11/17/2006 CMSC 131 Fall 2006
Rance Cleaveland
©2006 Univeristy of Maryland
Lecture 34:
Inheritance
Last time:
1. Inheritance
Today:
1. Project #7 assigned
2. Method overriding
3. Shadowing
4. Access issues in inheritance
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Lecture on Inheritance - Object-Oriented Programming I | CMSC 131, Study Guides, Projects, Research of Computer Science
Material Type: Project; Professor: Cleaveland; Class: OBJECT-ORIENTED PROG I; Subject: Computer Science; University: University of Maryland; Term: Fall 2006;
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11/17/
CMSC 131 Fall 2006Rance Cleaveland
Lecture 34:Inheritance ©2006 Univeristy of Maryland
Last time: 1.
Inheritance
Today: 1.
Project #7 assigned
Method overriding
Shadowing
Access issues in inheritance
CMSC 131 Fall 2006Rance Cleaveland ©2006 University of Maryland
Project #7 Assigned!
Project due Wednesday, 11/29 at 11 pm
Project is
closed
You must complete the project by yourself
Assistance can only be provided by teaching assistants(TAs) and instructors
You must not look at other students' code
Start now!
Read entire assignment from beginning to end beforestarting to code
Check out assignment now from CVS
Follow the instructions
exactly
, as much of grading is
automated
CMSC 131 Fall 2006Rance Cleaveland ©2006 University of Maryland
Recap ^
Inheritance occurs when one class (
derived class, subclass
) is
defined from another class (
base / parent class, superclass
).
^
To derive a class D from a base class B, use:
public class
D^
extends
B^
{ …
}
^
Derived class inherits all instance variables, methods from baseclass. It can also define new instance variables, methods ^
In derived-class constructor,
super
(
…
)
can be used to invoke
constructor from base class ^
Derived class can explicitly refer to entities from base class using super
, e.g.
super.toString(
)
^
Polymorphism
: object in derived class can be used anywhere
base class is expected (a
Student
“
is a
”^
Person
!)
CMSC 131 Fall 2006Rance Cleaveland ©2006 University of Maryland
University Person Recap
class
:^
Person
instance variables
:
String
name
String
idNum
methods
:
Person( … )
[various]
String
getName( )
String
getIdNum( )
void
setName(
String
)
void
setIdNum(
String
)
String
toString( )
boolean
equals(
Person
)
class
:^
Student
instance variables
:
int
admitYear double
gpa
methods
:
Student( … )
[various]
int
getAdmitYear( ) double
getGpa( )
void
setAdmitYear(
int
)
void
setGpa(
double
)
String
toString( )
boolean
equals(
Student
)
extends Person
class
:^
Faculty
instance variables
:
int
hireYear methods
:
Faculty( … )
[various]
int
hireYear( ) void
setHireYear(
int
)
String
toString( )
boolean
equals(
Student
)
extends Person
CMSC 131 Fall 2006Rance Cleaveland ©2006 University of Maryland
super
vs.
this
^
super
: refers to the base class
^
Can invoke any base class constructor using
super( …
^
Can access data and methods in base class (
Person
) via
super
E.g.,
toString(
,^ equals(
invoke the corresponding methods from
Person
base class using
super.toString(
and
super.equals(
^
this
: refers to current class / object
^
Can refer to own data and methods using
this
(usually unnecessary)
^
Can invoke any of own constructors using
this( …
. Like
super:
^
Can only be done within a constructor ^
Must be the first statement of the constructor ^
Example public
Faculty(
Faculty
f
this(
f.getName(
f.getIdNum(
f.hireYear
CMSC 131 Fall 2006Rance Cleaveland ©2006 University of Maryland
Method Overriding ^
A derived class can define new instance variables and methods (e.g. hireYear
and
getHireYear( )
^
A derived class can also redefine (
override
) existing methods
public
class
Person
… public
String
toString(
} public
class
Student
extends
Person
… public
String
toString(
} Student
bob
new
Student("Bob
Goodstudent","123-45-6789",2004,4.
System.out.println(
"Bob's
info:
bob.toString(
Overrides base-classdefinition of this method
Since bob is Student,Student toString used
CMSC 131 Fall 2006Rance Cleaveland ©2006 University of Maryland
Early vs. Late Binding ^
Consider:
Faculty
carol
new
Faculty("Carol
Tuffteacher","999-99-9999",
Person
p
carol;
System.out.println(
p.toString()
^
Which version of
toString
Person
or
Faculty
- is called?
^
Early (static) binding ^
p^
is declared to be of type
Person
^
Therefore, the
Person
version of
toString
is used
^
Late (dynamic) binding ^
The object to which
p
refers was created as
Faculty
object
^
Therefore, the
Faculty
version of
toString
is used
^
Java uses late binding
(C++ uses early binding)
^
Early binding is more efficient (decisions about method versions can be madeat compile time)
^
Late binding respects encapsulation (object defines its operations when it iscreated)
CMSC 131 Fall 2006Rance Cleaveland ©2006 University of Maryland
Disabling Overriding with final
Sometimes you do not want to allow methodoverriding.
Correctness Redefining a method for a derived class might break it
Efficiency ^
Late binding is less efficient than early binding ^
If you know that no subclass will redefine your method, youcan force early binding by disabling overriding
We can disable overriding by declaring a method tobe
final
CMSC 131 Fall 2006Rance Cleaveland ©2006 University of Maryland
Shadowing ^
Can we override instance variables just like methods? ^
Yes, but be careful! ^
Overriding instance variable is called
shadowing
^
Shadowing hides instance variables of base class (can stillaccess them using
super.varName
public
class Person
String
name;
} public
class Administrator extends Person
String
name;
name
refers
to
Administrator’s
name
^
Confusing! Better to pick a new variable name
CMSC 131 Fall 2006Rance Cleaveland ©2006 University of Maryland
Inheritance and
private
^
Student
inherits all private data (
name
and
idNum
) from
Person
^
However, private members of base class cannot be accessed directly
public
class
Student
extends
Person
… public
void
someMethod(
name
“Mr.
Foobar”;
Illegal!
} public
void
someMethod2(
setName(
“Mr.
Foobar”
OK
^
Why? ^
Although
Student
inherits from
Person
^
… they are
different
classes
CMSC 131 Fall 2006Rance Cleaveland ©2006 University of Maryland
Package access
When a class member is not given any accessmodifier (
private, public, protected
) it is
said to have
package access
Such members are accessible any class in the samepackage
Example:
package
person;
public
class
Person
{
int computeAge
()
{
…
}
//
Package
access
}
CMSC 131 Fall 2006Rance Cleaveland ©2006 University of Maryland
Which Access Policy To Use? ^
public ^
Methods in the public interface
^
Constant variables (
final
,^ static final
^
private ^
Variables other than constants
^
Internal helper/utility methods not intended for use outside class
^
protected ^
Helper / utility methods that may be useful in subclasses ^
Some style gurus discourage
protected
.^
They consider package
access safer, since trouble can be localized to current package.
^
Package ^