Java Reference
In-Depth Information
C
OMMON
E
RROR
10.1: Confusing Super- and Subclasses
If you compare an object of type
SavingsAccount
with an object of type
BankAccount
, then you find that
ȗ The keyword
extends
suggests that the
SavingsAccount
object is an
extended version of a
BankAccount
.
ȗ The
SavingsAccount
object is larger; it has an added instance field
interestRate
.
ȗ The
SavingsAccount
object is more capable; it has an
addInterest
method.
It seems a superior object in every way. So why is
SavingsAccount
called the
subclass and
BankAccount
the superclass?
The super/sub terminology comes from set theory. Look at the set of all bank
accounts. Not all of them are
SavingsAccount
objects; some of them are other
kinds of bank accounts. Therefore, the set of
SavingsAccount
objects is a
subset of the set of all
BankAccount
objects, and the set of
BankAccount
objects is a superset of the set of
SavingsAccount
objects. The more
specialized objects in the subset have a richer state and more capabilities.
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10.2 Inheritance Hierarchies
In the real world, you often categorize concepts into hierarchies. Hierarchies are
frequently represented as trees, with the most general concepts at the root of the
hierarchy and more specialized ones towards the branches.
Figure 3
shows a typical
example.
In Java it is equally common to group classes in complex inheritance hierarchies.
The classes representing the most general concepts are near the root, more specialized
classes towards the branches. For example,
Figure 4
shows part of the hierarchy of
Swing user interface components in Java.
Sets of classes can form complex inheritance hierarchies.
