Question

I’m trying to get a better understanding than I’ve been able to so far…it’s to help me with class, but not with any specific homework assignment; I’m looking for more conceptual information that I’ve not found in my textbook or in the tutorials. On a purely theoretical level, I get why encapsulation is powerful for OO programming, but need a better understanding of what, specifically is going on. I keep confusing myself about the path through which data flows, and wonder where we’re getting and setting data FROM. Hopefully this is not the exception to the “there are no stupid questions” rule.

Consider:

 public class Employee
 {
    private int empNum;
    public int getEmpNum()
    {   return empNum;   }
    public void setEmpNum(int Emp)
    {   empNum = emp;   }
 }

My interpretation of this: Using int Emp passed from somewhere, we set the declared empNum variable in the set... method. We then get... that variable and return it to the calling method. Is that a good understanding?

Further Consider:

import.javax.*
public class CreateEmp
{
    public static void main(String[] args)
    {
         int ssn;
         ssn = JOptionPane.showInputDialog(null,"enter ssn");
         Employee newEmp = new Employee(123456789);
         JOptionPane.showMessageDialog(null,"Employee ID " + newEmp.getEmpNum);
     }
 }

With this class, I don’t need to explicitly set a variable because declaring new Employee(123456789) automagically sends that value to the set... method. The return in the Employee method returns the resulting value to the calling method and makes that variable available, in this case to my dialog box output. Do I have a handle on that?

Here’s where I get confused. With my understanding as noted above–which may be off–I can never decide the best place to put logic. For instance: I would place the “enter employee number” dialog box in the Employee class, with a condition to run ONLY if the parameter passed to the method was null. In my mind, as I understand the data flow, the purpose of the class is to get an employee number. If one is not supplied as a default, one needs to be created. I’m not sure if this thinking is best practices, or if I’m setting myself up for trouble later.

I’d love to hear some thoughts about whether or not I’m properly understanding what’s happening.

Answer 1

First, this site is to be used more for questions that can be concretely answered; not so much for theoretical questions.

Anyways, the first part with the ‘get’ and ‘set’ you understand fine. Now when you said that you do not need to explicitly set the employee number because you do ‘new Employee …’ is not quite true. That would work if you had explicitly defined a constructor that takes an integer for an argument and then sets ’empNum’ equal to the argued integer value.

Getters and setters are usually used in conjunction with the ‘private’ modifier. Setters can be used to control the input. For example, you can check if the provided value is greater than zero, where as you cannot if you give direct access. Getters just allow values to be retrieved, but to make changes you force the use of the corresponding setter (if any).

You are right about how if a value is not supplied you should set one by default. Consider that the value that you retrieve (if any) is to be used in a different class. What if the user never set a value? What if it’s not something you expected? The different class, whatever it may be, still requires a value and you still need to pass something. By setting a default value, the different class can recognize that, “oh, this is a default value – this means that the user probably did not supply the correct input.” By doing so, you create a relatively fail-safe application; you can keep on working even if the input is not what it is expected to be.