Java Reference
In-Depth Information
TIP: Inner and Outer Classes Have Access to Each Other's
Private Members
Within the definition of a method of an inner class, it is legal to reference a private instance
variable of the outer class and to invoke a private method of the outer class. To facilitate this,
Java follows this convention: If a method is invoked in an inner class and the inner class has
no such method, then it is assumed to be an invocation of the method by that name in the
outer class. (If the outer class also has no method by that name, that is, of course, an error.)
Similarly, an inner class can use the name of an instance variable of the outer class.
The reverse situation, invoking a method of the inner class from the outer class, is
not so simple. To invoke a (nonstatic) method of the inner class from within a method
of the outer class, you need an object of the inner class to use as a calling object, as we
did in Display 13.9 (Part 2).
As long as you are within the definition of the inner or outer classes, the modifiers
public
and
private
(used within the inner or outer classes) are equivalent.
These sorts of invocations and variable references that cross between inner and
outer classes can get confusing. So, it is best to confine such invocations and variable
references to cases that are clear and straightforward. It is easy to tie your code in knots
if you get carried away with this sort of thing.
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Access Privileges between Inner and Outer Classes
Inner and outer classes have access to each other's private members.
EXAMPLE:
A Bank Account Class
Display 13.9 contains a simplified bank account program with an inner class for
amounts of money. The bank account class uses values of type
String
to obtain or
return amounts of money, such as the amount of a deposit or the answer to a query for
the account balance. However, inside the class it stores amounts of money as values of
type
Money
, which is an inner class. Values of type
Money
are not stored as
String
s, which
would be difficult to do arithmetic on, nor are they stored as values of type
double
,
which would allow round-off errors that would not be acceptable in banking transac-
tions. Instead, the class
Money
stores amounts of money as two integers, one for the dol-
lars and one for the cents. In a real banking program, the class
Money
might have a larger
collection of methods, such as methods to do addition, subtraction, and compute per-
centages, but in this simple example we have only included the method for adding an
amount of money to the calling object. The outer class
BankAccount
would also have
more methods in a real class, but here we have only included methods to deposit an
amount of money to the account and to obtain the account balance. Display 13.10 con-
tains a simple demonstration program using the class
BankAccount
.
(continued)
