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MIT AITI 2004

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Title: MIT AITI 2004

1
MIT AITI 2004 Lecture 13

Abstract Classes and Interfaces

2
What is an Abstract Class?

An abstract class is a class that cannot be
instantiatedwe cannot create instances of an
abstract class.
One or more methods may be declared, but not
defined. (The programmer has not yet written code
for a few methods).
The declared methods and classes have the keyword
abstract in their signature.

3
Employee Class

public class Employee
private String name
private double salary
public Employee(String n, double s)
name n
salary s
public String getName() return name
public double getSalary() return salary
public String description()
return "employee with a salary of "
salary

4
Student Class

public class Student
private String name
private String course
public Student(String n, String c)
name n
course c
public String getName() return name
public String getCourse() return course
public String description()
return "a student majoring in " course

5
Common Functionality

Student and Employee may have common fields and
methods.
private String name
getName()
Instead of repeating code, introduce a superclass

6
Example Hierarchy

Consider the following class structure

Person
Superclass
Employee
Student
Subclasses
7
Person Class

public class Person
String name
public Person(String n)
name n
public String getName()
return name

8
Employee Subclass of Person

public class Employee extends Person
// private String name
private double salary
public Employee(String n, double s)
super(n)
salary s
// public String getName() return name
public double getSalary()
return salary
public String description()
return "an employee with a salary of "
salary

9
Revised Student

public class Student extends Person
// private String name
private String course
public Student(String n, String c)
super(n)
course c
// public String getName() return name
public String getCourse()
return course
public String description()
return "a student majoring in " course

10
description() Method

Lets create Person, Employee, and Student object
Person kwame new Student("Kwame", "CS")
Employee kojo new Employee("Kojo", 200000)
Student yaa new Student("Yaa", "Math")
Description of an Employee and a Student returns
employee with a salary of 200000
student majoring in Math
Can we say kwame.description()
NO! the variable kwame is of type Person, which
does not have a description() method defined

11
Let's Revise Person

public class Person
String name
public Person(String n)
name n
public String getName()
return name
// add a method to return the
// description of a Person

12
Revised Person

public class Person
String name
public Person(String n)
name n
public String getName()
return name
public String description()
return "person named " name

13
description() Revisited

Now we can call the description() method on
Objects that are of type Person (instances of a
Student, Employee, or Person)
Person kwame new
Person("Kwame")
Person kojo new
Employee("Kojo", 200000)
Person yaa new Student("Yaa", "Math")
kwame.description()
kojo.description()
yaa.description()

14
description Method Revisited

Now we can call the description() method on
Objects that are of type Person (instances of a
Student, Employee, or Person)
Person kwame new Person("Kwame")
Person kojo new Employee("Kojo", 20000)
Person yaa new Student("Yaa", "Math")
kwame.description() // method in Person
kojo.description() // method in Employee
yaa.description() // method in Student
PROBLEM We dont want to create instances of
Person, just Students and Employee

15
Abstract Methods

Solution Use the keyword abstract
A method labeled abstract is declared but not
implemented
Rule An abstract class can have zero or more
abstract methods
Make description() an abstract method in the
class Person

16
Abstract Person Class

public abstract class Person
String name
public Person(String n)
name n
public String getName()
return name
public abstract String description()

17
Abstract Classes

Cannot instantiate or create an object of an
abstract class
Person jacob new Person("Jacob") // ERROR!!
An abstract class can have both abstract and
non-abstract methods
Abstract methods need to be defined in concrete
subclasses (classes that can be instantiated)

18
Using Abstract Classes

Variables can be objects of abstract types
Person p new Student("Greg", "CS")
Here p has the type Person, but references an
instance of a non-abstract class, Student

19
Calls to Abstract Methods

Person people new Person2
people0 new Employee("Evita", 2000000.0)
people1 new Student("Greg", "CS")
for (int i 0 i lt people.length i)
Person p peoplei
System.out.println(p.getName() ", "
p.description())
What is the output?
Evita, an employee with a salary of 200000
Greg, a student majoring in CS

20
Abstract Person Class

public abstract class Person
String name
public Person(String n)
name n
public String getName()
return name
// must declare in order to call
// method on variable of type Person
public abstract String description()

21
Advantages

Classes can now be very general in a class/type
hierarchy.
This allows more abstraction in object oriented
programming.
Have more control over inheritance in a
class/type hierarchy.
Make a class abstract even if there are no
abstract methods

22
Summary of Abstract Classes

Partial implementation of a class
Cannot be instantiated
Use the abstract keyword in their signature.
Abstract methods are defined in subclasses

23
Problem Situation

Consider creating an Object that represents an
Intern.
An Intern behaves like both an Employee and a
Student.
Problem a class can only extend ONE other class

24
Interfaces

Solution Use an interface, which is a set of
requirements for a class
A class can implement more than one interface
Methods in an interface are automatically public
and abstract
Make Employee an interface

25
Interface Details

An interface is a contract for a class.
An interface specifies a set of methods a class
must implement.
An interface, similar to an abstract class,
cannot be instantiated
An interface has no constructors, only constants
and method declarations.
Classes implement interfaces using the keyword
implements

26
Employee Interface

public interface Employee
// fields are public static final constants
double STARTING_SALARY 200000.0
// methods are automatically public and
// abstract must be overridden in
// classes that implement the interface
String description()
double getSalary()

27
Student Class Revisted

public class Student
private String name
private String course
public Student(String n, String c)
name n
course c
public String getName() return name
public String getCourse() return course
public String description()
return "a student majoring in " course

28
Intern Class

class Intern extends Student implements Employee
private double income
public Intern(String n, String c)
super(n, c)
income STARTING_SALARY
public double getSalary() return income
public String description()
return "intern majoring in "
super.getCourse()
"with an income of " income

29
Using Intern Class

public static void main(String args)
Intern irish new Intern("Conor", "Math")
System.out.println(irish.getName() " ,"
irish.description())
Output
Conor, intern majoring in Math
with an income of 200000.0

30
Variable Types

A variable may have the type of an abstract
class, an interface, or a concrete class (a
non-abstract class).
Because only a concrete class can be
instantiated, an object may only have the type of
a concrete class.
All of these are valid variable declarations
Intern a new Intern("Conor", "Math")
Student b new Intern("Conor", "Math")
Employee c new Intern("Conor", "Math")

31
Variable vs Object Types (Again)