CSSE501 Object-Oriented Development

1. CSSE501 Object-Oriented Development

2. Chapter 4: Classes and Methods • Chapters 4 and 5 present two sides of OOP: • Chapter 4 discusses the static, compile time representation of object-oriented programs. • Chapter 5 discusses the dynamic, run time behavior • Both are important, and both chapters should be understood before you begin further investigation of object-oriented programming

3. 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 • message passing, method lookup, member function invocation, method binding • methods, member function, method function • inheritance, subclassing

4. Encapsulation and Instantiation • What is a class? • A class is a category of objects; it is a new data type you create that is more complex than the basic data types • Classes provide a number of very important capabilities: • Encapsulation - The purposeful hiding of information, thereby reducing the amount of details that need 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

5. Examples • Student • Book • Vehicle • Banking Account • ……

6. Behavior and State • A class can also be viewed as a combination of behavior and state • Behavior: The actions that an instance can perform in response to a request. Implemented by methods • State: The data that an object must maintain in order to successfully complete its behavior. Stored in instance variables (also known as data members, or data fields)

7. Take the Student Class as an Example • Student • Behavior • Register/withdraw courses • Provide or change his/her name, ID, average grade, • Provide or change his/her contact information • … • State • Courses that are taking/taken • Name, ID number, Average grade • Address & Phone Number • ….

8. Class Definitions • We will use the Student class as a running example for the class definition, and show how this appears in several languages

9. C++ Class Definition (Partial) class Student { int idNum; string name; double gradePointAverage; void displayStudentData(); };

10. Java Class Definition (Partial) class Student { int idNum; string name; double gradePointAverage; void displayStudentData(); }

11. Visibility Modifiers • Accessibility to data fields (state) and methods (behavior) • Purpose • Information hiding • Visibility of a class (interface, field, method) should be as restricted as possible • Clear abstraction of program components • Does visibility of program components affect security? • Intuitively, it should • A secret may be embedded in an object as its private field • Untrusted code can be prevented from executing sensitive code by placing it in a package-local class

12. Visibility Modifiers • The terms public and private are used to differentiate the internal and external aspects of a class. • Public • Public features can be seen and manipulated by anybody (All part of the application) -- they are the external (interface or service) view • Private • Private features can be manipulated only within a class. They are the internal (implementation) view • Protected (Only in some, Not in all OO languages) • Protected features can be used only in the same package and from subclasses in other packages

13. Visibility Modifiers • Typically in object-oriented technology • All the data fields should be private • All the methods that are accessible by other should be public • Protected should only be used when a subclass needs access to what would otherwise be a private member

14. C++ Class Definition (Partial) class Student { private: int idNum; string name; double gradePointAverage; public: void displayStudentData(); };

15. Java Class Definition (Partial) class Student { private int idNum; private string name; private double gradePointAverage; public void displayStudentData(); }

16. 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

17. Body C++ Class Definition (Partial) //declaration section: class Student { private: int idNum; string name; double gradePointAverage; public: void displayStudentData(); }; //implementation section: void Student::displayStudentData() { cout<<“Student #”<<idNum<< “’s name is “<<name<<endl; cout<<“The grade point average for this student is “ <<gradePointAverage<<endl; } Name: dispalyStudentData Signature: void Student::displayStudentData()

18. Body Java Class Definition (Partial) class Student { private int idNum; private string name; private double gradePointAverage; public void displayStudentData() { system.out.println(“Student #” + idNum +” ‘s name is” + name); system.out.println(“The grade point average for this student is “ + gradePointAverage); } } Name: dispalyStudentData Signature: publicvoid Student::displayStudentData()

19. Constructor • A constructor • Is a method that is used to initialize a newly constructed object • Is called automatically each time an object is created • In C++, Java, C# and many other languages it has the same name as the class • A constructor cannot have a return type

20. C++ Class Definition (Partial) //declaration section: class Student { private: int idNum; string name; double gradePointAverage; public: Student(); void displayStudentData(); }; Student::Student() { idNum = 9999; name = “J. Smith”; gradePointAverage = 0.0; } void Student::displayStudentData() { …… } A constructor without arguments

21. Java Class Definition (Partial) class Student { private int idNum; private string name; private double gradePointAverage; public Student() { idNum = 9999; name = “J. Smith”; gradePointAverage = 0.0; } public void displayStudentData() { system.out.println(“Student #” + idNum +” ‘s name is” + name); system.out.println(“The grade point average for this student is “ + gradePointAverage); } } A constructor without arguments

22. Accessor (or getter) Methods • Since all data fields should be private, how can we update or present a student’s information? • Answer: through accessor or setter methods • An accessor (or getter) is a method that simply returns an internal data value • Therefore, we can present private data fields

23. 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 (private) • 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

24. C++ Class Definition (Partial) //declaration section: class Student { private: int idNum; string name; double gradePointAverage; public: Student(); void displayStudentData(); int getIDNum(); string getName(); double getGradePointAverage(); }; Student::Student() { …… } void Student::displayStudentData() { …… } int Student::getIdNum() { return idNum; } string Student::getName() { return name; } double Sudent::getGradePointAverage() { return gradePointAverage; } Accessors

25. Java Class Definition (Partial) class Student { private int idNum; private string name; private double gradePointAverage; public Student() { …… } public void displayStudentData() { …. } public int getIdNum() { return idNum; } public string getName() { return name; } public double getGradePointAverage() { return gradePointAverage; } } Accessors

26. 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

27. C++ Class Definition (Partial) //declaration section: class Student { private: int idNum; string name; double gradePointAverage; public: Student(); void displayStudentData(); int getIDNum(); string getName(); double getGradePointAverage(); void setIDNum(int); void setName(string); void setGradePointAverage(double); }; …… void Student::setIdNum(int num) { const int MAX_NUM = 9999; if (num <= MAX_NUM) idNum = num; else idNum = MAX_NUM; } void Student::setName(string nameIn) { name = nameIn; } void Sudent::setGradePointAverage(double grade) { gradePointAverage = grade: } setters

28. Java Class Definition (Partial) class Student { private int idNum; private string name; private double gradePointAverage; public Student() { …… } public void displayStudentData() { …. } public int getIdNum() { return idNum; } public string getName() { return name; } public double getGradePointAverage() { return gradePointAverage; } } public void setIdNum(int num) { const int MAX_NUM = 9999; if (num <= MAX_NUM) idNum = num; else idNum = MAX_NUM; } publicvoid setName(string nameIn) { name = nameIn; } publicvoid setGradePointAverage(double grade) { gradePointAverage = grade: } setters

29. 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 class PlayingCard { // Java example ... public static final int Spade = 1; public static final int Diamond = 2; public static final int Club = 3; public static final int Heart = 4; }

30. Order of Methods • For the most part, languages don't care about the order that methods are declared. Here are some guidelines: • List important topics 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 readers, and make it easy for them

31. 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

32. 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 class PlayingCard { public: ... Colors color () ; ... }; PlayingCard::Colors PlayingCard::color ( ) { // return the face color of a playing card if ((suit == Diamond) || (suit == Heart)) return Red; return Black; }

33. 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 (at the discretion of the compiler) be expanded in-line. Another reason for only defining very small methods this way.

34. Interfaces in Java • An interface is like a class, but it provides no implementation. Later, another class can declare that it supports the interface, and it must then give an implementation. We will have much more to say about interfaces later after we discuss inheritance. public interface Storing { void writeOut (Stream s); void readFrom (Stream s); }; public class BitImage implements Storing { void writeOut (Stream s) { // ... } void readFrom (Stream s) { // ... } };