Chapter 3 Writing classes

Writing Classes Writing Classes Chapter 3 Chapter 3 1 Out lin e • Introduction to Object-Oriented Programming (OOP) • Writing a Java class • Encapsulation and accessor methods 2 Procedural Programming Procedural Programming and and Object-Oriented Programming Object-Oriented Programming  Procedural Programming consists of designing a set of functions (or algorithms) to solve a problem • Algorithms + Data Structures = Programs  Object-Oriented Programming (OOP) puts data first, then looks at the algorithms that operate on the data 3 Concepts in OOP Concepts in OOP  Class • A class is the model, pattern, or blueprint from which objects are actually made • A class means a category of things • A class can be used in Java as the data type  Object • Object means a particular item that belongs to a class • Object also called an “instance”  Example: • String s1 = "Hello"; Here, String is the class, and the variable s1 is objects (or “instances of the String class”) 4 Concepts in OOP Concepts in OOP (cont.) (cont.)  Attributes • The properties that describe an object  Behaviors • Activities of an object  Each object has a set of attributes (state) and a set of behaviors 5  2003 Prentice Hall, Inc. All rights reserved. Example: Class and Object Class: Ball Attributes: radius, color Behavior: thrown, bounced, rolled Object1 radius = 5 color = red thrown, bounced, rolled Object3 radius = 3 color = green thrown, bounced, rolled Object2 radius = 10 color = blue thrown, bounced, rolled 6 Examples Examples  List attributes and behaviors of students • Student code, name, gender, birthday, birthplace, class, grade 1, grade 2,… • Calculate average grade, view grade, pay tuition,…  Suppose that you write a program to calculate area and perimeter of a rectangle. Find objects and list attributes and behaviors of them • Object: Rectangle • Attributes: width, height • Behaviors: Calculate area, calculator perimeter 7 Examples Examples (cont.) (cont.)  Suppose that you want to write a program to monthly pay the employees of a company. Paying employees based on products they made, in addition, you have to compute Social Security and Medicare taxes. Find objects and list attributes and behaviors of them • Object: Employee • Attributes: employee number, name, address, social security number, products,… • Behaviors: compute employee pay, compute employee Social Security and Medicare taxes, mail employee a paycheck once a month,… 8 Objects model Objects model ClassName - Attributes + Behaviors Properties Data Fields Variables Instance data Methods 9 Objects model: Objects model: Example Example Rectangle - width - height + Calculate area + Calculator perimeter Student - Student code - Name - Gender - Birthday - Birthplace - Class - Grade 1 - Grade 2 + Calculate average + View grade + Pay tuition 10 [...]...Outline • Introduction to Object-Oriented Programming (OOP) • Writing a Java class • Encapsulation and accessor methods 11 Writing a Java Class class ClassName { property1 property2 constructor1 constructor2 method1 method2 } 12 Writing class Rectangle Rectangle - width: double - height: double class ClassName { property1 class Rectangle { double... the names of your classes and methods reflect their responsibilities 30 Questions  Syntax to create a class in Java?  What is constructor? How many constructors can we have in a class?  What purpose of using this keyword?  Why to make all instance variables private?  How to access private instance variables?  Write a class to calculate area of a circle with radius Test this class 31 Read more ... constructor can invoke other constructor, by using this(agrs), note the this statement must appear as the first line of code in the constructor (otherwise, a compiler error will occur) 21 Example: Writing class Rectangle (3) Using this Rectangle.java public class Rectangle { double width, height; public Rectangle( double width, double height ) { this.width = width; this.height = height; } public Rectangle()... peri = r.getPerimeter(); System.out.println("Area is "+ area); System.out.println("Perimeter is "+ peri); } } 22 Outline • Introduction to Object-Oriented Programming (OOP) • Writing a Java class • Encapsulation and accessor methods 23 Encapsulation  Encapsulation is the technique of making the fields in a class private and providing access to the fields via public methods Client (user) Methods Data ... double + getPerimeter() : double property2 public double getArea() { return width*height; } constructor1 constructor2 public double getPerimeter() { return (width+height)*2; } method1 method2 } } 13 Writing class Student Student class Student{ - studentCode: String String studentCode, name, birthplace, classID; class NameOfClass { - name: String attribute1 boolean gender; attribute2 Date birthday;... fields would be 0, all booleans would be false, and all object variables would be set to null  If you have your own constructor, you should provide a default constructor, it can do nothing 19 Example: Writing class Rectangle (2) Class with Constructors Rectangle.java public class Rectangle { double width, height; public Rectangle( double x, double y ) { width = x; height = y; } public double getArea()... client  service methods • private method: cannot be invoked from outside the class The only purpose of a private method is to help the other methods of the class do their job  support methods 25 Example: Writing class Rectangle (4) Write accessor methods Rectangle.java public class Rectangle { private double width, height; public Rectangle (double width, double height) { this.width = width; this.height... return height; } public void setHeight(double height) { this.height = height; } public double getArea() { return width * height; } public double getPerimeter() { return (width+height) * 2; } } 26 Example: Writing class Rectangle (4) Write accessor methods RectangleTest.java public class RectangleTest { public static void main( String[] args ) { Rectangle rect = new Rectangle(); rect.setWidth(10); rect.setHeight(5);... shipName = …; } public String getShipName() { return shipName; } // No setShipName method }  Getter/setter names need not correspond to instance variable names 28 Note: Java naming conventions  Start classes with uppercase letters  Start other things with lowercase letters • Instance variables, local variables, methods, parameters to methods public class MyClass { public String firstName, lastName;... of an object, using the dot operator objectName.fieldName objectName.methodName(argumentsToMethod)  Example: r.width = 10; r.height = 5; double x= r.getArea(); double y= r.getPerimeter(); 16 Example: Writing class Rectangle (1) Rectangle.java public class Rectangle { double width, height; public double getArea() { return width * height; } public double getPerimeter() { return (width+height) * 2; } . Writing Classes Writing Classes Chapter 3 Chapter 3 1 Out lin e • Introduction to Object-Oriented Programming (OOP) • Writing a Java class • Encapsulation. e • Introduction to Object-Oriented Programming (OOP) • Writing a Java class • Encapsulation and accessor methods 11 Writing a Java Class Writing a Java Class class ClassName { property1 property2 ClassName { property1 property2 . . . constructor1 constructor2 . . . method1 method2 . . . } 12 Writing class Rectangle Writing class Rectangle class ClassName { property1 property2 . . . constructor1 constructor2 .