Chapter 2.pdf

Ahmad Khoj King Abdulaziz University KING ABDULAZIZ UNIVERSITY ‫جامعة الملك عبد العزيز‬ KING ABDULAZIZ UNIVERSITY Chapter 2 Object Oriented programming concepts KING ABDULAZIZ UNIVERSITY ‫جامعة الملك عبد العزيز‬ ...

Ahmad Khoj King Abdulaziz University KING ABDULAZIZ UNIVERSITY ‫جامعة الملك عبد العزيز‬ KING ABDULAZIZ UNIVERSITY Chapter 2 Object Oriented programming concepts KING ABDULAZIZ UNIVERSITY ‫جامعة الملك عبد العزيز‬  OO Programming Concepts  Object-oriented programming (OOP) involves programming using objects.  An object represents an entity in the real world that can be distinctly identified.  For example, 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 (also known as properties) with their current values.  The behavior of an object is defined by a set of methods. 3 KING ABDULAZIZ UNIVERSITY ‫جامعة الملك عبد العزيز‬  Objects Class Name: Person A class template Data Fields: name is _______ Methods: getName Person O1 Person O2 Person O3 Three objects of the Person class Data Fields: Data Fields: Data Fields: name is Ahmad name is Zaki name is Rashad  An object has both a state and behavior.  The state defines the object.  The behavior defines what the object does. 4 KING ABDULAZIZ UNIVERSITY ‫جامعة الملك عبد العزيز‬  Classes  Classes are constructs that define objects of the same type.  A Java class uses:  variables to define data fields.  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. 5 KING ABDULAZIZ UNIVERSITY ‫جامعة الملك عبد العزيز‬  Classes class Person { String name ; Data field Person() { } Constructors Person(String n) { name = n; } double getName() { Method return name; } } 6 KING ABDULAZIZ UNIVERSITY ‫جامعة الملك عبد العزيز‬  UML Class Diagram UML Class Diagram Person Class name name: String Data fields Person() Constructors and Person(n: String) Methods getName(): String O2: Person O3: Person UML notation O1: Person for objects name: Ahmad name: Zaki name: Rashad 7 KING ABDULAZIZ UNIVERSITY ‫جامعة الملك عبد العزيز‬  Declaring Object Reference Variables  To reference an object, assign the object to a reference variable.  To declare a reference variable, use the syntax: ClassName objectRefVar; Example: Person o1; 8 KING ABDULAZIZ UNIVERSITY ‫جامعة الملك عبد العزيز‬  Constructors  Constructors are a special kind of methods that are invoked to construct objects. Person() { } Person(String n) { name = n; } KING ABDULAZIZ UNIVERSITY ‫جامعة الملك عبد العزيز‬ 9  Constructors, cont.  Constructors play the role of initializing objects.  A constructor with no parameters is referred to as a no-arg constructor.  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. 10 KING ABDULAZIZ UNIVERSITY ‫جامعة الملك عبد العزيز‬  Creating Objects Using Constructors new ClassName(); Example: new Person(); new Person(“Ahmad Khoj”); 11 KING ABDULAZIZ UNIVERSITY ‫جامعة الملك عبد العزيز‬  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. 12 KING ABDULAZIZ UNIVERSITY ‫جامعة الملك عبد العزيز‬  Declaring/Creating Objects in a Single Step ClassName objectRefVar = new ClassName(); Assign object reference Create an object Example: Person O1 = new Person(); 13 KING ABDULAZIZ UNIVERSITY ‫جامعة الملك عبد العزيز‬  Accessing Objects  Referencing the object’s data: objectRefVar.data e.g. o1.name  Invoking the object’s method: objectRefVar.methodName(arguments) e.g. o1.getName() 14 KING ABDULAZIZ UNIVERSITY ‫جامعة الملك عبد العزيز‬  Instance Variables, and Methods  Instance variables belong to a specific instance.  Instance methods are invoked by an instance of the class. 15 KING ABDULAZIZ UNIVERSITY ‫جامعة الملك عبد العزيز‬  Trace Code Declare o1 Person o1 = new Person (“Ahmad”); no value o1 Person o2 = new Person(); o2.name = “Zaki”; 16 KING ABDULAZIZ UNIVERSITY ‫جامعة الملك عبد العزيز‬ animation Trace Code, cont. Person o1 = new Person (“Ahmad”); no value O1 Person o2 = new Person(); o2.name = “Zaki”; : Person name: Ahmad Create a new Person object 17 KING ABDULAZIZ UNIVERSITY ‫جامعة الملك عبد العزيز‬ animation Trace Code, cont. Person o1 = new Person (“Ahmad”); reference value O1 Person o2 = new Person(); Assign object reference o2.name = “Zaki”; to o1 : Person name: Ahmad 18 KING ABDULAZIZ UNIVERSITY ‫جامعة الملك عبد العزيز‬ animation Trace Code, cont. o1 reference value Person o1 = new Person (“Ahmad”); : Person Person o2 = new Person(); name: Ahmad o2.name = “Zaki”; o2 no value Declare o2 19 KING ABDULAZIZ UNIVERSITY ‫جامعة الملك عبد العزيز‬ animation Trace Code, cont. o1 reference value Person o1 = new Person (“Ahmad”); : Person Person o2 = new Person(); name: Ahmad o2.name = “Zaki”; o2 no value : Person Create a new name: null Person object 20 KING ABDULAZIZ UNIVERSITY ‫جامعة الملك عبد العزيز‬ animation Trace Code, cont. o1 reference value Person o1 = new Person (“Ahmad”); : Person Person o2 = new Person(); name: Ahmad o2.name = “Zaki”; o2 reference value Assign object reference to o2 : Person name: null 21 KING ABDULAZIZ UNIVERSITY ‫جامعة الملك عبد العزيز‬ animation Trace Code, cont. o1 reference value Person o1 = new Person (“Ahmad”); : Person Person o2 = new Person(); name: Ahmad o2.name = “Zaki”; o2 reference value : Person Change name in name: Zaki o2 22 KING ABDULAZIZ UNIVERSITY ‫جامعة الملك عبد العزيز‬  Differences between Variables of Primitive Data Types and Object Types Created using new Circle() Primitive type int i = 1 i 1 Object type Circle c c reference c: Circle radius = 1 23 KING ABDULAZIZ UNIVERSITY ‫جامعة الملك عبد العزيز‬  Copying Variables of Primitive Data Types and Object Types Primitive type assignment i = j Before: After: i 1 i 2 j 2 j 2 Object type assignment c1 = c2 Before: After: c1 c1 c2 c2 c1: Circle C2: Circle c1: Circle C2: Circle radius = 5 radius = 9 radius = 5 radius = 9 24 KING ABDULAZIZ UNIVERSITY ‫جامعة الملك عبد العزيز‬  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. 25 KING ABDULAZIZ UNIVERSITY ‫جامعة الملك عبد العزيز‬  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. 26 KING ABDULAZIZ UNIVERSITY ‫جامعة الملك عبد العزيز‬  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. 27 KING ABDULAZIZ UNIVERSITY ‫جامعة الملك عبد العزيز‬ package p1; package p2; public class C1 { public class C2 { public class C3 { public int x; void aMethod() { void aMethod() { int y; C1 o = new C1(); C1 o = new C1(); private int z; can access o.x; can access o.x; can access o.y; cannot access o.y; public void m1() { cannot access o.z; cannot access o.z; } void m2() { can invoke o.m1(); can invoke o.m1(); } can invoke o.m2(); cannot invoke o.m2(); private void m3() { cannot invoke o.m3(); cannot invoke o.m3(); } } } } } } package p1; package p2; class C1 { public class C2 { public class C3 {... can access C1 cannot access C1; } } can access C2; }  The private modifier restricts access to within a class.  The default modifier restricts access to within a package.  The public modifier enables unrestricted access. 28 KING ABDULAZIZ UNIVERSITY ‫جامعة الملك عبد العزيز‬  NOTE An object cannot access its private members, as shown in (b). It is OK, however, if the object is declared in its own class, as shown in (a). public class Foo { public class Test { private boolean x; public static void main(String[] args) { Foo foo = new Foo(); public static void main(String[] args) { System.out.println(foo.x); Foo foo = new Foo(); System.out.println(foo.convert(foo.x)); System.out.println(foo.x); } System.out.println(foo.convert()); } } private int convert(boolean b) { return x ? 1 : -1; } } (a) This is OK because object foo is used inside the Foo class (b) This is wrong because x and convert are private in Foo. 29 KING ABDULAZIZ UNIVERSITY ‫جامعة الملك عبد العزيز‬  Why Data Fields Should Be private? 1. To protect data. 2. To make class easy to maintain. 30 KING ABDULAZIZ UNIVERSITY ‫جامعة الملك عبد العزيز‬  Static Variables, Constants, and Methods  Static variables are shared by all the instances of the class.  Static constants are final variables shared by all the instances of the class.  Static methods are not tied to a specific object.  To declare static variables, constants, and methods, use the static modifier. 31 KING ABDULAZIZ UNIVERSITY ‫جامعة الملك عبد العزيز‬  Static Variables, Constants, and Methods instantiate Memory circle1 radius = 1 1 radius Circle numberOfObjects = 2 radius: double numberOfObjects: int 2 numberOfObjects getNumberOfObjects(): int +getArea(): double instantiate circle2 radius = 5 5 radius UML Notation: numberOfObjects = 2 underline: static variables or methods 32 KING ABDULAZIZ UNIVERSITY ‫جامعة الملك عبد العزيز‬ Static Variables, Constants, and Methods 33 KING ABDULAZIZ UNIVERSITY ‫جامعة الملك عبد العزيز‬  Scope of Variables  The scope of a local variable starts from its declaration and continues to the end of the block that contains the variable.  A local variable must be initialized explicitly before it can be used.  The scope of instance and static variables is the entire class.  They can be declared anywhere inside a class. 34 KING ABDULAZIZ UNIVERSITY ‫جامعة الملك عبد العزيز‬  The this Keyword  Use this to refer to an instance data field.  Use this to invoke an overloaded constructor of the same class.  Use this to refer to the object that invokes the instance method. 35 KING ABDULAZIZ UNIVERSITY ‫جامعة الملك عبد العزيز‬  Calling Overloaded Constructor public class Circle { private double radius; public Circle(double radius) { this.radius = radius; } this must be explicitly used to reference the data field radius of the object being constructed public Circle() { this(1.0); } this is used to invoke another constructor public double getArea() { return this.radius * this.radius * Math.PI; } } Every instance variable belongs to an instance represented by this, which is normally omitted 36 KING ABDULAZIZ UNIVERSITY ‫جامعة الملك عبد العزيز‬  Array of Objects  An array of objects is actually an array of reference variables.  e.g. circleArray references to the entire array. Circle[] circleArray = new Circle; circleArray reference circleArray Circle object 0 circleArray … Circle object 1 circleArray Circle object 9 37 KING ABDULAZIZ UNIVERSITY ‫جامعة الملك عبد العزيز‬  Array of Objects, cont.  circleArray references to a Circle object.  So invoking circleArray.getArea() involves two levels of referencing as shown in the previous figure. 38 KING ABDULAZIZ UNIVERSITY ‫جامعة الملك عبد العزيز‬ Array of Objects Example Output: Ahmad Zaki Rashad 39 KING ABDULAZIZ UNIVERSITY ‫جامعة الملك عبد العزيز‬  UML Example 40 KING ABDULAZIZ UNIVERSITY ‫جامعة الملك عبد العزيز‬  UML Example, cont. Output: Ahmad 31 true Ahmad Khoj 60 false 41 KING ABDULAZIZ UNIVERSITY ‫جامعة الملك عبد العزيز‬  Exercise Build a java class called “Bill” contain a method “getTax” that calculate tax according to the following formula: Tax = invoice * 0.05 Create a suitable object to acquire the required result. 42 KING ABDULAZIZ UNIVERSITY ‫جامعة الملك عبد العزيز‬