When elements are added to a list, we say the size of a list is increased. The size of a list represents how many elements are in a list.

We can use the size() instance method to determine this:

List<String> emailList = new List<String>();
Integer emailListSize = emailList.size(); // Returns 0 
// There are 0 element in the list, so the size is 0

emailList.add('jerry@CampApex.com');
emailListSize = emailList.size(); // Returns 1
// There's 1 element in the list, so the size is 1

emailList.add('elaine@CampApex.com');
emailListSize = emailList.size(); // Returns 2
// There's 2 elements in the list, so the size is 2

This will come in handy in the next course when we loop through the elements of our list.

There are two ways to know if our List is empty. The first you already saw, if the size of a list is 0 then it's empty. A second way to check if using the isEmpty() instance method. This method returns true if a list is empty, and returns false if elements are present.

List<String> emailList = new List<String>();
Integer emailListSize = emailList.size(); // Returns 0 
Boolean isListEmpty = emailList.isEmpty(); // returns true

An empty list is not the same as a null list. As a refresher, this is the difference between null and empty using Strings:

String firstName; // firstName is Null - because it's never been initialized 
String middleName = ''; // middleName is empty, it's been initialized but not assigned a value 
String lastName = 'smith'; // lastName is both initialized and assigned a value 

firstName.capitalize(); // throws a null pointer exception because firstName is null
middleName.capitalize(); // no exception, but there's nothing to capitalize 
lastName.capitalize(); // capitalizes smith into Smith

The same concept applies to Lists:

List<String> firstNamesList;  // declared, but not in initialized
List<String> middleNamesList = new List<String>(); // initialized, but not assigned elements 
List<String> lastNamesList = new List<String>{'Smith'}; // initialized and assigned elements 

firstNamesList.size(); // throws a null pointer exception 
middleNamesList.size(); // no exception. the list is empty, so returns 0 
lastNamesList.size(); // list has been initialized and assigned elements, returns 1

We can check if a List is null like this:

List<String> firstNamesList;  
List<String> middleNamesList = new List<String>();
List<String> lastNamesList = new List<String>{'Smith'}; 

Boolean isFirstNameNull = firstNamesList == null; //returns TRUE
Boolean isMiddleNameNull = middleNamesList == null; //returns FALSE
Boolean isLastNameNull = lastNamesList == null; //returns FALSE