Inheritance, Interfaces, and Polymorphism - Java for COBOL Programmers - page 126

// Call the setErrorMsg function in the base class ErrorMsg.
super.setErrorMsg (inputMsg);
if (msgSize != 0) {
linesToSkip = 1;
}
}
}
S HARING V ARIABLES AND M ETHODS
When a class derives from another class, it inherits all of the class data members and
methods from its base classes if the scope of those data members is not private. This
means that all of the variables and methods defined in the base classes are available in
the derived classes. For example, look carefully at the statement in PrintfileErrorMsg
that sets linesToSkip :
if (msgSize != 0) {
linesToSkip = 1;
}
Where did the variable msgSize come from? PrintfileErrorMsg did not explicitly
define this variable. The only one defined is in TextMessage , which ErrorMsg inherits.
But remember, PrintfileErrorMsg inherits from ErrorMsg (which inherits from
TextMessage ). Therefore, ErrorMsg , PrintfileErrorMsg , and TextMessage all share
the variables defined in TextMessage .
Any of the class instances in this hierarchy can treat msgSize as if it is contained
in this class instance. At the same time, if any of these class instances modifies msg-
Size , then all of the class instances will see this modification. How does this work?
When an instance of PrintfileErrorMsg is created, it automatically creates an in-
stance of ErrorMsg , which automatically creates an instance of TextMessage . The
variable msgSize is created when TextMessage is created, and all of these instances
share this single copy of the variable msgSize .
H IDING V ARIABLES AND M ETHODS
A derived class can create its own copies of base class variables or methods. To un-
derstand this better, let's examine how variable declarations (class data members)
are handled.