PowerPoint Slides
Object-Oriented
Design Using JAVA
by Dale Skrien
Chapter 2
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Object-oriented Programming
•  Divides the program into a set of
communicating objects
•  Encapsulates in an object all the
behavior and knowledge related to one
concept
•  Distributes control and data since the
intelligence and knowledge is
distributed among the objects
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Object-oriented Languages
•  Support OO style of programming
•  Provide
–  classes
–  objects
–  messages
–  inheritance
–  polymorphism
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Sample Java class
public class Person {!
private String name;!
private Date birthdate;!
public Person(String name, Date birthdate) {!
this.name = name;!
this.birthdate = birthdate;!
}!
public String getName() {!
return name;!
}!
public Date getBirthdate() {!
return birthdate;!
}!
}!
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Figure 2.1. Java primitive & reference types.
The variable x stores the primitive integer value
3 whereas the variable s stores a reference to
the String object “hello”.
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Figure 2.2. The UML class diagram for the
Person class.
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Figure 2.3. The UML notation for subclass and
superclass.
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Implementation Inheritance
•  Also called “subclassing”
•  Indicated in Java by keyword “extends”
•  Uses:
–  Specialization by adding new behavior in a
subclass
–  Specialization by overriding existing
behavior in a subclass
–  Generalization by creating a superclass
and moving code up
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Figure 2.4. Specialization by adding new
behavior to the subclass.
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Inheritance Example
public class EnhancedRectangle extends Rectangle
{
public EnhancedRectangle(int x, int y, int w, int h) //constructor
{
super(x, y, w, h);
}
public Point getCenter()
{
return new Point((int) getCenterX(), (int) getCenterY());
}
public void setCenter(int x, int y)
{
setLocation( x – (int) getWidth()/2, y – (int) getHeight()/2);
}
}
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
FilledOval subclass of Oval
•  Example of specialization by overriding
existing methods
•  FilledOval class overrides the draw
method of the Oval class to draw a
filled-in black oval instead of a white
oval with black border
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Figure 2.5. Two white ovals with black borders
and a filled oval.
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Public class Oval
{
public int x, y, w, h;
public Oval(int x, int y, int w, int h) { //constructor
this.x = x; this.y = y; this.w = w; this.h = h;
}
public void draw(Graphics g) {
g.drawOval(x, y, w, h);
}
public int getWidth() {return w;}
public int getHeight() {return h;}
public Point getTopLeftPoint() {return new Point(x,y);}
// … other methods …
}
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Specialization Example
Public class FilledOval extends Oval
{
public FilledOval(int x, int y, int w, int h)
{
super(x, y, w, h);
}
public void draw(Graphics g)
{
g.fillOval(x, y, w, h);
}
}
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Figure 2.6. Generalization by moving common
behavior to a superclass.
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Generalization Example
public abstract class Customer
{…}
HumanCustomer human = new HumanCustomer(…);
BusinessCustomer business = new BusinessCustomer(…);
String humanName = human.getName();
String businessName = business.getName();
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Figure 2.7. A Person class with two
superclasses. This is not legal in Java.
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Figure 2.8. Class C inherits two foo()
methods.
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Types and interfaces
•  A type is a set of data values and the
operations that can be performed on
them.
•  Example: The Java integer type has
232 values and has operations such as
addition, subtraction, multiplication, and
division.
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Java classes as types
•  Each Java class is a type
•  The values of such a type are the
objects of that class or any subclasses
•  The operations are the methods of the
class
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Java interfaces as types
•  Each Java interface is a type
•  The values of such a type are the
objects of all classes that implement the
interface
•  The operations are the methods
declared in the interface
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Subtypes
•  A type S is a subtype of type T if the set
of objects of S form a subset of the set
of objects of T and the set of operations
of S are a superset of the set of
operations of T
•  Example: Any subclass defines a
subtype of its superclass
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Figure 2.9. A SimpleRunner class that
implements the Runnable interface and extends
the Object class.
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Subtype polymorphism
•  You can use an object of a subtype
wherever an object of a supertype is
expected.
•  Examples:
List l = new ArrayList();!
List l = new LinkedList();
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Figure 2.10. The List interface and two classes
that implement it.
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Abstract classes
•  Abstract classes are denoted by the key
word “abstract”.
•  Objects of such classes cannot be
created.
•  Some of the methods can be declared
abstract, in which case subclasses must
implement those methods (or be
abstract themselves).
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Abstract classes vs. interfaces
•  Abstract classes can include method
implementations and non-final fields
•  Abstract classes with no
implementations are like interfaces
except a class can extend only one
superclass in Java.
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Dynamic method invocation
•  In a method call v.foo(), the Java
runtime environment looks at the actual
class of the value of v, not the declared
type of v, to determine which
implementation of foo to execute.
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Dynamic method invocation
example
•  Suppose a class A has a method foo
that prints “A” and a subclass B has a
method foo that prints “B”.
A v = new B();
!v.foo();
•  The implementation of foo in class B is
the one that is executed (“B” is printed).
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Figure 2.11. The abstract Automobile class and
its subclasses.
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Dynamic method invocation
for Automobiles
Automobile[] fleet = new Automobile[]{!
new Sedan(Color.black), !
new Minivan(Color.blue), !
new SportsCar(Color.red)};!
int totalCapacity = 0;!
for(Automobile car : fleet) {!
totalCapacity += car.getCapacity();!
}!
Three different getCapacity methods are
executed in this loop (one in each subclass)
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Overloading vs. Overriding
•  Overloading refers to two methods in
the same class with the same name, but
different parameter lists.
•  Overriding concerns two methods, one
in a subclass and one in a superclass,
with identical signatures (name and
parameter list).
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Figure 2.12. The Automobile class has two
equals methods, one inherited and one
defined in Automobile.
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Figure 2.13. The Automobile class does not
inherit the Object class’ equals method.
Instead, it overrides that method.
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Overloaded method example
Object o = new Object();!
Automobile auto = new Automobile();!
Object autoObject = new Automobile();!
auto.equals(o);!
auto.equals(auto);!
auto.equals(autoObject);!
Which of the two equals methods in the
Automobile class is executed in each of the
last 3 lines above?!
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.