Announcements

1
Announcements

• Project 4 due last night
• Project 5 has been posted
• Milestone due March 1st
• Final Submission due March 8th

2
Inheritance

• Chapter 7

3
Introduction to Inheritance

• Inheritance allows us to define a general class
  and then define 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.

4
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 to
  define more specialized classes Boat and Airplane.

5
Programming Example A Base Class

• class Person
• private String name
• public Person()
• name no name
• public Person(String _name)
• name _name
• //set and get name methods
• //sameName method
• //writeOutput method

6
Derived Classes

• Consider a college record-keeping system with
  records about students, faculty and staff.

7
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.

8
Derived Classes, cont.
public class Student extends Person private int
studentNumber //default constructor goes
here public Student(String _name, int
_num) super(_name) studentNumber
_num //more methods not in Person
class //writeOutput ? overwrites the method in
Person class

• class Student is a derived class of class Person
  and class Person is called the base class.

9
Derived Classes, cont.

• 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.
• Protected variables are available only to classes
  derived from the class ? different than public

10
Derived Classes, cont.

• class InheritanceDemo

11
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.

12
Overriding Method Definitions, cont.

• When overriding a method, you can change the
  method definition to anything you wish, but you
  cannot change the methods heading or the
  methods return type.

13
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.

14
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.

15
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.

16
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.
• super(initialName)
• not
• Person(initialName) //ILLEGAL

17
Using super, cont.

• equivalent definitions
• public Student()
• super()
• studentNumber 0
• and
• public Student()
• studentNumber 0

18
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.
• Saves effort if code is changed later
• example
• this(initialName, 0)

19
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.

20
Calling an Overridden Method

• super can be used to call a method in the base
  class that has been overridden in the derived
  class.
• Does not have to be the first line of the method
  like in the constructor
• example
• 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.

21
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.

22
Programming Example Multilevel Derived Classes,
cont.
class Undergraduate private int level public
Undergraduate() super() level
1 //more methods specific to an
undergraduate //overwrite writeOutput
23
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.

24
An Object Can Have More than One Type, cont.

• Given
• public static void compareNumbers (Student s1,
  Student s2)
• then either
• SomeClass.compareNumbers
• (studentObject, undergradObject)
• or
• SomeClass.compareNumbers
• (undergradObject, studentObject)
• could be used.

25
An Object Can Have More than One Type, cont.

• 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.

26
An Object Can Have More than One Type, cont.

• Hence, given
• public static void compareNumbers (Student s1,
  Student s2)
• neither
• SomeClass.compareNumbers
• (studentObject, personObject)
• nor
• SomeClass.compareNumbers
• (personObject, studentObject)
• could be used.

27
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.

28
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.

29
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.

30
Abstract Classes

• An abstract class is not intended to be used to
  create objects.
• 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.

31
Abstract Classes, cont.
Public abstract class Figure private int
offset public abstract void drawHere() public
void drawAt(int num) int count for(count
0 count lt num count) System.out.println()
drawHere()
32
Interfaces

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

33
Interfaces, cont.

• Interface Writeable

34
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.

35
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 if b and t are
  declared to be objects of type Figure.

36
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.

37
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.

38
Type Checking and Dynamic 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.

39
Type Checking and Dynamic 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)

40
Type Checking and Dynamic 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.

41
Another Dynamic Binding Example

• Recall the Person class
• Person x new Person100
• x0 new Student()
• x1 new Graduate()
• x2 new Staff()
• How does the compiler decide which method
  xi.writeOutput() is calling?

42
Determining Class Type

• In order to access a method in a subclass from an
  object with a static type of the superclass, need
  to downcast
• Downcasting is unsafe! Need to know that
  downcast is legal.
• Use instanceof keyword or other methods
• x instanceof A true if x is an instance of
  class A or a subclass of A
• x.getClass().equals(A.class) true if x is an
  instance of class A
• A.class.isAssignableFrom(x.getClass()) is true if
  x is an instance of class A or a subclass of A

43
Determining Class Type
public class Animal protected int
age public void makeSound() SOP(Make
sound!) public class Ferret extends Animal
public Ferret() super() public void
eatSweets() SOP(Yummy!) public void
makeSound() SOP(Squeal!) public class
Platypus extends Animal public Platypus()
super() public void swim() SOP(Swim!)
public void makeSound() SOP(Quack!)
Animal a new Ferret() if (a instanceof
Ferret) ((Ferret)a).eatSweets() a new
Platypus() if (a.getClass()
Platypus.class) ((Platypus)a).swim() if
(Platypus.class.isAssignableFrom(a)) ((Platypus)a
).swim() ________________________________ Animal
a new Ferret() ((Platypus)a).swim() //
compiles, but runtime error!
44
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.

45
The ActionListener Interface

• The ActionListener interface has only one method
  heading that must be implemented.
• public void actionPerformed
• (ActionEvent e)
• A listener that responds to button clicks in an
  applet or in a JFrame must implement the
  ActionListener interface.

46
Programming Example 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 a
  figure.

47
Programming Example Smiley Face as a Jframe,
cont.

• class HappyFace

48
Programming Example Smiley Face as a Jframe,
cont.

• class ShowHappyFace

49
Programming Example Smiley Face as a Jframe,
cont.
50
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.