Liang, Introduction to Java Programming, Eighth Edition, (c) 2011 Pearson Education, Inc. All rights reserved Chapter 7 Objects and Classes

Liang, Introduction to Java Programming, Eighth Edition, (c) 2011 Pearson Education, Inc. All rights reserved Motivations After learning the preceding chapters, you are capable of solving many programming problems using tools such as selections, loops, methods, and arrays. However, these Java features are not sufficient for developing graphical user interfaces (GUI) and large scale software systems. Suppose you want to develop a graphical user interface as shown below. How do you program it?

Liang, Introduction to Java Programming, Eighth Edition, (c) 2011 Pearson Education, Inc. All rights reserved Objectives F To describe objects and classes, and use classes to model objects (§8.2). F To use UML graphical notations to describe classes and objects (§8.2). F To demonstrate defining classes and creating objects (§8.3). F To create objects using constructors (§8.4). F To access objects via object reference variables (§8.5). F To define a reference variable using a reference type (§8.5.1). F To access an object’s data and methods using the object member access operator (.) (§8.5.2). F To define data fields of reference types and assign default values for an object’s data fields (§8.5.3). F To distinguish between object reference variables and primitive data type variables (§8.5.4). F To use classes Date, Random, and JFrame in the Java library (§8.6). F To distinguish between instance and static variables and methods (§8.7). F To define private data fields with appropriate get and set methods (§8.8). F To encapsulate data fields to make classes easy to maintain (§8.9). F To develop methods with object arguments and differentiate between primitive- type arguments and object-type arguments (§8.10). F To store and process objects in arrays (§8.11).

Liang, Introduction to Java Programming, Eighth Edition, (c) 2011 Pearson Education, Inc. All rights reserved 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.

Liang, Introduction to Java Programming, Eighth Edition, (c) 2011 Pearson Education, Inc. All rights reserved 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. The behavior of an object also known as its actions which is define by methods. Additionally, a class provides a special type of methods, known as constructors, which are invoked to construct objects from the class.

Liang, Introduction to Java Programming, Eighth Edition, (c) 2011 Pearson Education, Inc. All rights reserved More Classes 6 The access level modifiers will determine whether other classes can use a particular field or invoke a particular method. There are two level of access control: public, or package-private //top level public, private, protected, or package-private // member level A class may be declared with the modifier public, which is visible to all classes anywhere. However, if a class doe not have modifier (public or private, or else), then, it is only visible within its own package.

Liang, Introduction to Java Programming, Eighth Edition, (c) 2011 Pearson Education, Inc. All rights reserved Objects An object has both a state and behavior. The state defines the object, and the behavior defines what the object does. The State of an object, know as properties or attributes is represent as data fields. A circle object has data fields of radius, which is the property define circle. A rectangle object has data fields width and height, which are properties and characterize a rectangle.

Liang, Introduction to Java Programming, Eighth Edition, (c) 2011 Pearson Education, Inc. All rights reserved More Objects 8 A Java class uses variable to define data fields and methods. A class provides methods of special type, know as constructors which is invoked to create a new object. A constructor can performs any actions, but they do initial action. Example : Circle3 myCircle = new Circle3 (5.0); class object constructor ( keyword new that creates new object myCirlce from Circle 3 class with the argument of 5.0)

Liang, Introduction to Java Programming, Eighth Edition, (c) 2011 Pearson Education, Inc. All rights reserved Constructors The declaration for a method or a constructor type of the arguments for that method or constructor. Constructors are a special kind of methods that are invoked to construct objects. Constructors 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 new ClassName(); Example: new Circle(); new Circle(5.0);

Liang, Introduction to Java Programming, Eighth Edition, (c) 2011 Pearson Education, Inc. All rights reserved Create Objects 10 A class provides the blueprint for objects; that you create an object from a class. Each statement below taken from CreateObjectDemo program creates an object and assigns it to a variable:CreateObjectDemo program Point originOne = new Point (2, 9); //create object Point Rectangle recOne = new rectangle (originOne, 10, 20); //create object Rectangle Rectangle rectwo = new Rectangle (50, 10); //Create object Rectangle A class provides the blueprint for objects; that you create an object from a class. Each statement below taken from CreateObjectDemo program creates an object and assigns it to a variable:CreateObjectDemo program Point originOne = new Point (2, 9); //create object Point Rectangle recOne = new rectangle (originOne, 10, 20); //create object Rectangle Rectangle rectwo = new Rectangle (50, 10); //Create object Rectangle

Liang, Introduction to Java Programming, Eighth Edition, (c) 2011 Pearson Education, Inc. All rights reserved Classes

Liang, Introduction to Java Programming, Eighth Edition, (c) 2011 Pearson Education, Inc. All rights reserved UML Class Diagram Unified Modeling Language (UML) Unified Modeling Language (UML) is an object-oriented analysis and design language from the Object Management Group (OMG). The constructor is denoted as C lassName (parameterName: parameterType) The Method is denoted as methodName(parameterName: parameterType): retrunType

Liang, Introduction to Java Programming, Eighth Edition, (c) 2011 Pearson Education, Inc. All rights reserved public class Circle3 { /** The radius of the circle */ private double radius = 1; /** The number of the objects created */ private static int numberOfObjects = 0; /** Construct a circle with radius 1 */ public Circle3() { numberOfObjects++; } public Circle3(double newRadius) {/** Construct a circle with a specified radius */ radius = newRadius; numberOfObjects++; } public double getRadius() {/** Return radius */ return radius; } public void setRadius(double newRadius) { /** Set a new radius */ radius = (newRadius >= 0) ? newRadius : 0; } /** Return numberOfObjects */ public static int getNumberOfObjects() { return numberOfObjects; } /** Return the area of this circle */ public double getArea() { return radius * radius * Math.PI; } }

Liang, Introduction to Java Programming, Eighth Edition, (c) 2011 Pearson Education, Inc. All rights reserved public class TestCircle3 { 3 /** Main method */ 4 public static void main(String[] args) { 5 // Create a Circle with radius Circle3 myCircle = new Circle3(5.0); 8 System.out.println("The area of the circle of radius " 9 + myCircle.getRadius() + " is " + myCircle.getArea()); Circle3 yourCircle = new Circle3(6.0); 13 System.out.println("The area of the yourcircle of radius " 14 + yourCircle.getRadius() + " is " + yourCircle.getArea()); Circle3 hisCircle = new Circle3(); 18 System.out.println("The area of the hiscircle of radius " 19 + hisCircle.getRadius() + " is " + hisCircle.getArea()); } 22 }

Liang, Introduction to Java Programming, Eighth Edition, (c) 2011 Pearson Education, Inc. All rights reserved Example: Defining Classes and Creating Objects  Objective: Demonstrate creating objects, accessing data, and using methods. TestCircle1 Run 1 public class TestCircle1 { 2 /** Main method */ 3 public static void main(String[] args) { 4 // Create a circle with radius Circle1 myCircle = new Circle1(5.0); // create new object 6 System.out.println("The area of the circle of radius " 7 + myCircle.radius + " is " + myCircle.getArea()); 9 // Create a circle with radius 1 10 Circle1 yourCircle = new Circle1(); // create new object 11 System.out.println("The area of the circle of radius " 12 + yourCircle.radius + " is " + yourCircle.getArea()); // Modify circle radius 15 yourCircle.radius = 100; //Changes new object's with new value of System.out.println("The area of the circle of radius " 17 + yourCircle.radius + " is " + yourCircle.getArea()); 18 } 19 } 21 // Define the circle class with two constructors 22 class Circle1 { 23 double radius; 25 /** Construct a circle with radius 1 */ 26 Circle1() { 27 radius = 1.0; 28 } /** Construct a circle with a specified radius */ 31 Circle1(double newRadius) { 32 radius = newRadius; 33 } 34 /** Return the area of this circle */ 36 double getArea() { 37 return radius * radius * Math.PI; 38 } 39 }

Liang, Introduction to Java Programming, Eighth Edition, (c) 2011 Pearson Education, Inc. All rights reserved Example: Defining Classes and Creating Objects, Accessing Data and Using Methods F public class TV { int channel = 1; // Default channel is 1 int volumeLevel = 1; // Default volume level is 1 boolean on = false; // By default TV is off public TV() { } public void turnOn() { //Turn on TV on = true; } public void turnOff() { //Turn off TV on = false; } public void setChannel(int newChannel) { // set a new channel if (on && newChannel >= 1 && newChannel = 1 && newVolumeLevel 1) channel--; } public void volumeUp() { //increase volume if (on && volumeLevel 1) volumeLevel--; } }

Liang, Introduction to Java Programming, Eighth Edition, (c) 2011 Pearson Education, Inc. All rights reserved public class TestTV { // Main method public static void main(String[] args) { TV tv1 = new TV(); // create a TV tv1.turnOn(); // turn on tv1.setChannel(30); //set a new channel tv1.setVolume(3); //set a new volume TV tv2 = new TV(); //create a TV - object tv2 - TV class tv2.turnOn(); //turn on tv2.channelUp(); // trun on tv2.channelUp(); //increase channel tv2.volumeUp(); // increase volume System.out.println("tv1's channel is " + tv1.channel //display state + " and volume level is " + tv1.volumeLevel); System.out.println("tv2's channel is " + tv2.channel + " and volume level is " + tv2.volumeLevel); } }

Liang, Introduction to Java Programming, Eighth Edition, (c) 2011 Pearson Education, Inc. All rights reserved Encapsulation 18 Encapsulation is one of the four fundamental OOP concepts. The other three are inheritance (is the capability of a class to use the properties and methods of another class ), polymorphism ( more than one form ), and abstraction ( simplifying complex reality by modeling classes ). Encapsulation can be described as a protective barrier that prevents the code and data being randomly accessed by other code defined outside the class. Access to the data and code is tightly controlled by an interface. Encapsulation provides a technique of making the fields in a class private and providing access to the fields via public methods. If a field is declared private, it cannot be accessed by anyone outside the class, thereby hiding the fields within the class. For this reason, encapsulation is also referred to as data hiding. The main benefit of encapsulation is the ability to modify our implemented code without breaking the code of others who use our code. With this feature Encapsulation gives maintainability, flexibility and extensibility to our code.

Liang, Introduction to Java Programming, Eighth Edition, (c) 2011 Pearson Education, Inc. All rights reserved Benefits of Encapsulation: The fields of a class can be made read-only or write-only. A class can have total control over what is stored in its fields. The users of a class do not know (like a blackbox) how the class stores its data. A class can change the data type of a field, and users of the class do not need to change any of their code. Link – example of EncapsulationLink – example of Encapsulation program Benefits of Encapsulation: The fields of a class can be made read-only or write-only. A class can have total control over what is stored in its fields. The users of a class do not know (like a blackbox) how the class stores its data. A class can change the data type of a field, and users of the class do not need to change any of their code. Link – example of EncapsulationLink – example of Encapsulation program

Liang, Introduction to Java Programming, Eighth Edition, (c) 2011 Pearson Education, Inc. All rights reserved Example of Data Field Encapsulation Circle3 RunTestCircle3

Liang, Introduction to Java Programming, Eighth Edition, (c) 2011 Pearson Education, Inc. All rights reserved Declaring/Creating Objects in a Single Step ClassName objectRefVar = new ClassName(); Example: Circle myCircle = new Circle(); //the variable myCirlce holds a reference to a Circle object. Create an object Assign object reference

Liang, Introduction to Java Programming, Eighth Edition, (c) 2011 Pearson Education, Inc. All rights reserved Trace Code Circle myCircle = new Circle(5.0); SCircle yourCircle = new Circle(); yourCircle.radius = 100; Declare myCircle no value myCircle animation // Circle & SCircle - class // myCircle & yourCircle– object // new – keyword that creates object from the class Circle // new Circle(5.0) - constructor // (5.0)– argument(s) or instance variable value of 5.0. //() - is null parameter, goes to default main method parameter value.

Liang, Introduction to Java Programming, Eighth Edition, (c) 2011 Pearson Education, Inc. All rights reserved Trace Code, cont. Circle myCircle = new Circle(5.0); Circle yourCircle = new Circle(); yourCircle.radius = 100; no value myCircle Create a circle animation

Liang, Introduction to Java Programming, Eighth Edition, (c) 2011 Pearson Education, Inc. All rights reserved Trace Code, cont. Circle myCircle = new Circle(5.0); Circle yourCircle = new Circle(); yourCircle.radius = 100; reference value myCircle Assign object reference to myCircle animation

Liang, Introduction to Java Programming, Eighth Edition, (c) 2011 Pearson Education, Inc. All rights reserved Trace Code, cont. Circle myCircle = new Circle(5.0); Circle yourCircle = new Circle(); yourCircle.radius = 100; reference value myCircle no value yourCircle Declare yourCircle animation

Liang, Introduction to Java Programming, Eighth Edition, (c) 2011 Pearson Education, Inc. All rights reserved Trace Code, cont. Circle myCircle = new Circle(5.0); Circle yourCircle = new Circle(); yourCircle.radius = 100; reference value myCircle no value yourCircle Create a new Circle object animation

Liang, Introduction to Java Programming, Eighth Edition, (c) 2011 Pearson Education, Inc. All rights reserved Trace Code, cont. Circle myCircle = new Circle(5.0); Circle yourCircle = new Circle(); yourCircle.radius = 100; reference value myCircle reference value yourCircle Assign object reference to yourCircle animation

Liang, Introduction to Java Programming, Eighth Edition, (c) 2011 Pearson Education, Inc. All rights reserved Trace Code, cont. Circle myCircle = new Circle(5.0); Circle yourCircle = new Circle(); yourCircle.radius = 100; reference value myCircle reference value yourCircle Change radius in yourCircle animation

Liang, Introduction to Java Programming, Eighth Edition, (c) 2011 Pearson Education, Inc. All rights reserved 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' } If a data field of a reference type does not reference any object, the data field holds a special Java value, null. Null is a literal just like true and false, Boolean type, null is a literal for a reference type.

Liang, Introduction to Java Programming, Eighth Edition, (c) 2011 Pearson Education, Inc. All rights reserved 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. public class Test { public static void main(String[] args) { Student student = new Student(); /* default values of data fields name, aga, isScienceMajor, and gender for a Student object.*/ System.out.println("name? " + student.name); System.out.println("age? " + student.age); System.out.println("isScienceMajor? " + student.isScienceMajor); System.out.println("gender? " + student.gender); }

Liang, Introduction to Java Programming, Eighth Edition, (c) 2011 Pearson Education, Inc. All rights reserved Differences between Variables of Primitive Data Types and Object Types

Liang, Introduction to Java Programming, Eighth Edition, (c) 2011 Pearson Education, Inc. All rights reserved Copying Variables of Primitive Data Types and Object Types

Liang, Introduction to Java Programming, Eighth Edition, (c) 2011 Pearson Education, Inc. All rights reserved 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.

Liang, Introduction to Java Programming, Eighth Edition, (c) 2011 Pearson Education, Inc. All rights reserved 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.

Liang, Introduction to Java Programming, Eighth Edition, (c) 2011 Pearson Education, Inc. All rights reserved The Date Class Java provides a system-independent encapsulation of date and time in the java.util.Date class. You can use the Date class to create an instance for the current date and time and use its toString method to return the date and time as a string.

Liang, Introduction to Java Programming, Eighth Edition, (c) 2011 Pearson Education, Inc. All rights reserved The Date Class Example For example, the following code java.util.Date date = new java.util.Date(); System.out.println(date.toString()); // date is an object for the class library of java.util.Date displays a string like Sun Mar 09 13:50:19 EST 2003.

Liang, Introduction to Java Programming, Eighth Edition, (c) 2011 Pearson Education, Inc. All rights reserved The Random Class You have used Math.random() to obtain a random double value between 0.0 and 1.0 (excluding 1.0). A more useful random number generator is provided in the java.util.Random class.

Liang, Introduction to Java Programming, Eighth Edition, (c) 2011 Pearson Education, Inc. All rights reserved The Random Class Example If two Random objects have the same seed, they will generate identical sequences of numbers. For example, the following code creates two Random objects with the same seed 3. Random random1 = new Random(3); System.out.print("From random1: "); for (int i = 0; i < 10; i++) System.out.print(random1.nextInt(1000) + " "); Random random2 = new Random(3); System.out.print("\nFrom random2: "); for (int i = 0; i < 10; i++) System.out.print(random2.nextInt(1000) + " "); From random1: From random2:

Liang, Introduction to Java Programming, Eighth Edition, (c) 2011 Pearson Education, Inc. All rights reserved Displaying GUI Components When you develop programs to create graphical user interfaces, you will use Java classes such as JFrame, JButton, JRadioButton, JComboBox, and JList to create frames, buttons, radio buttons, combo boxes, lists, and so on. Here is an example that creates two windows using the JFrame class. TestFrame Run

Liang, Introduction to Java Programming, Eighth Edition, (c) 2011 Pearson Education, Inc. All rights reserved Trace Code JFrame frame1 = new JFrame(); frame1.setTitle("Window 1"); frame1.setSize(200, 150); frame1.setVisible(true); JFrame frame2 = new JFrame(); frame2.setTitle("Window 2"); frame2.setSize(200, 150); frame2.setVisible(true); Declare, create, and assign in one statement reference frame1 : JFrame title: width: height: visible: animation frame1 & frame2 – object created from Jframe class setTtile, setSize, setVisible, setTitle – methods that set the properties of the objects.

Liang, Introduction to Java Programming, Eighth Edition, (c) 2011 Pearson Education, Inc. All rights reserved Trace Code JFrame frame1 = new JFrame(); frame1.setTitle("Window 1"); frame1.setSize(200, 150); frame1.setVisible(true); JFrame frame2 = new JFrame(); frame2.setTitle("Window 2"); frame2.setSize(200, 150); frame2.setVisible(true); reference frame1 : JFrame title: "Window 1" width: height: visible: Set title property animation frame1.setTtile(“Window1”); // setTitle method sets a title for Window 1

Liang, Introduction to Java Programming, Eighth Edition, (c) 2011 Pearson Education, Inc. All rights reserved Trace Code JFrame frame1 = new JFrame(); frame1.setTitle("Window 1"); frame1.setSize(200, 150); frame1.setVisible(true); JFrame frame2 = new JFrame(); frame2.setTitle("Window 2"); frame2.setSize(200, 150); frame2.setVisible(true); reference frame1 : JFrame title: "Window 1" width: 200 height: 150 visible: Set size property animation //setSize method defined the size of 200 & 150 (width, height).

Liang, Introduction to Java Programming, Eighth Edition, (c) 2011 Pearson Education, Inc. All rights reserved Trace Code JFrame frame1 = new JFrame(); frame1.setTitle("Window 1"); frame1.setSize(200, 150); frame1.setVisible(true); JFrame frame2 = new JFrame(); frame2.setTitle("Window 2"); frame2.setSize(200, 150); frame2.setVisible(true); reference frame1 : JFrame title: "Window 1" width: 200 height: 150 visible: true Set visible property animation //setVisible method displays the window.

Liang, Introduction to Java Programming, Eighth Edition, (c) 2011 Pearson Education, Inc. All rights reserved Trace Code JFrame frame1 = new JFrame(); frame1.setTitle("Window 1"); frame1.setSize(200, 150); frame1.setVisible(true); JFrame frame2 = new JFrame(); frame2.setTitle("Window 2"); frame2.setSize(200, 150); frame2.setVisible(true); reference frame1 : JFrame title: "Window 1" width: 200 height: 150 visible: true Declare, create, and assign in one statement reference frame2 : JFrame title: width: height: visible: animation frame2 – object created from Jframe class with another sets of methods; setTitle, setSize, setVisible

Liang, Introduction to Java Programming, Eighth Edition, (c) 2011 Pearson Education, Inc. All rights reserved Trace Code JFrame frame1 = new JFrame(); frame1.setTitle("Window 1"); frame1.setSize(200, 150); frame1.setVisible(true); JFrame frame2 = new JFrame(); frame2.setTitle("Window 2"); frame2.setSize(200, 150); frame2.setVisible(true); reference frame1 : JFrame title: "Window 1" width: 200 height: 150 visible: true reference frame2 : JFrame title: "Window 2" width: height: visible: Set title property animation

Liang, Introduction to Java Programming, Eighth Edition, (c) 2011 Pearson Education, Inc. All rights reserved Trace Code JFrame frame1 = new JFrame(); frame1.setTitle("Window 1"); frame1.setSize(200, 150); frame1.setVisible(true); JFrame frame2 = new JFrame(); frame2.setTitle("Window 2"); frame2.setSize(200, 150); frame2.setVisible(true); reference frame1 : JFrame title: "Window 1" width: 200 height: 150 visible: true reference frame2 : JFrame title: "Window 2" width: 200 height: 150 visible: Set size property animation

Liang, Introduction to Java Programming, Eighth Edition, (c) 2011 Pearson Education, Inc. All rights reserved Trace Code JFrame frame1 = new JFrame(); frame1.setTitle("Window 1"); frame1.setSize(200, 150); frame1.setVisible(true); JFrame frame2 = new JFrame(); frame2.setTitle("Window 2"); frame2.setSize(200, 150); frame2.setVisible(true); reference frame1 : JFrame title: "Window 1" width: 200 height: 150 visible: true reference frame2 : JFrame title: "Window 2" width: 200 height: 150 visible: true Set visible property animation

Liang, Introduction to Java Programming, Eighth Edition, (c) 2011 Pearson Education, Inc. All rights reserved Adding GUI Components to Window You can add graphical user interface components, such as buttons, labels, text fields, combo boxes, lists, and menus, to the window. The components are defined using classes. Here is an example to create buttons, labels, text fields, check boxes, radio buttons, and combo boxes. GUIComponents Run