Question

Why are private members not inherited?

Asked by: csar

The Java Specification (second edition) says:
"Members of a class that are declared private are not inherited by subclasses of that class."

Why are private members treated differently from public and protected members? Usually one declares them final if they shall not be overridden.

Strange effect of this definition: You can override such a member but it is not used if called from a member of the base class. There is no warning/error issued from the compiler!

I think there is a good reason why it has been defined the way it is right now, but I really like to know this reason.

Carsten

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Asked On
2003-10-11 at 10:20:23ID20763874
Topics

Java Programming Language

,

New to Java Programming

Participating Experts
5
Points
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Comments
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Answers

 

by: CEHJPosted on 2003-10-11 at 10:57:35ID: 9533386

>>Why are private members treated differently from public and protected members

Probably because it would compromise encapsulation - if they were inherited, they would be accessible and therefore not completely encapsulated

>>You can override such a member but it is not used if called from a member of the base class

This behaviour is expected, since the member would merely be a replacement, much in the same way a constructor would be.

 

by: dualsoulPosted on 2003-10-11 at 12:50:07ID: 9533686

>> Probably because it would compromise encapsulation

yes it's right, private members - is the part of class implementation, both variables and methods.  If it can be inhereted, then subclass can see details of implementation - this is violation of main OO principle.

>> You can override such a member but it is not used if called from >>a member of the base class.here is no warning/error issued from >> the compiler!

it's not overriding - it's just define new member of subclass.
Let's imaging you have:
    private int id;  in your base class, and you override it by:
    private String id; in your subclass
so, if try in base class use the overriding member (String instead of int), i think, it will cause troubles :)

 

by: objectsPosted on 2003-10-11 at 20:35:59ID: 9534690

private members are not visible to sub-classes so even if they were inherited you couldn't call them.

> You can override such a member but it is not used
> if called from a member of the base class.

And that's the reason they aren't inherited. To stop subclasses overriding functionality that is private to the class.



 

by: iboPosted on 2003-10-12 at 00:51:38ID: 9535135

u dont want ur privates being accessed by someone else.

 

by: anthony_castillonPosted on 2003-10-13 at 12:19:42ID: 9542432

Private members are inherited, though are not visible.  This enables full encapsulation of the members of the class so that other objects cannot change the internals of the class.  Objects are then known and used for their interface.

By analogy, in real life, we use objects (e.g. phone, car, watch) without really knowing what the internals of those objects are comprised of.  We simply use things because of their interface, i.e. how we know they should "behave".  For phones, we know them because they enable us to call people from a distance.  That is the phone's interface.

Anyway, interfaces are another topic that can be discussed in a different thread.

20120131-EE-VQP-002

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