protected Members
Chapter 8 discussed access modifiers public and private. A class's public members are accessible wherever the program has a reference to an object of that class or one of its subclasses. A class's private members are accessible only from within the class itself. A superclass's private members are not inherited by its subclasses. In this section, we introduce access modifier protected. Using protected access offers an intermediate level of access between public and private. A superclass's protected members can be accessed by members of that superclass, by members of its subclasses and by members of other classes in the same package (i.e., protected members also have package access).
All public and protected superclass members retain their original access modifier when they become members of the subclass (i.e., public members of the superclass become public members of the subclass, and protected members of the superclass become protected members of the subclass).
Subclass methods can refer to public and protected members inherited from the superclass simply by using the member names. When a subclass method overrides a superclass method, the superclass method can be accessed from the subclass by preceding the superclass method name with keyword super and a dot (.) separator. We discuss accessing overridden members of the superclass in Section 9.4.
Software Engineering Observation 9.1
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Methods of a subclass cannot directly access private members of their superclass. A subclass can change the state of private superclass instance variables only through non-private methods provided in the superclass and inherited by the subclass. |
Software Engineering Observation 9.2
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Declaring private instance variables helps programmers test, debug and correctly modify systems. If a subclass could access its superclass's private instance variables, classes that inherit from that subclass could access the instance variables as well. This would propagate access to what should be private instance variables, and the benefits of information hiding would be lost. |