Learn inheritance, derived classes, polymorphism in Java

Objectives
• become acquainted with inheritance
• learn how to define and use derived classes
• learn about dynamic binding and
polymorphism
• (optional) learn about the class JFrame used to
produce windowing interfaces

Outline
•
•
•
•

Inheritance Basics
Programming with Inheritance
Dynamic Binding and Polymorphism
(optional) Graphics Supplement

Inheritance Basics: Outline
•
•
•
•
•
•

Introduction to Inheritance
Derived Classes
Overriding Method Definitions
Overriding vs. Overloading
The final Modifier
UML Inheritance Diagrams

Introduction to Inheritance
• Inheritance allows us to define a general
class and then more specialized classes
simply by adding new details to the more
general class definition.
• A more specialized class inherits the
properties of the more general class, so that
only new features need to be programmed.

Introduction to Inheritance,
cont.
• example
– General class Vehicle might have instance
variables for weight and maximum
occupancy.
– More specialized class Automobile might
add instance variables for wheels, engine
size, and license plate number.
– General class Vehicle might also be used
define more specialized classes Boat and
Airplane.

Programming Example: A
Base Class
• class Person

Derived Classes
• Consider a college record-keeping system
with records about students, faculty and
(nonteaching) staff.

Derived Classes, cont.
• Even though your program may not need any
Person or Employee objects, these classes can
be useful for consolidating and representing
features common to all subclasses.
– For example, all students, faculty, and staff
have names, and these names may need
to be initialized, changed, retrieved, or
printed.

is a derived class of class Person
Person is called the base class.

• class Student

and class

• public class Student extends Person

• syntax
public class Derived_Class_Name extends
Base_Class_Name
{
Declarations_of_Added_Instance
_Variables
Definitions_of_Added_and_Overridden_Method
s
}

• When you define a derived class, you declare
only the added instance variables and you
define only the added and overridden
methods.
• The variables and methods of the parent
class which are not declared private are
inherited automatically.

• class InheritanceDemo

Overriding Method Definitions
• Notice that class Student has a method
writeOutput with no parameters, and class
Person also has a method writeOutput with no
parameters, that class Student inherits.
• When a derived class defines a method with
the same name and the same number and
types of parameters as a method in the base
class, the method in the derived class
overrides the method in the base class.

Overriding Method Definitions,
cont.
• When overriding a method, you can change
the method definition to anything you wish,
but you cannot change the method’s heading
or the method’s return type.

Overriding vs. Overloading
• When you override a method, the new
method definition in the derived class has the
same name and the same number of types of
parameters as the method definition in the
base class.
• When the name is the same, but the number
or types of the parameters differs, whether in
the base class or in the derived class, the
method is overloaded in the derived class.

Overriding vs. Overloading,
cont.
• example:
public String getName(String title)

in class Student and
public String getName()

in class Person overload method getName
since the two methods have different
parameter lists.
– Both methods are available in class
Student.

The final Modifier
• You can prevent a method definition from
being overridden by adding the word final to
the method heading.
• example
public final void someMethod()
{
…

• This is used rarely, but it produces more
efficient code.

The final Modifier, cont.
• An entire class can be declared final, in
which case it cannot be used as a base class
to derive another class.

Private Instance Variables in
the Base Class
• Private instance variables inherited from a
base class cannot be accessed directly.
• Instead, they must be accessed using a
method that is not declared private.
• While this may seem inconvenient, it provides
an important mechanism for controlling
access and changes to instance variables in
the base class.

Private Methods in the Base
Class
• Like private instance variables, private
method inherited from a base class cannot be
accessed directly.
• Instead, they, too, must be accessed using a
method that is not declared private.
• This, too, provides an important mechanism
for controlling access to methods in the base
class.

Private Methods in the Base
Class
• Since private methods typically serve as
helping methods, their use always is limited to
the class in which they are defined.

UML Inheritance Diagrams
• Typically, a UML class diagram shows only
as much as is needed for the design task at
hand.

UML Inheritance Diagrams,
cont.

cont.
• An arrowhead points from a derived class to
its base class, indicating an is-a relationship.
– For example, a Student is-a Person.
• Inherited instance variables and methods can
be found by following the arrowheads.

cont.
• more details of the inheritance hierarchy

Programming with Inheritance:
Outline
•
•
•
•

Constructors in Derived Classes
(optional) The this Method
Calling an Overridden Method
(optional) A Subtle Point About Overloding
and Overriding
• The class Object
• Abstract Classes
• Interfaces

Constructors in Derived
Classes
• A base class has its own constructors.
– Their purpose typically is to initialize the
instance variables declared in the base
class.
• A derived class has its own constructors.
– Their purpose typically is to call a
constructor in the base class, and then to
initialize the instance variables declared in
the derived class.

Constructors in Derived
Classes, cont.
• To call a constructor in the base class, use
super(Values_for_Instance_Variables
_Declared_in_the_Base_Class);

• example
super(initialName);

not
Person(initialName); //ILLEGAL

Using super
• The call to the constructor in the base class
(using super) must be the first action taken in
the constructor of a derived class.
• When no call to the constructor in the base
class is included, Java automatically includes
a call to the default constructor in the base
class.

Using super, cont.
• equivalent definitions:
public Student()
{
super();
studentNumber= 0;
}

and
public Student()
{
studentNumber= 0;
}

(optional) The this Method
• Within the definition of one constructor, it can
be appropriate to call another constructor in
the same class.
• The keyword this is used to call another
constructor in the same class.
• example
this(initialName, 0)

(optional) The this Method,
cont.
• Any use of this must be the first action in the
constructor definition.
– Thus, a constructor definition cannot
contain a call using super and a call using
this.
• To use both super and this, include a call
using this in one constructor and a call using
super in the constructor called using this.

Calling an Overridden Method
can be used to call a method in the
base class that has been overridden in the
derived class.
• example
• super

super.writeOutput();

• However, you cannot repeat the use of super
to invoke a method in some ancestor class
other than the immediate base (parent) class.

Programming Example:
Multilevel Derived Classes
• Class Undergraduate can be derived from class
Student which is derived from class Person.
• Class Undergraduate will have all the instance
variables and methods of class Student which
has all the instance variables and methods of
class Person.

Multilevel Derived Classes,
cont.

• class Undergraduate

cont.

cont.
• A chain of derived classes permits code to be
reused which improves efficiency.

Terminology
• A base class often is called a parent class.
• A derived class then is called a child class.
• A class that is a parent of a parent of…a
parent of (with one or more “parent of”s)
another class often is called an ancestor
class.
• A class than is a child of a child of…a child of
(with one or more “child of”s) often is called a
descendant.

(optional) A Subtle Point
About Overloading and
Overriding
• Even when two methods have the same
number of parameters, a difference in
parameter type is sufficient to qualify for
overloading.
• But, a reference to an ancestor class type can
refer to a descendant type.
• Hence an overridden method in an ancestor
class sometimes needs to be invoked
explicitly using super.

(optional) A Subtle Point
About Overloading and
Overriding, cont.
• method equals

An Object Can Have More
than One Type
• If class Undergraduate is derived from class
Student and class Student is derived from class
Person, then every object of class Undergraduate
is also an object of class Student and an
object of class Person.
• A reference to an object of class Undergraduate
can be substituted for a reference to an
object of class Student or a reference to an
object of class Person.

than One Type, cont.
• Given
public static void compareNumbers (Student s1,
Student s2)

then either
SomeClass.compareNumbers
(studentObject, undergradObject);

or
(undergradObject, studentObject);

could be used.

• However, a reference to an object of class
person cannot be substituted for a reference to
an object of class Student or an object of class
Undergraduate.
• A reference to an object of an ancestor
cannot be substituted for a reference to an
object of a derived class.

• Hence, given
public static void compareNumbers (Student s1,
Student s2)

neither
(studentObject, personObject);

nor
(personObject, studentObject);

could be used.

“Is a” and “Has a”
Relationships
• A derived class is a more specialized class
than its base class or any of its ancestor
classes.
– For example, a student is a person.
• A more complex object has as one or more of
its instance variables one or more references
to objects of a simple class, exhibiting a has a
relationship.
– For example, a person has a name.

The Class Object
• In Java, every class descends from (and
inherits features from) the Object class.
• Therefore, every object of every class is of
type Object.
• Unless a class is declared explicitly to be a
descendant of some other class, it is an
immediate descendant of the class Object.

The Class Object, cont.
• An object of any class can substituted when a
parameter of type Object is expected.
• Every class inherits some methods from the
class Object:
– equals()
– toString()

but usually these methods are overridden by
the derived class or by an intermediate
ancestor class.

Method toString
• Inherited method toString takes no
arguments.
• Typically, method toString is coded to
produce and return a string which contains
everything of interest about the object.

Method toString, cont.
• example
public String toString();
{
return (“Name: “ + getName()+ n
+ “Number: “ + getNumber());
}

• Whenever a new class is created, a suitable
toString method should be defined.

Method toString, cont.
• Method toString can be called my the
conventional means, Object_Name.toString, or
by using only Object_Name.
• example
System.out.println(s.toString());

or
System.out.println(s);

Method clone
• Method clone also is inherited from the class
Object.
• Method clone takes no arguments and returns
a copy of the calling object.
– Even though the data is identical, the
objects are distinct.
• Typically, method clone needs to be
overridden to function properly.
– See Appendix 8 for more details.

Case Study: Character
Graphics
• Sometimes terminals, especially older ones,
allow only text output.
• We’ll design three simple classes for simple
graphics using only text output.
– One class will produce a box.
– One class will produce a triangle.
– One class will draw a pine tree using the
other two classes.

Graphics, cont.
• Box and triangle figures will have an offset
from the edge of the screen.
• A box figure will have a size, expressed as a
number of characters.
• A triangle figure will have a size, given by its
base, with the sides determined
automatically.

Graphics, cont.

Graphics, cont.
• A base class named Figure will provide the
offset property common to a box and a
triangle and will be used to derive class Box
and class Triangle.
• The base class Figure will set the parameters
for all figures and will draw the figures.

Graphics, cont.
• class Figure

Graphics, cont.
• To draw a box, top to bottom
– draw the top line
– draw the side lines
– draw the bottom line

Graphics, cont.
• class Box

Graphics, cont.
• class Box, contd.

• Each new method should be tested upon
completion.

Graphics, cont.
• To draw a triangle, top to bottom
– draw an inverted “V”
– draw a horizontal line for the base

Graphics, cont.
• class Triangle

Graphics, cont.
• class Triangle, contd.

Graphics, cont.
• To complete the project, produce a driver.

Graphics, cont.
class GraphicsDemo

Abstract Classes
• An abstract class is not intended to be used
to create objects.
• Class Figure is an example of a class used as
a base class to derive other classes including
class Box and class Triangle.
– Nevertheless, as written, objects of class
Figure could be instantiated and its
methods used, even though that was not
our intention.

Abstract Classes, cont.
• By declaring one or more methods to be
abstract and by omitting the method body,
only objects of derived classes which override
the method(s) can be instantiated.
• example
public abstract void drawHere();

• A class that has at least one abstract method
must be declared abstract.

• example
public abstract class Figure
{
…

• class Figure

• An abstract class serves as a placeholder.
• An abstract class makes it simpler to define
derived classes.
• An abstract class assures that all its derived
classes implement its abstract method(s), or
they too will be abstract.

Interfaces
• An interface specifies the headings for
methods that must be defined for any class
that implements the interface.

Interfaces, cont.
• Interface Writeable

Interfaces, cont.
• To implement an interface, a class must
– include the phrase
implements Interface_Name

at the start of the class definition
• example
implements MyInterface, YourInterface

– implement all the method headings listed in
the definition of the interface.

Interfaces, cont.
• class WriteableUndergraduate

Interfaces, cont.
• An interface is a type.
• A method may have a parameter of an
interface type.
• Any class that implements the interface can
be substitute for the parameter.
• A class can implement any number of
interfaces by providing implementations for all
the methods of all the interfaces it claims to
implement.

Dynamic Binding and
Polymorphism: Outline
•
•
•
•

Dynamic Binding
Type Checking and Dynamic Binding
Dynamic Binding with toString
Polymorphism

Dynamic Binding
• Different objects can invoke different method
definitions using the same method name.
• For example, if b references a Box and t
references a Triangle, b and t invoke different
definitions of method drawAt even of b and t
are declared to be objects of type Figure.

Dynamic Binding, cont.
• Handling the invocation of a method that may
be overridden later is called dynamic binding
or late binding.
• The type of object being referenced at the
time of the method call, not the type of
reference that was declared, determines
which method is invoked.

• Consider
Figure f;
Box b = new Box(1, 4, 4);
f = b;
f.drawAt(2);
Triangle t = new Triangle(1,2);
f = t;
f.drawAt(2);

• Method drawAt is inherited from class Figure
and is not overridden.
• But, method drawHere is invoked within the
definition of method drawAt, and method
drawHere is overridden.
• The type of object referred to by f determines
which method drawHere is invoked.

Type Checking and Dynamic
Binding
• Recall that an object reference to an ancestor
class can refer to an object of a descendant
class.
Employee e = new Employee();
Person p;
p = e;

• However, you can invoke only a method in
class Person with the variable p.

Binding, cont.
• However, if a method is overridden in the
class Employee, and variable p references an
Employee object, then the method in class
Employee is used.
• The variable determines what methods can
be used, but the type referenced by the object
determines which definition of the method will
be used.

Binding, cont.
• To use a method name in the class Employee
with an object named by the variable p of type
Person, use a type cast.
• example
Employee e = (Employee)p;
e.setEmployeeNumber(5678);

Binding, cont.
• However, even a type cast cannot fool Java
– example
Box b = new Box (1, 4, 4);
Figure f = (Figure)b;
f. drawHere()

will use the definition of drawHere given in
class Box, not the definition of drawHere
given in class Figure.

Binding, cont.
• You are unlikely to assign an object of a
descendant type to a variable of a parent
type, at least not directly.
• But, such an assignment can occur indirectly
by providing an argument of a descendant
type for a method that has a parameter of an
ancestor type.

Dynamic Binding with
toString
• Recall method toString typically is used to
prepare and return a string, describing an
object, for output to the screen.
• The name of this method can be omitted,
thanks to dynamic binding, because one
definition of method println expects a single
argument of type Object which it uses to
invoke the method toString associated with
the object.

Polymorphism
• Polymorphism comes from Greek meaning
“many forms.”
• In Java, polymorphism refers to the dynamic
binding mechanism that determines which
method definition will be used when a method
name has been overridden.
• Thus, polymorphism refers to dynamic
binding.

Subtle Difference
• Dynamic binding refers to the process carried
out by the computer.
• Polymorphism can be thought of as
something objects do.
• Polymorphism, encapsulation, and
inheritance, and considered to be the main
features of object-oriented programming.

(optional) A Better equals
Method
• Sometimes method equals from class Object is
overloaded when it should have been
overridden.
– This occurs when its parameter is not of
type Object.
• Usually, this is all right.

Method, cont.
• But, if method equals is called with an object
of class Object as its argument, method equals
from class Object will be invoked.
• The problem is fixed by changing the formal
parameter in the overriding method so that it
is a parameter of type Object.

Method, cont.
• However, this allows the argument to be any
type of object, which can produce a run-time
error.
• But, we can determine if an object is of the
correct type using
Object instanceof Class_Name

• Finally, we should return false when
comparing an object to a null reference.

Method, cont.
• method equals

(optional) Graphics
Supplement: Outline
•
•
•
•

The Class JApplet
The Class JFrame
Window Events and Window Listeners
The ActionListener Interface

The Class JApplet
• An applet is a derived class from the class
JApplet
public class LabelDemo extends JApplet

• Class JApplet has methods named init and
paint.
• When you define methods init or paint in an
applet, you are overriding inherited methods.

The Class JApplet, cont.
• Methods init and/or paint can be invoked
without having to (re)define them.
• Thanks to polymorphism, methods init
and/or paint defined in an applet will be
invoked by library class methods and other
methods when you run the applet with the
applet (of type JApplet) as the parameter to
the method.

The Class JApplet, cont.
• The class JApplet is used as a base class to
derive applets to run from a webpage.

The Class JFrame
• A Graphical User Interface or GUI is simply a
windowing interface for some kind of
program.
• The class JFrame, rather than the class JApplet,
is used to produce GUIs that can run as
regular Java applications.

The Class Jframe, cont.
• class ButtonDemo

The Class Jframe, cont
class ShowButtonDemo

The Class JFrame, cont.
• The class JFrame, and every class derived
from it, has a method named setVisible.
– When its argument has the value true, the
GUI is visible.
• Labels, buttons, JFrames, and other
components inherit method setVisible from a
common ancestor.

The Class JFrame, cont.
• A class derived from JFrame has no init
method, but it does use a constructor.
• Code that would be in the init method of an
applet is placed in the constructor of the class
derived from JFrame.
• A GUI derived from a JFrame sets an initial
size
setSize(WIDTH, HEIGHT);

Window Events and Window
Listeners
• With a JFrame, a GUI’s close-window button
needs to be programmed.
• A close-window button generates a window
event which is handled by a window listener.
• Class WindowAdapter is a window listener, so
every class derived from class WindowAdapter is
a window listener.

Listeners, cont.
• A window listener is registered with a
GUI using method
addWindowListener.

JFrame

Listeners, cont.
• class WindowDestroyer

Listeners, cont.
• A WindowDestroyer object is created and
registered as a window listener for our JFrame
GUI using
WindowDestroyer listener =
new WindowDestroyer();
addWindowListener(listener);

• When the window-close button is clicked, the
JFrame GUI ends.

The ActionListener
Interface
• The ActionListener interface has only one
method heading that must be implemented.
public void actionPerformed
(ActionEvent e)

• A listener that response to button clicks in an
applet or in a JFrame must implement the
ActionListener interface.

Smiley Face as a JFrame
• Class JFrame and every class derived from
class JFrame has a paint method.
• The paint method can be redefined to draw
figure.

Smiley Face as a Jframe,
cont.
• class HappyFace

cont.
• class ShowHappyFace

cont.

Summary
• You have become acquainted with
inheritance.
• You have learned how to define and use
derived classes.
• You have learned about dynamic binding and
polymorphism.
• (optional) You have learned about the class
JFrame used to produce windowing interfaces.

Learn inheritance, derived classes, polymorphism in Java

Recomendados

Recomendados

Más contenido relacionado

Similar a Learn inheritance, derived classes, polymorphism in Java

Similar a Learn inheritance, derived classes, polymorphism in Java (20)

Último

Último (20)

Learn inheritance, derived classes, polymorphism in Java