Writing Classes: Object Definitions, Encapsulation, and Modifiers

1. Chapter 5 – Writing Classes We've been using predefined classes. Now we will learn to write our own classes to define objects Chapter 5 focuses on class definitions and relationships instance data encapsulation and Java modifiers method declaration and parameter passing constructors method design and overloading 1-1

2. Outline • Classes and Objects Revisited • Anatomy of a Class • Encapsulation • Anatomy of a Method • Static Class Members • Class Relationships • Method Design • Method Overloading • Testing and Debugging

3. 5.1 – Classes and Object Revisited The programs we’ve written in previous examples have used classes defined in the Java standard class library Now we will begin to design programs that rely on classes that we write ourselves The class that contains the main method is just the starting point of a program True object-oriented programming is based on defining classes that represent objects with well-defined characteristics and functionality

4. 5.1 – Identifying Classes and Objects Remember that a class represents a group (classification) of objects with the same behaviors Generally, classes that represent objects should be given names that are singular nouns Examples: Coin, Student, Message A class represents the concept of one such object We are free to instantiate as many of each object as needed

5. 5.1 – Identifying Classes and Objects Sometimes it is challenging to decide whether something should be represented as a class For example, should an employee's address be represented as a set of instance variables or as an Address object The more you examine the problem and its details the more clear these issues become When a class becomes too complex, it often should be decomposed into multiple smaller classes to distribute the responsibilities

6. 5.1 – Identifying Classes and Objects We want to define classes with the proper amount of detail For example, it may be unnecessary to create separate classes for each type of appliance in a house It may be sufficient to define a more general Appliance class with appropriate instance data It all depends on the details of the problem being solved

7. 5.1 – Identifying Classes and Objects Part of identifying the classes we need is the process of assigning responsibilities to each class Every activity that a program must accomplish must be represented by one or more methods in one or more classes We generally use verbs for the names of methods In early stages it is not necessary to determine every method of every class – begin with primary responsibilities and evolve the design

8. 5.1 – Identifying Classes and Objects Recall from our overview of objects in Chapter 1 that an object has state and behavior Consider a university student object it’s state can be defined as the student’s name, address, major, and GPA it’s primary behavior may be to update the address or to recalculate the GPA We can represent a student in software by designing a class called Student that models this state and behavior the class serves as the blueprint for a student object We can then instantiate as many student objects as we need for any particular program

9. 5.1 – Classes A class can contain data declarations and method declarations int size, weight; char category; Data declarations Method declarations

10. 5.1 – Classes The values of the data define the state of an object created from the class The functionality of the methods define the behaviors of the object For our Student class, we might declare a String that represents the student’s major One of the methods would update the student’s major by setting that value to the value of a new string (the new major).

11. Outline • Classes and Objects Revisited • Anatomy of a Class • Encapsulation • Anatomy of a Method • Static Class Members • Class Relationships • Method Design • Method Overloading • Testing and Debugging

12. 5.2 – Anatomy of a Class Consider a six-sided die (singular of dice) It’s state can be defined as which face is showing It’s primary behavior is that it can be rolled We can represent a die in software by designing a class called Die that models this state and behavior We’ll want to design the Die class with other data and methods to make it a versatile and reusable resource Any given program will not necessarily use all aspects of a given class

13. 5.2 – SnakeEyes.java //******************************************************************** // SnakeEyes.java Java Foundations // // Demonstrates the use of a programmer-defined class. //******************************************************************** public class SnakeEyes { //----------------------------------------------------------------- // Creates two Die objects and rolls them several times, counting // the number of snake eyes that occur. //----------------------------------------------------------------- public static void main (String[] args) { final int ROLLS = 500; int num1, num2, count = 0; Die die1 = new Die(); Die die2 = new Die(); (more…)

14. 5.2 – SnakeEyes.java for (int roll=1; roll <= ROLLS; roll++) { num1 = die1.roll(); num2 = die2.roll(); if (num1 == 1 && num2 == 1) // check for snake eyes count++; } System.out.println ("Number of rolls: " + ROLLS); System.out.println ("Number of snake eyes: " + count); System.out.println ("Ratio: " + (float)count / ROLLS); } }

15. 5.2 – Die.java //******************************************************************** // Die.java Java Foundations // // Represents one die (singular of dice) with faces showing values // between 1 and 6. //******************************************************************** public class Die { private final int MAX = 6; // maximum face value private int faceValue; // current value showing on the die //----------------------------------------------------------------- // Constructor: Sets the initial face value of this die. //----------------------------------------------------------------- public Die() { faceValue = 1; } (more…)

16. 5.2 – Die.java //----------------------------------------------------------------- // Computes a new face value for this die and returns the result. //----------------------------------------------------------------- public int roll() { faceValue = (int)(Math.random() * MAX) + 1; return faceValue; } //----------------------------------------------------------------- // Face value mutator. The face value is not modified if the // specified value is not valid. //----------------------------------------------------------------- public void setFaceValue (int value) { if (value > 0 && value <= MAX) faceValue = value; } (more…)

17. 5.2 – Die.java //----------------------------------------------------------------- // Face value accessor. //----------------------------------------------------------------- public int getFaceValue() { return faceValue; } //----------------------------------------------------------------- // Returns a string representation of this die. //----------------------------------------------------------------- public String toString() { String result = Integer.toString(faceValue); return result; } }

18. 5.2 – The Die Class The Die class contains two data values a constant MAX that represents the maximum face value an integer faceValue that represents the current face value The roll method uses the random method of the Math class to determine a new face value There are also methods to explicitly set and retrieve the current face value at any time

19. 5.2 – The toString Method All classes that represent objects should define a toString method The toString method returns a character string that represents the object in some way It is called automatically when an object is concatenated to a string or when it is passed to the println method

20. 5.2 – Constructors As mentioned previously, a constructor is a special method that is used to set up an object when it is initially created A constructor has the same name as the class The Die constructor is used to set the initial face value of each new die object to one We examine constructors in more detail later in this chapter

21. 5.2 – Data Scope The scope of data is the area in a program in which that data can be referenced (used) Data declared at the class level can be referenced by all methods in that class Data declared within a method can be used only in that method Data declared within a method is called local data In the Die class, the variable result is declared inside the toString method -- it is local to that method and cannot be referenced anywhere else

22. 5.2 – Instance Data The faceValue variable in the Die class is called instance data because each instance (object) that is created has its own version of it A class declares the type of the data, but it does not reserve any memory space for it Every time a Die object is created, a new faceValue variable is created as well The objects of a class share the method definitions, but each object has its own data space That's the only way two objects can have different states

23. 5.2 – Instance Data We can depict the two Die objects from the SnakeEyes program as follows die1 faceValue 5 die2 faceValue 2 Each object maintains its own faceValue variable, and thus its own state

24. 5.2 – UML Diagrams UML stands for the Unified Modeling Language UML diagrams show relationships among classes and objects A UML class diagram consists of one or more classes, each with sections for the class name, attributes (data), and operations (methods) Lines between classes represent associations A solid arrow shows that one class uses the other (calls its methods)

25. 5.2 – UML Class Diagrams A UML class diagram for the SnakeEyes program:

26. Outline • Classes and Objects Revisited • Anatomy of a Class • Encapsulation • Anatomy of a Method • Static Class Members • Class Relationships • Method Design • Method Overloading • Testing and Debugging

27. 5.3 – Encapsulation We can take one of two views of an object internal - the details of the variables and methods of the class that defines it external - the services that an object provides and how the object interacts with the rest of the system From the external view, an object is an encapsulated entity, providing a set of specific services These services define the interface to the object 1-27

28. 5.3 – Encapsulation One object (called the client) may use another object for the services it provides The client of an object may request its services (call its methods), but it should not have to be aware of how those services are accomplished Any changes to the object's state (its variables) should be made by that object's methods We should make it difficult, if not impossible, for a client to access an object’s variables directly That is, an object should be self-governing 1-28

29. 5.3 – Encapsulation An encapsulated object can be thought of as a black box– its inner workings are hidden from the client The client invokes the interface methods of the object, which manages the instance data Methods Client Data 1-29

30. 5.3 – Visibility Modifiers In Java, we accomplish encapsulation through the appropriate use of visibility modifiers A modifier is a Java reserved word that specifies particular characteristics of a method or data We've used the final modifier to define constants Java has three visibility modifiers: public, protected, and private The protected modifier involves inheritance, which we will discuss later 1-30

31. 5.3 – Visibility Modifiers Members of a class that are declared with public visibility can be referenced anywhere Members of a class that are declared with private visibility can be referenced only within that class Members declared without a visibility modifier have default visibility and can be referenced by any class in the same package An overview of all Java modifiers is presented in Appendix E 1-31

32. 5.3 – Visibility Modifiers Public variables violate encapsulation because they allow the client to “reach in” and modify the values directly Therefore instance variables should not be declared with public visibility It is acceptable to give a constant public visibility, which allows it to be used outside of the class Public constants do not violate encapsulation because, although the client can access it, its value cannot be changed 1-32

33. 5.3 – Visibility Modifiers Methods that provide the object's services are declared with public visibility so that they can be invoked by clients Public methods are also called service methods A method created simply to assist a service method is called a support method Since a support method is not intended to be called by a client, it should not be declared with public visibility 1-33

34. 5.3 – Visibility Modifiers public private Variables Methods Violate encapsulation Enforce encapsulation Support other methods in the class Provide services to clients

35. 5.3 – Accessors and Mutators Because instance data is private, a class usually provides services to access and modify data values An accessor method returns the current value of a variable A mutator method changes the value of a variable The names of accessor and mutator methods take the form getX and setX, respectively, where X is the name of the value

36. 5.3 – Accessors and Mutators They are sometimes called “getters” and “setters” Coin.java The isHeads method is an accessor The flip method is a mutator

37. 5.3 – Coin.java //******************************************************************** // Coin.java Java Foundations // // Represents a coin with two sides that can be flipped. //******************************************************************** public class Coin { private final int HEADS = 0; // tails is 1 private int face; // current side showing //----------------------------------------------------------------- // Sets up this coin by flipping it initially. //----------------------------------------------------------------- public Coin () { flip(); } (more…)

38. 5.3 – Coin.java //----------------------------------------------------------------- // Flips this coin by randomly choosing a face value. //----------------------------------------------------------------- public void flip () { face = (int) (Math.random() * 2); } //----------------------------------------------------------------- // Returns true if the current face of this coin is heads. //----------------------------------------------------------------- public boolean isHeads () { return (face == HEADS); } //----------------------------------------------------------------- // Returns the current face of this coin as a string. //----------------------------------------------------------------- public String toString() { return (face == HEADS) ? "Heads" : "Tails"; } }

39. 5.3 – CountFlips.java //******************************************************************** // CountFlips.java Java Foundations // // Demonstrates the use of programmer-defined class. //******************************************************************** public class CountFlips { //----------------------------------------------------------------- // Flips a coin multiple times and counts the number of heads // and tails that result. //----------------------------------------------------------------- public static void main (String[] args) { final int FLIPS = 1000; int heads = 0, tails = 0; Coin myCoin = new Coin(); (more…)

40. 5.3 – CountFlips.java for (int count=1; count <= FLIPS; count++) { myCoin.flip(); if (myCoin.isHeads()) heads++; else tails++; } System.out.println ("Number of flips: " + FLIPS); System.out.println ("Number of heads: " + heads); System.out.println ("Number of tails: " + tails); } }

41. 5.3 – FlipRace.java //******************************************************************** // FlipRace.java Java Foundations // // Demonstrates the reuse of programmer-defined class. //******************************************************************** public class FlipRace { //----------------------------------------------------------------- // Flips two coins until one of them comes up heads three times // in a row. //----------------------------------------------------------------- public static void main (String[] args) { final int GOAL = 3; int count1 = 0, count2 = 0; Coin coin1 = new Coin(), coin2 = new Coin(); (more…)

42. 5.3 – FlipRace.java while (count1 < GOAL && count2 < GOAL) { coin1.flip(); coin2.flip(); System.out.println ("Coin 1: " + coin1 + "\tCoin 2: " + coin2); // Increment or reset the counters count1 = (coin1.isHeads()) ? count1+1 : 0; count2 = (coin2.isHeads()) ? count2+1 : 0; } if (count1 < GOAL) System.out.println ("Coin 2 Wins!"); else if (count2 < GOAL) System.out.println ("Coin 1 Wins!"); else System.out.println ("It's a TIE!"); } }

43. 5.3 – Mutator Restrictions The use of mutators gives the class designer the ability to restrict a client’s options to modify an object’s state A mutator is often designed so that the values of variables can be set only within particular limits For example, the setFaceValue mutator of the Die class should have restricted the value to the valid range (1 to MAX) Such restrictions can be implemented through the use of an if statement in the body of the constructor.

44. Outline • Classes and Objects Revisited • Anatomy of a Class • Encapsulation • Anatomy of a Method • Static Class Members • Class Relationships • Method Design • Method Overloading • Testing and Debugging

45. 5.4 – Method Declarations Let’s now examine method declarations in more detail A method declaration specifies the code that will be executed when the method is invoked (called) When a method is invoked, the flow of control jumps to the method and executes its code When complete, the flow returns to the place where the method was called and continues The invocation may or may not return a value, depending on how the method is defined

46. 5.4 – Method Control Flow If the called method is in the same class, only the method name is needed compute myMethod myMethod();

47. 5.4 – Method Control Flow The called method is often part of another class or object main doIt helpMe obj.doIt(); helpMe();

48. 5.4 – Method Header A method declaration begins with a method header char calc (int num1, int num2, String message) method name parameter list The parameter list specifies the type and name of each parameter The name of a parameter in the method declaration is called a formal parameter return type

49. 5.4 – Method Body The method header is followed by the method body char calc (int num1, int num2, String message) { int sum = num1 + num2; char result = message.charAt (sum); return result; } sumand result are local data They are created each time the method is called, and are destroyed when it finishes executing The return expression must be consistent with the return type

50. 5.4 – The return Statement The return type of a method indicates the type of value that the method sends back to the calling location A method that does not return a value has a void return type A return statement specifies the value that will be returned return expression; Its expression must conform to the return type 1-50