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More on Hierarchies
1. When an object of a subclass is instantiated, is
memory allocated for only the data members of the
subclass or also for the members of the superclass?
Answer: for data members of the superclass, also.
NOTE: They can be accessed directly by the subclass
only if they have the scope modifier protected in the
super class.
2. DEF: protected is the scope modifier that permits
access to a data field in a superclass by a subclass but
no other class.
3. Can the same identifier be used for a local
variable in a method, a data member in a
subclass, and a data member in a superclass?
Answer: yes
a. Which will be accessed from within the
method?
Answer: the local variable
b. How can the subclass data member be
accessed then from within that method?
Answer: this.identifier
c. How can the superclass data member be
accessed?
Answer: super.identifier
4. Can an object of a subclass be referenced by a
variable declared to be a reference to the
superclass?
Answer: yes
NOTE: when this happens only methods with
names in the superclass identical to those in the
subclass can be used BUT it will be the subclass
method that is executed!!
Casting can be used to improve this. See ex. 6.4
p.384
5. Can an object of a superclass be referenced
by a variable that was declared to be a
reference to the subclass?
Answer: NO (see ex. 6.2 p.385)
Casting can be used to do this in some cases: see
p.385
6. Can a subclass variable reference an object of
another subclass of the same superclass on the
same level?
Answer: NO
7. What does it mean when we say primitives
are passed to parameters in method headings by
value?
Answer: a copy of the value in the argument
(that is in the method call) is made in the
parameter (that’s in the method heading).
Therefore, any changes made to the parameter
do not change the argument (the original data).
8. Since objects are always passed by
reference, that is, the parameter receives the
address of the object only, changes to the
parameter do change the original object.
9. RULE: An argument of a subclass type can
be passed to a parameter which is its
superclass, BUT not the other way around
(that is the converse is false).
CAUTION: It can appear that this is not the
case when methods in subclasses and
superclasses have the same name.
Examples:
1. equals()
2. toString()
see p.718, 720, 722 for classes: Object & String
Fig. 6.16 p.386 shows an equals() method defined
in the class NewEmployee, a subclass of the class
Object, receiving an instance of the class Object
as a parameter! (emp)
This will only work if emp had previously
received the address of an object of the
NewEmployee class. (see ex. P.381)
In Fig 6.16 checking with the operator
instanceOf and type casting, allows this method
to work,
thereby, overriding the equals() method in class
Object.
A call such as clerk.equals(obj) is not breaking
the above rule. It is calling the equals() method
in class Object (the superclass), which can be
done by any subclass.
Why is clerk not calling the equals() method in
the subclass? (answer: different argument!)
10. Review: Arrays must contain only 1 type
component.
EXCEPTION: An array of objects may contain
subclass objects also, since a superclass variable
can reference objects of its own type OR objects
of its subclass types.
Ex: employees [ i ] . toString()
See NewCompany class p. 390.
NOTE: With a method such as toString( )
whose name is the same in different classes,
Java cannot determine at compile time which
toString( ) mthod to call.
It is not known at that time whether
employee[i] references an object of the
superclass or one of its subclasses.
However, at run-time, the Java interpreter
determines this based on the address then
stored in employee[i].
11. Def: dynamic binding - the
programming language feature that permits
selection of a code fragment at execution
time.
NOTE: This is another example of
polymorphism. What are other?
method overloading
method overriding
12. Def: abstract class - a class that is never
instantiated. It has the purpose of being a
superclass (parent) of a group of classes. It is a
place to store data fields and methods common
to all subclasses.
NOTE: an abstract class must contain at least 1
abstract method (its header only) which must be
implemented by each subclass.
See p. 406 - abstract class: GeoFigure hierarchy
top p.408 - abstract class code
p. 412 - test class showing array containing
different subclasses.
13. Does Java support multiple inheritance?
No. (a class can extend only one superclass.)
However, Java provides a mechanism called an
interface.
An interface is similar to a class but the only
members of an interface are abstract methods and
final class data fields. (See p.766 ActionListener)
A class implements an interface (not extend it).
Such classes are guaranteed to have certain
functionality (behavior) and constants. Often used
in GUI and graphic programs
14. What is a GUI?
A Graphical User Interface is a collection of
instances (objects) of certain classes called
components.
Examples of components are:
buttons, check boxes, text fields and labels.
Each is able to access data fields and methods
defined somewhere in a hierarchy.
Each handles an action performed by a user called
an event which is usually a click of a mouse.
Note: packages that include GUI’s are
AWT (Abstract Window Toolkit) and
Swing (which an improved alternate to AWT).
See example DistanceGUI p.448 and method
actionPerformed() p. 458.
On website “boxProjects” see boxApplet.java
15. Java provides a collection of graphics
methods that enable you to draw pictures on a
drawing surface (computer screen).
The class Applet has an associated drawing
surface accessed through its paint() method.
This method requires one argument which is an
object of the Graphics class defined in the AWT
package.
Therefore, we must import both packages.
See ex. Intersection p.159, House p.162
drawBox (website).

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