1. Object-Oriented Programming & Design Lecture 14 Martin van Bommel
CSCI 383
Object-Oriented Programming & Design
Lecture 14
Martin van Bommel

2. Same Ideas, Different Terms
All OOP languages have the following concepts, although the terms they use may differ
classes, object type, factory object
instances, objects
methods, member function, method function
message passing, method lookup, method binding, member function invocation
inheritance, subclassing
Fall 2010
CSCI 383 Lecture M. van Bommel

3. Encapsulation and Instantiation
Classes provide a number of important capabilities
Encapsulation
The purposeful hiding of information
Reduces the amount of detail that needs to be remembered/communicated among programmers
A Service View
The ability to characterize an object by the service it provides, without knowing how it performs its task
Instantiation
The ability to create multiple instances of an abstraction
Fall 2010
CSCI 383 Lecture M. van Bommel

4. Internal and External Views
As we noted in the last chapter, encapsulation means there are two views of the same system.
The outside, or service view, describes what an object does
The inside, or implementation view, describes how it does it
Fall 2010
CSCI 383 Lecture M. van Bommel

5. CSCI 383 Lecture 14 M. van Bommel
Behavior and State
A class can also be viewed as a combination of behavior and state
Behavior
The actions an instance performs in response to a request
Implemented by methods
State
The data that an object maintains in order to complete its behavior
Stored in instance variables (also known as data members, or data fields)
Fall 2010
CSCI 383 Lecture M. van Bommel

6. CSCI 383 Lecture 14 M. van Bommel
Class Definitions
We will use as running example the class definition for a playing card abstraction, and show how this appears in several languages
Languages considered in the book include:
Java, C++, C#
Delphi Pascal, Apple Pascal
Ruby, Python, Eiffel
Objective-C and Smalltalk
Fall 2010
CSCI 383 Lecture M. van Bommel

7. CSCI 383 Lecture 14 M. van Bommel
A C++ Class Definition
class PlayingCard {
public:
enum Suits {Spades, Diamonds, Clubs, Hearts};
Suits suit() { return suitValue; }
int rank() { return rankValue; }
private:
suit suitValue;
rank rankValue; };
Fall 2010
CSCI 383 Lecture M. van Bommel

8. CSCI 383 Lecture 14 M. van Bommel
Visibility Modifiers
The terms public and private differentiate the internal and external aspects of a class
public features can be seen and manipulated by anybody
they are the external (interface or service) view
private features can be manipulated only within a class.
They are the internal (implementation) view
Typically methods are public and data fields are private, but either can be placed in either category
Fall 2010
CSCI 383 Lecture M. van Bommel

9. CSCI 383 Lecture 14 M. van Bommel
A C# Class Definition
enum Suits {Spades, Diamonds, Clubs, Hearts};
class PlayingCard {
public Suits suit() { return suitValue; }
public int rank() { return rankValue; }
private Suits suitValue;
private rank rankValue; }
C# class definitions have minor differences:
no semicolon at the end
enum cannot be nested inside a class
visibility modifiers are applied to methods and data fields individually
Fall 2010
CSCI 383 Lecture M. van Bommel

10. CSCI 383 Lecture 14 M. van Bommel
Java Class Definition
class PlayingCard {
public int suit () { return suitValue; }
public int rank () { return rankValue; }
private int SuitValue;
private int rankValue;
public static final int Spades = 1;
public static final int Diamonds = 2;
public static final int Clubs = 3;
public static final int Hearts = 4; }
Java also applies visibility modifiers to each item individually. Does not have enumerated data types, uses symbolic constants instead
Fall 2010
CSCI 383 Lecture M. van Bommel

11. CSCI 383 Lecture 14 M. van Bommel
Static and Final
Notice how symbolic constants are defined in Java
static means that all instance share the same value. One per class. Similar meaning in many languages
final is Java specific, and means it will not be reassigned. (C++ has const keyword that is similar, although not exactly the same).
public static final int Spades = 1;
public static final int Diamonds = 2;
public static final int Clubs = 3;
public static final int Hearts = 4;
Fall 2010
CSCI 383 Lecture M. van Bommel

12. CSCI 383 Lecture 14 M. van Bommel
Pascal Dialects
We will consider two dialects of Pascal, both descended from the earlier language
Apple Object Pascal, defined by Apple Computer
once widely used on the Macintosh
much less commonly used now
Delphi Pascal, defined by Borland on the PC
still fairly widely used on that platform
called Kylix on the Linux platform
Many similarities due to the common heritage, but some important differences
Fall 2010
CSCI 383 Lecture M. van Bommel

13. Class Definition in Apple Object Pascal
Type
Suits {Spades, Diamonds, Clubs, Hearts};
PlayingCard = object
suit : Suits;
rank : integer;
end;
No explicit visibility modifiers (will see syntax for methods later)
Fall 2010
CSCI 383 Lecture M. van Bommel

14. CSCI 383 Lecture 14 M. van Bommel
Delphi Pascal
Type
Suits {Spades, Diamonds, Clubs, Hearts};
TPlayingCard = class (TObject)
public
constructor Create(r:integer, s:Suits);
function suit : Suits;
function rank : integer;
private
suitValue : Suits;
rankValue : integer;
end;
Slightly different syntax, must name parent class, has visibility modifier, requires creation of a constructor
Fall 2010
CSCI 383 Lecture M. van Bommel

15. CSCI 383 Lecture 14 M. van Bommel
Smalltalk
Smalltalk does not have textual description for classes, but instead you define classes in a visual interface
revolutionary idea in 1980, but now Visual Basic and Delphi programmers are used to similar facilities
Fall 2010
CSCI 383 Lecture M. van Bommel

16. CSCI 383 Lecture 14 M. van Bommel
Methods
Although syntax will differ depending upon language, all methods have the following
A name that will be matched to a message to determine when the method should be executed
A signature, which is the combination of return type and argument types. Methods with the same name can be distinguished by different signatures
A body, which is the code that will be executed when the method is invoked in response to a message
Fall 2010
CSCI 383 Lecture M. van Bommel

17. CSCI 383 Lecture 14 M. van Bommel
An Example, from C#
Fall 2010
CSCI 383 Lecture M. van Bommel

18. CSCI 383 Lecture 14 M. van Bommel
Constructor
A constructor is a method that is used to initialize a newly constructed object.
In C++, Java, C# and many other languages, it has the same name as the class
Fall 2010
CSCI 383 Lecture M. van Bommel

19. Accessor (or getter) Methods
An accessor (or getter) is a method that simply returns an internal data value
Fall 2010
CSCI 383 Lecture M. van Bommel

20. CSCI 383 Lecture 14 M. van Bommel
Why Use an Accessor?
There are many reasons why an accessor is preferable to providing direct access to a data field
You can make the data field read-only
It provides better documentation that the data field is accessible
It makes it easier to later change the access behavior (count number of accesses, whatever)
Some conventions encourage the use of a name that begins with get, (as in getRank()), but this is not universally followed
Fall 2010
CSCI 383 Lecture M. van Bommel

21. CSCI 383 Lecture 14 M. van Bommel
Setters (or mutators)
A setter (sometimes called a mutator method) is a method that is used to change the state of an object
Mutators are less common than accessors, but reasons for using are similar
Fall 2010
CSCI 383 Lecture M. van Bommel

22. CSCI 383 Lecture 14 M. van Bommel
Constant Data Fields
Some languages allow data fields to be declared as constant (const modifier in C++, final in Java, other languages have other conventions).
Constant data fields can be declared as public, since they cannot be changed
Fall 2010
CSCI 383 Lecture M. van Bommel

23. CSCI 383 Lecture 14 M. van Bommel
Order of Methods
For the most part, languages don't care about the order that methods are declared.
Here are some guidelines
List important methods first
Constructors are generally very important, list them first
Put public features before private ones
Break long lists into groups
List items in alphabetical order to make it easier to search
Remember that class definitions will often be read by people other than the original programmer.
Remember the reader, and make it easy for them
Fall 2010
CSCI 383 Lecture M. van Bommel

24. Separation of Definition and Implementation
In some languages (such as C++ or Object Pascal) the definition of a method can be separated from its implementation. They may even be in a different file (e.g., the “header” file and the “implementation” file)
Fall 2010
CSCI 383 Lecture M. van Bommel

25. Considerations in Method Definitions
In C++ you have a choice to define a method in the class interface, or separately in an implementation file.
How do you decide?
Readability
Only put very small methods in the class definition, so that it is easier to read
Semantics.
Methods defined in class interface may be expanded in-line (at the discretion of the compiler).
Another reason for only defining very small methods this way
Fall 2010
CSCI 383 Lecture M. van Bommel

26. CSCI 383 Lecture 14 M. van Bommel
Variations on Classes
We will consider a few of the mostly language-specific variations on the idea of a class
Methods without classes in Oberon
Interfaces in Java (methods without implementations)
Nested classes in Java and C++
Fall 2010
CSCI 383 Lecture M. van Bommel

27. Methods without Classes in Oberon
Oberon does not have classes, per se, but allows methods to be defined as a funny type of function
Fall 2010
CSCI 383 Lecture M. van Bommel

28. CSCI 383 Lecture 14 M. van Bommel
Interfaces in Java
An interface is like a class with no implementation.
Later, another class declared that supports the interface, and it must then give an implementation.
We will have much more to say about interfaces later after we discuss inheritance
Fall 2010
CSCI 383 Lecture M. van Bommel

29. Inner or Nested Classes
Some languages (C++ or Java) allow a class definition to be given inside another class definition.
Whether the inner class can access features of the outer class is different in different languages
Fall 2010
CSCI 383 Lecture M. van Bommel