1. Unit 6 - Classes

2. WHAT

3. Classes Classes allow us to encapsulate behavior and state that could be useful for the future We have been using classes for some time now: Scanner, String, ArrayList We have already seen an example of how classes can be useful Our FractionCalculator project Wouldn’t you have liked to have returned an object that contained both the numerator and denominator instead of using class fields/variables? We have actually been declaring classes all along! public class MyClass {... }

4. Class Definition Classes are defined using the class keyword: public class { } Note that static is missing. From here on out, static classes will be used for client code. Remember our rules/conventions: Class names are like variable names: Letters, digits, underscores only Start with letter Class names begin with a capital letter Use another capital letter for each new word e.g. Date, MixedFraction, TicTacToeBoard

5. Class Definition Classes consist of three components: [Functionality]Methods Provide class behavior Defined just like we have been, except don’t use static [Data] Variables Store class state Declared like we’re used to, but usually prefix with private Scope is the entire class [Functionality] Constructors Define how to create instances of the class Really a special case of methods

6. WHY

7. Why Some programming problems are best thought of as a collection of objects that interact with each other.

8. A programming problem Given a file of cities' (x, y) coordinates, which begins with the number of cities: 6 50 20 90 60 10 72 74 98 5 136 150 91 Write a program to draw the coordinates for cities, then display whether your car can make a given city by turning all cities red that are within a given radius: Current location x? 100 Current location y? 100 Driving range? 75

9. A bad solution Scanner input = new Scanner(new File("cities.txt")); int cityCount = input.nextInt(); int[] xCoords = new int[cityCount]; int[] yCoords = new int[cityCount]; for (int i = 0; i < cityCount; i++) { xCoords[i] = input.nextInt(); // read each city yCoords[i] = input.nextInt(); }... parallel arrays: 2+ arrays with related data at same indexes. Considered poor style.

10. Observations The data in this problem is a set of points. It would be better stored as Point objects. A Point would store a city's x/y data. We could compare distances between Point s to see whether a car can reach a given city. Each Point would know how to draw itself. The overall program would be shorter and cleaner.

11. Classes and objects class: A program entity that represents either: 1.A program / module, or 2.A template for a new type of objects. The DrawingPanel class is a template for creating DrawingPanel objects. object: An entity that combines state and behavior. object-oriented programming (OOP): Programs that perform their behavior as interactions between objects.

12. Blueprint analogy iPod blueprint state: current song volume battery life behavior: power on/off change station/song change volume choose random song creates iPod #1 state: song = " 1,000,000 Miles " volume = 17 battery life = 2.5 hrs behavior: power on/off change station/song change volume choose random song iPod #2 state: song = "Letting You" volume = 9 battery life = 3.41 hrs behavior: power on/off change station/song change volume choose random song iPod #3 state: song = "Discipline" volume = 24 battery life = 1.7 hrs behavior: power on/off change station/song change volume choose random song

13. Clients of objects client program: A program that uses objects. Example: DrivingRange is a client of DrawingPanel and Graphics. DrivingRange.java (client program) public class DrivingRange { main(String[] args) { new DrawingPanel(...)... } DrawingPanel.java (class) public class DrawingPanel {... }

14. Classes Summary What are the 3 components of a class? Methods Fields Constructors What is difference between a class and an object? A class is a blueprint for how to create an instance of an object.

15. OBJECT STATE: FIELDS

16. Point class, version 1 public class Point { private int x; private int y; } The above code creates a new type named Point. Each Point object contains two pieces of data: an int named x, and an int named y. Point objects do not contain any behavior (yet). As a class: Create a class named Point

17. Fields field: A variable inside an object that is part of its state. Each object has its own copy of each field. Declaration syntax: type name ; Example: public class Student { private String name; / / each Student object has private double gpa; // a name and gpa field }

18. Accessing fields Other classes can access/modify an object's fields. access:variable. field modify:variable. field = value ; Public fields tend to link you to a particular implementation and limit your flexibility in changing your code. Example: Point p1 = new Point(); Point p2 = new Point(); System.out.println("the x-coord is " + p1.x); // access p2.y = 13; // modify

19. Access Control We’ve been using public in a lot of places public is a Java access specifier Others are private and protected Access specifiers tell Java who should be able to see and use the class/method/variable we’re defining public : can be seen/used by everyone anywhere private : can only be seen/used by the class itself Ignore protected for now– we won’t be using it.

20. Access Control Now that we’re defining classes, we’ll start using private for some things Like with variable names, there are no rules about this, just conventions We’ll use the following conventions: Almost all fields will be private Almost all constructors will be public Methods will be public if other classes will want to use them, and private otherwise If you’re not sure, feel free to ask

21. Methods instance method (or object method): Exists inside each object of a class and gives behavior to each object. public type name ( parameters ) { statements ; } same syntax as static methods, but without static keyword Example: public void shout() { System.out.println("HELLO THERE!"); } accessor:A method that lets clients examine object state. Examples: distance, distanceFromOrigin often has a non- void return type mutator:A method that modifies an object's state. Examples: setLocation, translate

22. Accessor Methods Because we declare our variables as private, others cannot access them Movie megamind = new Movie(“Megamind”); System.out.print(megamind.title); // won’t work Instead, we use accessor methods to provide access to the data This provides a layer of abstraction over our data Accessor methods typically names of the form getVariable and don’t take arguments

23. Accessor (Getter) Methods // accessor method public String getTitle() { return title; } // simple accessor method public String[] getActors() { return actors; } As a class: Add getX and getY accessor methods

24. Other Accessor Methods We can also provide accessor methods for data for which we don’t store a variable This is another reason we use methods instead of direct variable access These getters are named similarly, but do not have a corresponding variable public String getLeadActor() { // lead actor always stored first in list return actors[0]; }

25. Mutator Methods Mutator methods are the analog of getter methods They are used to set the value of a variable Mutator methods typically have names of the form setVariable, have a void return type, and take a single argument of the relevant type We can also define mutator methods that don’t simply set a variable, but modify something more complex

26. Mutator Methods // simple mutator method public void setTitle(String title) { this.title = title; } // mutator method public void addActor(String actor) { String[] newActors = new String[actors.length + 1]; for (int i = 0; i < actors.length; i++) { newActors[i] = actors[i]; } newActors[actors.length] = actor; actors = newActors; } As a class: Add a setLocation mutator method

27. Other Methods We can also provide any other methods we might want or that might be useful These other methods will use our variables, and can possibly call other methods public boolean isInFilm(String actor) { for (int i = 0; i < actors.length; i++) { if (actors[i].equals(actor)) { return true; } return false; }

28. Constructors Constructors are defined just like other methods, but with no return type or value Constructors can be overloaded, just like other methods public Movie(String title) { title = title; } public Movie(String title, String[] actors) { title = title; actors = actors; } These constructors won’t work– why not? Parameters are shadowing the fields As a class: Add two constructors: one that takes no parameters and one that takes x and y values.

29. this keyword We can access the current object by using the this keyword In a constructor, this is the object being created In any other method, this is the target of the method call public Movie(String title) { this.title = title; } public Movie(String title, String[] actors) { this.title = title; this.actors = actors; }

30. Exercise Exercise: Define a class to represent a high school course. Your class should include methods to provide the following information about the course: Course title Teacher name Room number Period Is a particular student enrolled in the course? Bonus: Does this course meet at the same time as another course? Bonus: Does this course have the same teacher as another course? Think about what variables, methods, and constructors your class will need Exercise 2: Create a client program that creates multiple instances of your class and calls the “Is a particular student enrolled in the course” method and the bonus methods, if you did those.

31. IMPLICIT PARAMETER, ARRAYS AND TOSTRING

32. The toString method tells Java how to convert an object into a String Point p1 = new Point(7, 2); System.out.println("p1: " + p1); // the above code is really calling the following: System.out.println("p1: " + p1.toString()); Every class has a toString, even if it isn't in your code. Default: class's name @ object's memory address (base 16) Point@9e8c34

33. toString syntax public String toString() { code that returns a String representing this object ; } Method name, return, and parameters must match exactly. Example: // Returns a String representing this Point. public String toString() { return "(" + x + ", " + y + ")"; }

34. The implicit parameter implicit parameter: The object on which an instance method is called. During the call p1.draw(g); the object referred to by p1 is the implicit parameter. During the call p2.draw(g); the object referred to by p2 is the implicit parameter. The instance method can refer to that object's fields. We say that it executes in the context of a particular object. draw can refer to the x and y of the object it was called on.

35. Calling another constructor public class Point { private int x; private int y; public Point() { this(0, 0); // calls (x, y) constructor } public Point(int x, int y) { this.x = x; this.y = y; }... } Avoids redundancy between constructors Only a constructor (not a method) can call another constructor

36. Summary What method does print and println call on an object to be printed? toString In the following code, what is the implicit parameter? p1.draw(g) p1 How does a constructor call another constructor? public Point() { this(0, 0); }

37. Exercise Exercise: Extend your high school course class by adding a class constructor to create a course from user input. Use multiple constructors, if appropriate and re-use code as much as possible. i.e. have the constructors call one main one, if possible. Add a toString method that outputs a reasonable reflection of a course. Add code to your client code to print out a course Your client code could create an array of courses that represent the courses that you are taking in this semester. In that case, print out all of the courses in the array. Add a field that represents the students in the class. Add a addStudent method.

38. Exercise Exercise: Add a field that represents the students in the class. Add a addStudent method.

39. ENCAPSULATION

40. Abstraction abstraction: A distancing between ideas and details. We can use objects without knowing how they work. abstraction in an iPod: You understand its external behavior (buttons, screen). You don't understand its inner details, and you don't need to.

41. Abstraction abstraction: A distancing between ideas and details. We can use objects without knowing how they work. abstraction in a Car You understand that you use the steering wheel to turn You understand that you press a pedal to go faster or to stop. When you press that pedal to go faster you are abstracted away from how that action is implemented. In a gas-powered engine, what happens? In an electric car, what happens? You don’t have to care. You just need to need to use the interface. What happens if a better way to implement the acceleration is discovered? Does the interface need to change? No! That’s an implementation detail that is abstracted away from the client.

42. Encapsulation abstraction: A distancing between ideas and details. encapsulation: Hiding implementation details from clients. We can use objects without knowing how they work. Encapsulation is a way of implementing abstraction. separates external view (behavior) from internal view (state) protects the integrity of an object's data

43. Accessing private state // A "read-only" access to the x field ("accessor") public int getX() { return x; } // Allows clients to change the x field ("mutator") public void setX(int newX) { x = newX; } Client code will look more like this: System.out.println(p1.getX()); p1.setX(14);

44. Benefits of encapsulation Abstraction between object and clients Protects object from unwanted access Example: Can't fraudulently increase an Account 's balance. Can change the class implementation later Example: Point could be rewritten in polar coordinates (r, θ) with the same methods. Can constrain objects' state (class invariants) Example: Only allow Account s with non-negative balance. Example: Only allow Date s with a month from 1-12.

45. CONSTANTS

46. Constants Consider the following method we could add to our high school course class: public boolean mysteryMethod() { return (students.length > 30); } What would you call this method?

47. Constants What if the method looked like this? public boolean mysteryMethod() { return (students.length > MAX_CLASS_SIZE); } Now what would you call it?

48. Constants The number 30 in the original method is called a magic number A number literal that has no context Magic numbers are bad for a couple reasons: Poor documentation Potential for bugs if value changes Especially if the value is used in multiple places We avoid magic numbers by defining constants A constant is a variable whose value cannot change once it has been assigned

49. Constants Are defined using the final keyword Typically named in ALL_CAPS Follow normal scope rules for variables Can be defined at the class or method level public class HighSchoolCourse {... public static final int MAX_CLASS_SIZE = 30; public boolean isFull() { return (students.length <= MAX_CLASS_SIZE); }

50. Constants Class constants are typically declared as public and static Class constants can be initialized either at the declaration or in the constructor (but not both) Class constants cannot be initialized or assigned anywhere else

51. STATIC METHODS/FIELDS

52. Static members static: Part of a class, rather than part of an object. Object classes can have static methods and fields. Not copied into each object; shared by all objects of that class. class state: private static int staticFieldA private static String staticFieldB behavior: public static void someStaticMethodC() public static void someStaticMethodD() object #1 state: int field2 double field2 behavior: public void method3() public int method4() public void method5() object #2 state: int field1 double field2 behavior: public void method3() public int method4() public void method5() object #3 state: int field1 double field2 behavior: public void method3() public int method4() public void method5()

53. Static fields private static type name ; or, private static type name = value ; Example: private static int theAnswer = 42; static field: Stored in the class instead of each object. A "shared" global field that all objects can access and modify. Like a class constant, except that its value can be changed.

54. Accessing static fields From inside the class where the field was declared: fieldName // get the value fieldName = value ; // set the value From another class (if the field is public ): ClassName. fieldName // get the value ClassName. fieldName = value ; // set the value generally static fields are not public unless they are final Math.PI

55. Static methods // the same syntax you've already used for methods public static type name ( parameters ) { statements ; } static method: Stored in a class, not in an object. Shared by all objects of the class, not replicated. Does not have any implicit parameter, this ; therefore, cannot access any particular object's fields.

56. Static Method example public class BankAccount { // static count of how many accounts are created // (only one count shared for the whole class) private static int objectCount = 0; // clients can call this to find out # accounts created public static int getNumAccounts() { return objectCount; } // fields (replicated for each object) private String name; private int id; public BankAccount() { objectCount++; // advance the id, and id = objectCount; // give number to account }... public int getID() { // return this account's id return id; }

57. Summary How is abstraction and encapsulation related? Encapsulation is a way of implementing abstraction. How do you define a class constant? public class HighSchoolCourse {... public static final int MAX_CLASS_SIZE = 30; How does an instance method differs from a static method? Static methods are shared by all objects of the class, not replicated like instance methods are. Static methods do not have any implicit parameter, this ; therefore, cannot access any particular object's fields like instance methods can.

58. ARRAYS OF OBJECTS AND NULL

59. Two-phase initialization 1) initialize the array itself (each element is initially null ) 2) initialize each element of the array to be a new object // phase 1 Course[] courses = new Course[3]; for (int i = 0; i < courses.length; i++) { // phase 2 courses[i] = new Course(“CS” + (i +1), “teach”); } courses index012 value Course Object title = “cs1” teacher= “teach” Course Object title = “cs2” teacher= “teach” Course Object title = “cs3” teacher= “teach”

60. Arrays of objects null : A value that does not refer to any object. The elements of an array of objects are initialized to null. String[] words = new String[5]; Course[] courses = new Course[3]; index01234 valuenull index012 valuenull words courses

61. Things you can do w/ null store null in a variable or an array element String s = null; words[2] = null; print a null reference System.out.println(s); // null ask whether a variable or array element is null if (words[2] == null) {... pass null as a parameter to a method System.out.println(null); // null return null from a method (often to indicate failure) return null;

62. Null pointer exception dereference: To access data or methods of an object with the dot notation, such as s.length(). It is illegal to dereference null (causes an exception). null is not any object, so it has no methods or data. String[] words = new String[5]; System.out.println("word is: " + words[0]); words[0] = words[0].toUpperCase(); // ERROR Output: word is: null Exception in thread "main" java.lang.NullPointerException at Example.main(Example.java:8) index01234 valuenull

63. Looking before you leap You can check for null before calling an object's methods. String[] words = new String[5]; words[0] = "hello"; words[2] = "goodbye";// words[1],[3],[4] are null for (int i = 0; i < words.length; i++) { if (words[i] != null) { words[i] = words[i].toUpperCase(); } index01234 value "HELLO" null "GOODBYE" null words

64. Summary What is null? A value that does not refer to any object.

65. Exercise Exercise: Add a static method that accepts a Scanner object and returns a Course object by asking the user for input using the Scanner object. Add a field that represents the students in the class. Add a MAX_CLASS_SIZE constant to your high school course class. Add an isClassFull() method. Add an addStudent() that does not allow the student to be added if the class if full. Be sure to give the caller of the method an indication of whether or not adding the student succeeded. (Printing a message to the console will not accomplish this.)

66. Review 1.Classes vs objects Create a class named Ski that has at least 1 constructor, at least 1 method, and at least one private field 2.How is abstraction and encapsulation related? Create a class named Bobsled that has 1 constructor, 1 accessor method, and 1 private field 3.How do you define a class constant? Create a class named Olympics that has 1 constructor, 1 constant that represents the maximum # of participants, and 1 method that adds a participant 4.How does an instance method differs from a static method? Create a class named SpeedSkating that has at least one constructor, one instance method, and one static method. 5.What method does print and println call on an object to be printed? Create a class named Athlete that has at least 1 constructor and a toString method that returns the details of the athlete. 6.Implicit parameter and this Create a class named CrossCountrySkiing that has two constructors. In one of the constructors use the this keyword to call the other constructor. 7.What is null ? Create a class named Event. In a client program create an array of events, populate some, but not all of the array elements, and print out the contents of the array. If an array element is null, output “null”.

67. Classes and Objects What are the 3 components of a class? Methods Fields Constructors What is difference between a class and an object? A class is a blueprint for how to create an instance of an object.

68. Abstraction, Constants, instance methods How is abstraction and encapsulation related? Encapsulation is a way of implementing abstraction. How do you define a class constant? public class HighSchoolCourse {... public static final int MAX_CLASS_SIZE = 30; How does an instance method differs from a static method? Static methods are shared by all objects of the class, not replicated like instance methods are. Static methods do not have any implicit parameter, this ; therefore, cannot access any particular object's fields like instance methods can.

69. toString, implicit parameter and null What method does print and println call on an object to be printed? toString In the following code, what is the implicit parameter? p1.draw(g) p1 What is null? A value that does not refer to any object. How does a constructor call another constructor? public Point() { this(0, 0); }