Java™ Application Development on Linux - Dator

88Chapter 3An Experienced Programmer’s Introduction to Javaif they don’t extend this class). Classes in other packages (unless they extendthis class) can not get at such members.A public data member or method may be accessed by any code inany class.What if you do not put an access specifier on? Then the item (data member,method, or class) has package visibility. Such an item is accessible to anyother class within the package, but no further. Not even derived classes, unlessthey are in the same package, are allowed to see it. 12In terms of how restrictive the access is, you can think of the terms in orderof decreasing strictness as:• private• (package) 13• protected• publicTIPBeginner Java programmers often declare everything as public, so that theycan ignore such issues. But then they get the OO religion, and having experiencedreliability issues (others messing with their variables) they go to the otherextreme and declare private as much as possible. The problem here is thatthey often don’t know how others will want to reuse their code. Restricting everythingto private makes reuse more narrow. We prefer using private fordata members but protected for those internal helper methods that you mightotherwise make private; this hides your implementation from most otherclasses while allowing someone to override your methods, effectively providinga way for them to override your implementation. Allow those who would buildon your work the ability to do so without having to reimplement.Here is a simple example of each type of declaration:12. If you are a C++ programmer, the following description may mean something to you(if not, skip this): All classes within a package are essentially “friends.”13. Remember there is no keyword for package level protection, rather it is the absence of akeyword that denotes this level of protection. We had to write something in that space on thepage so you’d know what we’re talking about.