© 2007 Lawrenceville Press Slide 1 Chapter 8 Objects  A variable of a data type that is a class. Also called an instance of a class.  Stores data  Can perform actions and provide communication  State of object refers to the data it stores  Behavior of object refers to the action and communication it provides

© 2007 Lawrenceville Press Slide 2 Chapter 8 Class  An abstract data type  Used to create, or instantiate, objects  Provides encapsulation, also called information hiding

© 2007 Lawrenceville Press Slide 3 Chapter 8 The Circle Class

© 2007 Lawrenceville Press Slide 4 Chapter 8 A Class public class Circle { private static final double PI = 3.14; private double radius; public Circle() { radius = 1; } public void setRadius(double newRadius) { radius = newRadius; } access level class name body variables constructor method

© 2007 Lawrenceville Press Slide 5 Chapter 8 Methods in a Class  An accessor method is used to determine the value of a variable  A modifier method is used to change the value of a variable  A helper method is called from within a class by other methods

© 2007 Lawrenceville Press Slide 6 Chapter 8 Overloading Constructors  Constructors can be overloaded to provide more options for instantiating an object  The compiler uses the number and types of parameters to determine which constructor to execute

© 2007 Lawrenceville Press Slide 7 Chapter 8 Instance and Class Variables  Each object of a class has its own copy of the instance variables in the class.  A class variable is declared with the keyword static and only one copy of a class variable is maintained for all objects to refer to. Circle spot1 = new Circle(2);radiusPI Circle spot2 = new Circle(5);radiusPI

© 2007 Lawrenceville Press Slide 8 Chapter 8 Instance and Class Methods  Instance methods must be called from an instance of the class. Accessor and modifier methods are always instance methods because they change the state of an object.  Class methods are declared using the keyword static and can be called from the class itself.

© 2007 Lawrenceville Press Slide 9 Chapter 8 Differences Between Instance and Class Members  Instance variables are created each time an object is declared.  Class variables are created once for the class and then objects of the class refer to this copy.  Instance methods can only be called from an object of the class.  Class methods can be called from the class itself.

© 2007 Lawrenceville Press Slide 10 Chapter 8 The Object Class  Superclass of all other classes. Classes, such as Circle and String, are subclasses:  All subclasses inherit the Object methods, which include equals() and toString().  Inherited superclass methods can be redefined, or overridden, in subclasses.

© 2007 Lawrenceville Press Slide 11 Chapter 8 Classes Using Classes  A class containing a member variable that is a class data type.  Demonstrates a has-a relationship. The class "has-a" class.

© 2007 Lawrenceville Press Slide 12 Chapter 8 Object-Oriented Development  Objects are selected to model a program specification  Objects are created from new classes or from existing classes  Objects send messages to other objects to perform a task

© 2007 Lawrenceville Press Slide 13 Chapter 8 Features of Object-Oriented Programming  Reusability: existing classes can be used over and over again in different applications, which reduces development time and decreases the likelihood of bugs.  Modular: Components are separately written and maintained.