1. 1 COS240 O-O Languages AUBG, COS dept Lecture 12 Title: Java Classes and Objects Reference: COS240 Syllabus

2. 2 Lecture Contents: F Defining Classes for Objects F Constructing Objects using Constructors F Accessing Objects via Reference Variables F Visibility Modifiers, Accessors, Mutators F Data Field Encapsulation F Immutable Objects and Classes F Static Variables, Constants and Methods F The Scope of Variables  The this keyword F Class Abstraction and Encapsulation F Inner Classes

3. 3 Motivations After learning the preceding lectures, you are capable of solving problems using Java as conventional Prog Lang incl. selections, loops, methods, and arrays. However, these Java features are not sufficient for developing graphical user interfaces and large scale software systems. Suppose you want to develop a graphical user interface as shown below. How do you program it?

4. 4 OO Programming Concepts O-OP involves programming using objects. An object represents an entity in the world that can be distinctly identified like a student, a desk, a circle, a button, and even a loan can all be viewed as objects. An object has a unique identity - state and behaviors. The state of an object consists of a set of data fields (known as properties) with their current values. The behavior of an object is defined by a set of methods. Invoking a method is like sending a message to ask an object to perform a task.

5. 5 Defining Classes for Objects An object has both a state and behavior. The state defines the object. The behavior defines what the object does. Example: A Circle object has a data field, i.e. radius which is the property to characterize a circle. One behavior of a circle is that its area can be computed using the method getArea()

6. 6 Objects An object has both a state and behavior. The state defines the object. The behavior defines what the object does.

7. 7 Classes Classes are constructs that define objects of the same type. A Java class uses variables to define data fields and methods to define behaviors. Additionally, a class provides a special type of methods, known as constructors, which are invoked to construct objects from the class.

8. 8 Classes

9. 9 UML Class Diagram

10. 10 Constructors Circle() { } //---------------------------- Circle(double newRadius) { radius = newRadius; } Constructors are a special kind of methods that are invoked to construct objects. A constructor with no parameters is referred to as a no-arg constructor.

11. 11 Constructors Circle() { } //---------------------------- Circle(double radius) { this.radius = radius; } Constructors are a special kind of methods that are invoked to construct objects. A constructor with no parameters is referred to as a no-arg constructor.

12. 12 Constructor-method with three differences · Constructors must have the same name as the class itself. · Constructors do not have a return type—not even void. · Constructors are invoked using the new operator when an object is created. Constructors play the role of initializing objects.

13. 13 Creating Objects Using Constructors Circle class differs from other classes already discussed: It does not have a main() method and therefore cannot be run. It is a definition used to declare and create Circle objects. HOW? Using the new operator. See next slide

14. 14 Creating Objects Using Constructors Syntax: new ClassName(); Example: To create anonymous objects new Circle() new Circle(5.0)

15. 15 Default Constructor A class may be declared without constructors. In this case, a no-arg constructor with an empty body is implicitly declared in the class. This constructor, called a default constructor, is provided automatically only if no constructors are explicitly declared in the class.

16. 16 Declaring Object Reference Variables To reference an object, assign the object (being created using new operator) to a reference variable. To declare a reference variable, use the syntax: ClassName objectRefVar; Example: Circle myCircle;

17. 17 Declaring/Creating Objects in a Single Step ClassName objectRefVar = new ClassName(); Example: Circle myCircle = new Circle(); Create an object Assign object reference

18. 18 Accessing Objects F Referencing the object’s data: objectRefVar.data e.g., myCircle.radius F Invoking the object’s method: objectRefVar.methodName(arguments) e.g., myCircle.getArea()

19. 19 Practical problem F Write a Java program that constructs an object with radius 5 and an object with radius 10 and display the radius and area of each of the two circles. F Change the radius of the second object to 100 and display the new radius and area. F Add a new method to return the circumference of the circle. F Hints: read next slide

20. 20 Hints F Version 1: –One.java file, two classes: u Circle with methods two Circle constructors and method getArea() u TestCircle with methods main() –See attached file Circle2.java F Version 2: –One.java source file, one class Circle with methods main(), two Circle constructors and method getArea(). –See attached file Circle1.java

21. 21 Hints F When a program contains more than 1 class, how to proceed?  A program contains two classes. The first is TestCircle, is the main class. Its sole purpose is to test the second class Circle. Every time you run the program, JRE invokes the main() method in the main class. F You can put the two or more classes into one file, but only one class in the file can be a public class. Furthermore, the public class must have the same name as the file name.  The main class contains the main() method that creates objects of the second class. F There are many ways to write Java programs. For instance, you can combine two classes into one

22. 22 Caution Recall that you use Math.methodName(arguments) (e.g., Math.pow(3, 2.5)) to invoke a method in the Math class. Can you invoke getArea() using Circle.getArea()? The answer is no. All the methods used before this chapter are static methods, which are defined using the static keyword. However, getArea() is non-static. It must be invoked from an object using objectRefVar.methodName(arguments) (e.g., myCircle.getArea()). More explanations will be given in the section on “Static Variables, Constants, and Methods.”

23. 23 Reference Data Fields The data fields can be of reference types. For example, the following Student class contains a data field name of the String type. public class Student { String name; // name has default value null int age; // age has default value 0 boolean isScienceMajor; // isScienceMajor has default value false char gender; // c has default value '\u0000' }

24. 24 The null Value If a data field of a reference type does not reference any object, the data field holds a special literal value, null.

25. 25 Default Value for a Data Field The default value of a data field is null for a reference type, 0 for a numeric type, false for a boolean type, and '\u0000' for a char type. However, Java assigns no default value to a local variable inside a method (see next slide). public class Test { public static void main(String[] args) { Student student = new Student(); System.out.println("name? " + student.name); System.out.println("age? " + student.age); System.out.println("isScienceMajor? " + student.isScienceMajor); System.out.println("gender? " + student.gender); }

26. 26 Example public class Test { public static void main(String[] args) { int x; // x has no default value String y; // y has no default value System.out.println("x is " + x); System.out.println("y is " + y); } Compilation error: variables not initialized Java assigns no default value to a local variable inside a method.

27. 27 Differences between Variables of Primitive Data Types and Object Types int i = 1; Circle c = new Circle(1);

28. 28 Copying Variables of Primitive Data Types and Object Types

29. 29 Garbage Collection As shown in the previous figure, after the assignment statement c1 = c2, c1 points to the same object referenced by c2. The object previously referenced by c1 is no longer referenced. This object is known as garbage. Garbage is automatically collected by JVM.

30. 30 Garbage Collection, cont TIP: If you know that an object is no longer needed, you can explicitly assign null to a reference variable for the object. The JVM will automatically collect the space if the object is not referenced by any variable.

31. 31 Instance Variables and Instance Methods Instance variables belong to a specific instance. Instance methods are invoked by an instance of the class.

32. 32 Static Variables, Constants, and Methods Static variables are shared by all the instances of the class. Static methods are not tied to a specific object. Static constants are final variables shared by all the instances of the class. To declare static variables, constants, and methods, use the static modifier.

33. 33 Static Variables, Constants, and Methods, cont.

34. 34 Example of Using Instance and Class Variables and Method Objective: Demonstrate the roles of instance and class variables and their uses. Task: Modify the Circle program by adding a class variable numberOfObjects to track the number of Circle objects created.

35. 35 Visibility modifiers private : private members are accessible only in the class itself package : package members are accessible in classes in the same package and the class itself (by default) protected : protected members are accessible in classes in the same package, in subclasses of the class, and in the class itself public : public members are accessible anywhere the class is accessible

36. 36 public class Pencil { public String color = “red”; public int length; public float diameter; private float price; public static long nextID = 0; public void setPrice (float newPrice) { price = newPrice; } public class CreatePencil { public static void main (String args[]){ Pencil p1 = new Pencil(); p1.price = 0.5; // illegal } Pencil CreatePencil

37. 37 Visibility Modifiers and Accessor/Mutator Methods By default, the class, variable, or method can be accessed by any class in the same package.  public The class, data, or method is visible to any class in any package.  private The data or methods can be accessed only by the declaring class. The get and set methods are used to read and modify private properties.

38. 38 Accessor/Mutator Methods F Coloquially, –a get method is referred to as a getter (or accessor) –a set method is referred to as a setter (or mutator) F A get method has following signature: – public returntype getPropertyName() F If return type is boolean, get method is like: – public boolean isPropertyName() F A set method has following signature: – public void setPropertyName(dataType propertyValue)

39. 39 Accessor/Mutator Methods F See file ModernObjectProg1.java F Modify the Circle class with get /accessor/ method and set /mutator or mutators/ method(s)

40. 40 The private modifier restricts access to within a class, the default modifier restricts access to within a package, and the public modifier enables unrestricted access.

41. 41 NOTE An object cannot access its private members, as shown in (b). It is OK, if the object is declared in its own class, as shown in (a).

42. 42 Why Data Fields Should Be private? To protect data. To make class easy to maintain.

43. 43 Example of Data Field Encapsulation

44. 44 Immutable Objects and Classes F If the contents of an object cannot be changed once the object is created, the object s called an immutable object and the class is called an immutable class.  If you delete the set method(s) of the Circle class and the radius data field is qualified private, the class would be immutable.

45. 45 Passing Objects to Methods F Passing by value for primitive type value (the value is passed to the parameter) F Passing by value for reference type value (the value is the reference to the object)

46. 46 Passing Objects to Methods, cont.

47. 47 The this Reference  Need to reference a class’s hidden variable in a method. A property name is used as a parameter in a set method for the property F Hidden static variable is accessed using class name reference  Hidden instance variable is accessed using keyword this class Foo { int i= 5; static double k=0; void setI(int i) { this.i = i; } void setK(double k) { Foo.k = k; }

48. 48 The this Reference  Keyword this can also use inside a constructor to invoke another constructor of the same class. public class Circle { private double radius; public Circle (double radius) { this.radius = radius; } public Circle() { this(1.0); } public getArea() { return this.radius * this.radius * Math.PI; } // this(1.0) has to appear first in the constructor before any other statements.

49. 49 Class Abstraction F Java provides many levels of abstraction. F Class abstraction is the separation of class implementation from the use of a class.

50. 50 Array of Objects Circle[] circleArray = new Circle[10]; An array of objects is actually an array of reference variables. So invoking circleArray[1].getArea() involves two levels of referencing as shown in the next figure. circleArray references to the entire array. circleArray[1] references to a Circle object.

51. 51 Array of Objects, cont. Circle[] circleArray = new Circle[10];

52. 52 Practical session Problem: Write a program to develop ADT Counter class: Data field: int m_count Methods: no-arg constructor 1-arg constructor void incCount() get method /accessor/ set method /mutator/

53. 53 Practical session F // One class combines/includes both: F // ADT - user defined class Counter F // + F // tester class functionality as method Main(....) F //

54. 54 Practical session IDE NetBeans: Project name: Counter1 – user specified IDE generates: package name counter1 class name Counter1 Class includes ADT functionality + tester method Main

55. 55 Practical session F // Two separate classes, same package: F // one for ADT - user defined class Counter F // F // one tester class as method Main(....) F //

56. 56 Practical session IDE NetBeans: Project name: Counter2 – user specified IDE generates: package name counter2 class name Counter2 (main class To add new class: File > New File > Choose category: Java, File type : Java class Name: Counter

57. 57 Thank You for Your attention!