Chapter 8: Creating Simple ClassesChapter 8: Creating Simple ClassesOne of the principle goals of object oriented design was the creation and use of classes to represent real world objects in the programs we are designing. So far we have focused on using the Java API, and using existing classes provided by the Java language itself… But have yet to discuss what a class is and how it is composed.To provide the ability to create classes, an object oriented programming language must support the following key language features: Data Abstraction, information hiding, encapsulation, inheritance, and polymorphism. Lets begin by reviewing some terms introduced earlier in the semester:1) A Class: A class represents a group of similar (real world, or abstract) things (or objects) which share a set of common properties called attributes and a common set of behaviors which define how objects of the class can be use, these actions are implemented as methods. A class definition is a template containing both:a. variable declarations representing attributesb. methods representing object behavior. A class definition can be created to represent any entity in the solution of a problem space: a passenger, a ticket agent, a queue or a ticket.2) An object or instance: A technical term for a specific object of a class: ONE specific occurrence of an individual object with a specific set of values for the data members of the class. String name = new String(“Camille”);3) method: a procedure body implementing an operation, defined for a class of objects.4) message: A procedure call; a request to execute a method. Objects interact by sending messages to each other. A computation is characterized by the messages sent andreceived by objects in a program….name.length(); Math.pow(2,4);When designing programs, why do we need to design classes???Every program we create models some aspect of the real world. (an airline reservation system. Patient monitoring system, checkout line at a grocery store )There is a tremendous gap between the kinds of problems we are asked to solve using a computer and the BITS of information the computer can actually process. Not every aspect of the data requirements of a problem can be adequately expressed directly using the built-in data or primitive data types provided by any programming language.Another related problem is how to capture the behavior of objects so that they are consistent with the requirements of the real world problem. for example : A check-out line at the grocery store consists of a series of individuals, each of which have a set of items that they wish to purchase. These individuals enter a checkout line, wait for service, place their items on the checkout counter, and pay for their purchase. WE need a method of going from We need to come up with a process for taking the built-in data types we have available in any language and extending these type to create NEW application defined DATA TYPES that model objects and to capture their behavior. This is the idea behind Data Abstraction. Data Abstraction : is the creation and use of User defined Abstract Data Type as Conceptual Models of real world problems. We do this through the definition and implementation of classes:An Abstract Data Type (ADT): represents a group of related objects which have a common set of properties & a set of operations or behaviors. A CLASS IS AN ABSTRACT DATA TYPE. By defining classes we may create as many objects of a class as we need. ProgramRequirementsBuilt – in Objects (int)ProgramRequirementsBuilt – in Objects (int)AbstractDataTypes/ ClassesInformation HidingIs the practice of controlling access to the implementation details of a class, or ADT. This is based on the idea that we do not need to know implementation details to make effective use of anobject. For example, we do not need to know how STRINGS are stored internally in memory to make use of them in our program. We only need to know what they represent, and what operations may be performed on them. {They are a sequence of characters, that we may access individually, change the case of, search for the first occurrence of a character or substring.. etc..}We can declare objects using the types provided in the module and we make use of the operations provided in the module to manipulate the objects we’ve created with out knowing the exact implementation details. {String name = new String(“Camille Hayhurst”); name.charAt(10) }This is important because when ever a module is implemented which relies upon the detailed implementation of another it becomes vulnerable. It causes the situation where changes in one portion of a program can have rippling and unexpected errors in other portions. I don’t want a change in the implementation of class String to crash my program which uses String variables.Information Hiding is the separation of the behavior and use of an object from its implementation. This means that the implementation of an object can be changed with out the change rippling through other programs that use the class.Encapsulation Encapsulation is the concentration of all program text dependent on a particular design choice in one place. Encapsulation is a feature that allows a compiler to enforce information hiding. A class definition provides encapsulation.Inheritance and Polymorphism are advanced concepts relative to object oriented design that we will cover more fully later in the semester.Defining ClassesDefining a class is a two step process:1. Determining what attributes all objects in the class share in common.2. Define the interface for the class. The interface is the set of methods and possibly attributes that we will make available to users of the class. Along with the interface, we will need to define the set of attributes that define objects of the class.As an example, we will design a simple class to represent Accounts at a bank.Our class account represents accounts that customers will open at a bank. Although there are many potential data items that a real bank would collect about their customer accounts we will start with the following:Some of the class “members” we define will be instance data members and methods that apply to declared instances of the class, others will be class members which are static data members and methods, that apply to the class as a whole, and are usable whether or not ANY instances of the class are ever