<List<Task>>();
    For (User Us : Servicio){
    List <Task> ta = [Select Id, ActivityDate, Who.name, subject, status, 
                      OwnerId From task Where ActivityDate=:Date.today() and OwnerId=:us.id 
                      and 
                      status!='Completed'];
    if(!ta.isEmpty()){
    Tareas_Servicio.add(ta);
    }           
 }