Question

I am new to programming. I am learning Java now, there is something I am not really sure, that the use of private. Why programmer set the variable as private then write , getter and setter to access it. Why not put everything in public since we use it anyway.

public class BadOO {
    public int size;

    public int weight;
    ...
}

public class ExploitBadOO {
    public static void main (String [] args) {
        BadOO b = new BadOO();
        b.size = -5; // Legal but bad!!
    }
}

I found some code like this, and i saw the comment legal but bad. I don’t understand why, please explain me.

Answer 1

The main reason to not just make the variable public in the first place is that if you did make it public, you would create more headaches later on.

For example, one programmer writes public getters and setters around a private member variable. Three months later, he needs to verify that the variable is never “set” to null. He adds in a check in the “setFoo(…)” method, and all attempts to set the variable will then be checked for “setting it to null”. Case closed, and with little effort.

Another programmer realizes that putting in public getters and setters around a private member variable is violating the spirit of encapsulation, he sees the futility of the methods and decides to just make the member variable public. Perhaps this gains a bit of a performance boost, or perhaps the programmer just wants to “write it as it is used”. Three months later, he needs to verify that the variable is never “set” to null. He scans every access to the variable, effectively searching through the entire code base, including all code that might be accessing the variable via reflection. This includes all 3rd party libraries which has extended his code, and all newly written modules which used his code after it was written. He then either modifies all calls to guarantee that the variable is never set to null. The case is never closed, because he can’t effectively find all accesses to the exposed member, nor does he have access to all 3rd party source code. With imperfect knowledge of newly written modules, the survey is guaranteed to be incomplete. Finally he has no control over the future code which may access the public member, and that code may contain lines which set the member variable to null.

Of course the second programmer could then break all existing code by putting “get” and “set” methods around the variable and making it private, but hey, he could have done that three months earlier and saved himself the explanation of why he needed to break everyone else’s code.

Call it what you will, but putting public “get” and “set” methods around a private member variable is defensive programming which has been brought about by many years (i.e. decades) of experience.