Re: Accessor Methods
"Aki Laukkanen" <aki.laukkanen@helsinki.fi> wrote in message
news:e4hji7$33l$1@oravannahka.helsinki.fi...
fightingbull06@yahoo.com wrote:
Am a newbie to Java Programming and am studying Accessor Methods.
If you designate variables as "private", then you can change their
values using "accessor methods". So what's the point of making them
"private" if you can still manipulate them using accessor methods.
Two reasons:
1. It is a matter of coding style. Allowing direct manipulation of member
variables would violate the principles of Object Oriented Programming
(OOP). Generally only constants (public static members) used by accessors
of the class should be accessible from outside of the class. (I mean
things like the BorderLayout.SOUTH constant used to tell a BorderLayout
instance you want to place a component in its south pane.)
2. You can perform validity checks on the new values given through
accessor methods. This can't be done if the member variable is accessible
from outside of the class. Let's say you have a class called Dog:
public class Dog{
int age;
public void setAge(int a){
if(a>=0){
this.age=a;
}
else{
throw new IllegalArgumentException("Age cannot be negative.");
}
} }
Now, if you don't declare the variable "age" private, you (or someone else
using the Dog class without knowing anything about its inner workings)
could do something like:
Dog rufie = new Dog();
rufie.age = -3; //does not check for validity
instead of
rufie.setAge(-3); //throws IllegalArgumentException
Somewhere later on, this weird Dog with a negative age could cause some
unforeseen trouble.
--
-Aki Laukkanen
A third reason is that modification of some variables may imply
consequences or actions that must be taken. For example, consider
Component's variables "font" and "background". If they were public, and you
changed them in your code, nothing happens.
On the other hand, when you call the setter to change the value,
it changes the internal value, modifies the Graphics
to use the newly specified color or font, and then
invokes repaint() so that the component gets redrawn with the
new color or font.
--
Fred L. Kleinschmidt
Boeing Associate Technical Fellow
Technical Architect, Software Reuse Project