EDIT: I posted this the same time Henry posted his response so it is mostly reduntant.
In answer to your specific question, you have to loop through the list looking for the number.
As for your posted code, you didn't create a "list object". You created an array of List references called numbers. That is, you created 5 list objects. So if you tried numbers.add(6) you would get a compiler error because numbers is an array and an array does not have an add() method.