Lesson 4 – Classes & Objects

1. Lesson 4 – Classes & Objects Unit D1 – Introduction

2. Container vs. Definition classes • Generally, classes can be used for two purposes: • Container classes: • a collection of static methods that are not bound to any particular object (e.g., the main() method). • These static methods usually have something in common • Definition classes: • These classes define new objects, in a way that we will soon see.

3. Container vs. Definition Classes - Example • The Math class is an example of the first kind. It is a container for math utility methods: Math.sqrt(), Math.abs(), Math.max(), ... • The class Turtle is an example of the second kind. It defines a new type of objects, Turtle objects. • We will now focus on the second kind.

4. Class Abstraction • The most important stage in writing a class is to get an abstraction of the class done first. • What does an object of this type represent? • What is its interface? • What is its internal state? • This means you have to know what should be the internal state (variables) and also the behavior/interface (methods) of the class before you start to write you class code.

5. Objects and Classes • The relationship between Classes and Objects can be seen as the relationship between a blueprint/model and the the actual object built from it. • Example: Blueprint of a house (class) and the house (object) • The class defines: • The type of data that will be held in an object • The code for the methods of the object • In order to use a class you must instantiate an object from it. • This corresponds to building the house from the blueprint. • There may be many objects from a single class

6. Turtle Class Abstraction • A Turtlerepresents a creature moving on the screen with a pen attached to its tail • The internal state of a Turtle includes: • Location on screen; direction of movement; tail up/down • The interface of a Turtle is: Turtle(); void moveForward(double units); void moveBackward(double units); void turnLeft(double degrees); // …

7. Encapsulation • The internal state of an object is not directly accessible to other parts of the program • Other parts of the program can only access the object using its interface • We say that the state is encapsulated or hidden • This gives modularity to the program moveForward() . . . . . . • tailDown • x,y • direction turnRight()

8. Clock Class Abstraction • A Clockrepresents a 12-hour clock • Its internal state includes: hour, minute, second • Its interface allows telling the clock that a second has elapsed, and querying the clock for the time: Clock(int hours, int minutes, int seconds) void secondElapsed() int getSeconds() int getMinutes() int getHours() ...

9. Using a Clock class Clock newYorkTime = new Clock(12,59,59); for (int j = 0; j < 3; j++) newYorkTime.secondElapsed(); System.out.println(newYorkTime.getHours() + “:” + newYorkTime.getMinutes() + “:” + newYorkTime.getSeconds());

10. Structure of the Clock class • The private modifier specifies an identifier that is only visible within the class. Usually used for fields. • The public modifier specifies an identifier that is accessible to all other classes. Usually used for methods. public class Clock { private int hours, minutes, seconds; public Clock(int h, int m, int s){ … } publicvoid secondElapsed() { … } publicint getHours() { … } publicint getMinutes() { … } public int getSeconds() { … } }

11. Lesson 4 – Classes & Objects Unit D2 - Class Elements

12. Instance Variables • Recall that a class must define the state of an object and its behavior. • We declare state variables (variables that hold the state of the object) in a similar way to that of regular variables, only they appear outside methods, inside the class. • State variables are also called instance variables or fields. • Roughly speaking, the private modifier means that the variables are not part of the object’s interface. public class Clock { private int hours, minutes, seconds; // … }

13. Constructors • Objects must be initialized before they can be used. • We must specify what is the initial state of the object before we can use it. • Space for holding the object state data is only allocated when the object is constructed. • We specify the way an object is initialized using a constructor, which is a special method which is invoked every time we create a new object • The name of the constructor is always identical to the class name

14. Clock Constructor public Clock(int h, int m, int s){ hours = h; minutes = m; seconds = s; }

15. Clock constructor invoked Clockc; c = new Clock(10,45,0); C: public class Clock { private int hours, minutes,seconds; public Clock(int h, int m, int s){ hours = h; minutes = m; seconds = s; } // … hours: Minutes: Seconds: 10 45 0

16. Methods • To make the date object useful, we must provide methods that define its behavior. • We declare methods in a similar way to the way the method main was declared. • Only there’s a big difference: • the main method was static, which means it wasn’t bound to a specific object • we want to declare instance methods, which operate on a specific instance of an object

17. secondElapsed() method public void secondElapsed() { if (seconds < 59) seconds++; else { seconds=0; if (minutes < 59) minutes++ ; else { minutes = 0; hours = hours < 12 ? hours+1 : 1; } } }

18. Return Types • Methods may return values • The return type of a method indicates the type of value that the method sends back to the calling client • The return-type of getHours() is int. When a client ask for the hours read of a clock it gets the answer as an int value. • A method that does not return a value (such as secondElapsed()) has a void return type • The returnstatement specifies the value that should be returned, which must conform with the return type of the method.

19. Clock accessor methods public int getHours() { return hours; } public int getMinutes() { return minutes; } public int getSeconds() { return seconds; }

20. Method Context • The getHours() and secondElapsed() methods are instance methods, which means they act on a particular instance of the class • They cannot be invoked “out of the blue”. They must act on a particular object: • An instance method is executed in the context of the object it acts upon. Clock c = new Clock(1,2,3); getHours(); // Error!! of which clock? c.getHours(); // will return 1

21. ClockTest example public class ClockTest { public static void main(String[] args) { Clock swatch = new Clock(12,59,59); Clock seiko = new Clock(12,59,59); System.out.println(swatch.getHours()); // 12 System.out.println(seiko.getHours()); // 12 swatch.secondElapsed(); System.out.println(swatch.getHours()); // 1 System.out.println(seiko.getHours()); // 12 } }

22. Method Parameters • A method can be defined to accept zero or more parameters • Each parameter in the parameter list is defined by its type and name • The parameters in the method definition are called formal parameters • The values passed to a method when it is invoked are called actual parameters • The name of the method together with the list of its formal parameters is called the signature of the method public void setTime(int h, int m, int s)

23. Method setTime() public void setTime(int h, int m, int s){ if ((s >= 0) && (s < 60) && (m >= 0) && (m < 60) && (h > 0) && (h <= 12)) { hours = h; minutes = m; seconds = s; } // no effect if input is illegal }

24. Lesson 4 – Classes & Objects Unit D3 - Method writing details

25. Variable Scope • Variables may be declared in: • Class – state variables • Method/constructor – local variables (and parameters) • Inner block of a method – also local variables • A variable is recognized throughout the block in which it was defined. This is called the scope of the variable. • Local variables are allocated when the method is entered and freed when the method exits. • The same name may be used in different scopes, and refer to totally different things • If the same name is used in an outer and inner scope, then the inner scope definition “hides” the outer one.

26. The this reference • When appearing inside an instance method, the thiskeyword denotes a reference to the object that the method is acting upon. • The following are equivalent: public int getHours() { return hours; } public int getHours() { returnthis.hours; }

27. Using same names for parameters and fields • It is usually bad practice to use the same name for a state variable and a local variable or parameter. • Exception: parameters that correspond exactly to fields public void setTime(int hours, int minutes, int seconds) { if ((seconds >= 0) && (seconds < 60) && (minutes >= 0) && (minutes < 60) && (hours > 0) && (hours <= 12)) { this.hours = hours; this.minutes = minutes; this.seconds = seconds; } // no effect if input is illegal }

28. Passing Parameters • When a parameter is passed, a copy of the value is made and assigned to the formal parameter: Clock beritling = new Clock(1,2,3); int lunchHour = 12; breitling.setTime(lunchHour,32,14); hours = lunchHour minutes = 32 seconds = 14

29. Parameters are passed by value Clock c = new Clock(1,2,3); int a= 7; c.funnyGetHour(a); // still 7!! class Clock { // … public void funnyGetHour(int h){ h = hours; } }

30. Passing Object parameters • When an Object is a parameter, only the reference is passed by value. The parameter now is an alias of the object. class XX { // … public void setMidnight(Clock c) { c.setTime(12,0,0); } class YY { // … XX x = new XX(); Clock myClock = new Clock(1,2,3); x.setMidnight(myClock); // 12:0:0

31. Example: A Bank Account Object public BankAccount(long accountNumber) publicvoid deposit(double amount) public void withdraw(double amount) publicdouble getBalance() publicvoid transfer(double amount, BankAccount targetAccount) BankAccount

32. Bank Account - example public class BankAccount { private long accountNumber; private double balance; public BankAccount(long accountNumber){ this.accountNumber = accountNumber; balance = 0; } public double getBalance() { return balance; } // continued in the next slide... }

33. Bank Account - example public void deposit(double amount) { balance += amount; } public void withdraw(double amount) { balance -= amount; } public void transfer(double amount, BankAccount targetAccount){ withdraw(amount); targetAccount.deposit(amount); }

34. Bank Account – usage example BankAccount aliceAcc = new BankAccount(1398723); BankAccount bobAcc = new BankAccount(1978394); aliceAcc.deposit(900); aliceAcc.transfer(700,bobAcc); // Alice’s balance = 200 ; Bob’s balance = 700

35. Constructor and Method Overloading • A class can define several constructors -- several ways to initialize an instance of this class • These constructors differ by the number and/or type of parameters they get. • When we construct an object, the compiler decides which constructor to invoke according to the number and types of the actual arguments • A constructor with no parameters is called the default constructor • Different methods can also use the same name as long as they differ in the number or type of parameters. • When we invoke a method, the compiler decides which method to invoke according to the number and types of the actual arguments

36. Constructor Overloading example public Clock(int h, int m, int s){ hours = h; minutes = m; seconds = s; } public Clock(int h) { this(h, 0 ,0); } public Clock() { this(12); }

37. Lesson 4 – Classes & Objects Unit D4 - Visibility Modifiers

38. Visibility Modifiers • We accomplish encapsulation through the appropriate use of visibility modifiers • Visibility modifiers specify which parts of the program may see and use any particular class/method/field • Information hiding is good! • A modifier is a Java reserved word that specifies particular characteristics of a programming construct • We've used the modifier final to define a constant • Java has three visibility modifiers: public, private, and protected • We will discuss the protected modifier later in the course

39. Visibility Modifiers - Classes • A class can be defined either with the public modifier or without a visibility modifier. • If a class is declared as public it can be used by any other class • If a class is declared without a visibility modifier it has a default visibility. This draws a limit to which other classes can use this class (classes in the same package). We will discuss default visibility later in the course. • Classes that define a new type of objects, that are supposed to be used anywhere, should be declared public.

40. Visibility Modifiers - Members • A member is a field, a method or a constructor of the class. • Members of a class can be declared as private, protected, public or without a visibility modifier: • Members that are declared without a visibility modifier are said to have default visibility. We will discuss default and protected visibility later in the course. privateint hours; int hours; public int hours;

41. Public Visibility • Members that are declared as public can be accessed from any class that can access the class of the member • We expose methods that are part of the interface of the class by declaring them as public • Example: the methods getHours(), secondElapsed()andsetTime() are part of the interface of class Clock so we define them as public. • We do not want to reveal the internal representation of the object’s data. So we usually do not declare its state variables as public (encapsulation)

42. Private Visibility • A class member that is declared as private, can be accessed only by code that is within the class of this member. • We hide the internal implementation of the class by declaring its state variables and auxiliary methods as private. • Data hiding is essential for encapsulation.

43. Illegal Access - example // Example of illegal access class BankAccountTest { public static void main(String[] args) { BankAccount victim = new BankAccount(2398742); victim.balance = victim.balance - 500; // this will not compile! } } public class BankAccount { private long accountNumber; private double balance; // …

44. Encapsulation not Among Instances of Same Class • Encapsulation is to protect the programmers and is thus between the code of different classes • Sometimes object instances of the same class need to access each other’s “guts” (e.g., for state copying - if we want to create an identical instance of an object we have) • Example: • from within a BankAccount object, any private member of a different BankAccount object can be accessed.

45. Encapsulation - example public void transfer(double amount, BankAccount targetAccount) { withdraw(amount); targetAccount.deposit(amount); } // alternative version (valid, but not so nice) public void transfer(double amount, BankAccount targetAccount) { balance -= amount; targetAccount.balance += amount; }

46. Lesson 4 – Classes & Objects Unit D5 - API documentation

47. API Documentation • Your classes are often intended to be used by other programmers • Programmers that use your class are not interested in the way it is implemented. They want to use it as a whole and are only interested in what it does and how to use it. • API (Application Programmer Interface) documentation is a description of the interface of the class intended for the application programmer who wants to use it. • To use the class, we need not (and should not) look at the code. All that is needed is the class API.

48. API Documentation • The JDK contains a special tool for the generation of API documentation for your classes, called javadoc. • Any documentation which is part of the interface begins with /** (double asterick) and ends with */ • javadoc takes as input Java programs and automatically generates documentation using: • the public/protected method signatures • the documentation comments (enclosed by /** … */). • The output is an HTML file which can be viewed by an internet browser.

49. Clock API Documentation /** * A clock represents a point of time in a 12 * hour period within a precision of seconds. * Its range: 1:00:00 -- 12:59:59. */ public class Clock { private int hours; private int minutes; private int seconds; /** * Constructs a Clock: Sets the clock to the * specified time. */ public Clock(int hours, int minutes, int seconds){ //… }

50. Clock API Documentation – cont. /** * Constructs a Clock: Sets the clock to 12:00:00 */ public Clock(){ this(12,0,0); } /** * Advances this clock by 1 second. */ public void secondElapsed() { //… } //…