protected data in abstract class

Question

My question involves specifically Java, abstract classes, and the use of protected data. I am being told that all the data should be private, and protected getters/setters used only.

Now, I understand we want to shield data from direct manipulation by casual users of the class, and that public data members in general are a questionable practice. I have looked at "Java protected fields vs public getters" ( Java protected fields vs public getters ), but I still am dubious that:

protected int i;  

is worse in an abstract class than:

private int i;  
protected int geti();  
protected void seti(int j); 

I am just not seeing the down side when the abstract class is there precisely to provide parent/common facility to the children classes, and the protected scope is meant to provide access to children, while protecting the data from casual users. I note in the question referenced above, that most of the answers seem to address the issue of why data in general should be private rather than public. I am trying to focus my question specifically on data existing in an abstract parent intended for use by the children. The sole reasonable comment I have heard to date is that using the parents protected data (e.g., int i above) leaves you with code in the child class that references a variable not declared in the child class. Less compelling is the argument (see Common protected data member in base class? ) that you may want to change the access some day, and now you have to honor your interface. This is an abstract class, and is intended to be extended 100% of the time.

Thanks! Specific Title/page# references to books are far more helpful that references to "..any basic Java programming text..."

========================================== 10-13-2010
This was as much a question about abstract classes as it is about protected data. I find it disappointing that the focus seems to have shifted in the responses to whether data hiding is a good thing in OOP (answer: yes). There's a lot of depth here involving the nature of the abstract class, and how it differs from a regular non-final class, and what possible advantages there might be for fixing the names and types of data-items in the abstract parent for use by the child classes. I think there is the possibility here for innovation and greater control being extended down from the abstract parent to the implementing child classes. I am concerned that general principles, such as the advantages of data-hiding, can become dogma, and inhibit innovation and the development of new patterns and ideas.

Thanks to all who contributed.

Answer 1

If the field is private and access is through getters and setters, you will be able to reimplement getters and setters (for instance, dropping the field and updating/reading the value from an external source), and thus change how the "field" works without touching any child classes.

Whether this is worth it, that's up to you.