The ABCs of Classes and Objects

A Tutorial and Walkthrough of coding Classes and Objects

The purpose of this tutorial is to give the student an opportunity to write code and to explore the fundamentals of Classes and Objects. Specifically, this tutorial demonstrates fields, properties, constructors, and methods in class declarations along with object instantiation and the use of the member access operator.

About Classes and Objects

TIP

Understanding Classes and Objects is easier if you've already taken a look at the distinction between variables, values and data types.

A computer program is a set of instructions for manipulating information.

Information is central to the whole purpose of computer programs, and is the "thing" that has value in the eyes of the end-user. Information is often complex, being made up of many smaller pieces of related information. For example, the information could be the details of a bank account, the marks earned by a student, or information about a company or an employee. A key aspect of all of these is that the details of the information (First and Last name, for example) are bundled together and treated as a single unit - a composition of related information. In programming, those bundled sets of detailed information are known as objects.

A class defines

• What an object "looks like" (information)
• How an object "behaves" (instructions to manipulate information)

Objects embody more than just information, however. A person can do things with objects. For example, if you imagine a Bank Account as an object, it can be made up of a bunch of information, such as Account Number, Balance and Overdraft Limit. Now imagine the things you can do with a bank account: you can Deposit or Withdraw money. These actions are tightly related to bank accounts, largely because they directly affect the information in the bank account. These actions are carried out as a set of instructions - known as methods - which are carried out in a prescribed way through a set of one or more program statements.

Two objects can have the same set of characteristics, but different information. For example, consider two Bank Account objects where one account has a balance of $4,500.00 and another has a balance of $125.00. The actual information is different, but the characteristics (what Bank Accounts "look like") remain the same. The things you can do with the two bank accounts also remain the same: you can Deposit and Withdraw money. In programs, the characteristics of an object are defined in something called a Class. A class defines

• What an object "looks like" (information)
• How an object "behaves" (instructions to manipulate information)

For a computer program to be able to work with objects, it must first know the class definition of that object. Coding a class definition is like telling the computer about a new data type that you are defining. The name of the class is the name of the new data type. Once the data type is defined, then it can be used to create objects. A class definition (or "class" for short) acts as a blueprint for creating objects; from the blueprint, the computer is told what the object "looks like" and how it "behaves". Every object is said to be an "instance" of a class, in the same way that a physical car would be an instance of the car's blueprint. The term instantiation basically means "to instantiate", and it refers to the act of creating objects in code. An object is said to be "based on" a class when that class is the data type from which an object has been instantiated (just as a physical car would be "based on" the car's blueprint).

So, objects and classes are closely related, but not exactly the same. The relationship between objects and classes is pretty much the same as the relationship between ordinary values and data types.

Coding the Class Definition

A class definition can contain four parts: Fields, Properties, Constructors, and Methods. Fields and properties work together to describe what an object based on the class "looks like" (the object's information). Constructors and Methods describe how the object "behaves" (what instructions an object performs to manipulate information).

• Fields
  • Fields are variables that are shared by/accessible to all the other members of a class (properties, constructors, and methods).
  • Fields are almost always declared as private (meaning that they are not directly accessible from outside the class).
  • Although not commonly done, a field can be assigned an initial default value when it is declared.
• Properties
  • Properties are used like ordinary fields or variables, but work internally as methods.
  • Properties can have a Get method and/or a Set method.
    • The Get method is used when attempting to retrieve a value from the property.
    • The Set method is used when attempting to store a value in the property.
  • Properties often (but not always) have a corresponding field that is used to hold or store the property's value in the "background"; the field associated with a property is often referred to as the "backing store".
  • Properties that do not have a corresponding backing store will only have a Get method that calculates a value based on other values; there is no Set method because there is no single field in which to store a value for the property.
  • Properties are usually declared as public (meaning that they can be accessed directly from outside the class).
• Constructors
  • Constructors are called automatically whenever an object is created. Objects are created by the new keyword, whose general syntax is as follows:
    new ClassName(ArgList)
  • ArgList (or Argument List) is a comma-separated list of values that are sent to the constructor. The decision of which constructor can be called depends on the arguments (values) sent in when the new keyword is used; the compiler looks for a constructor with a parameter list that matches the argument list.
  • Constructors have one purpose: to make sure that the fields in the object have meaningful values. This usually means that a constructor will need some parameters to get those values from the code that creates the object.
  • Usually, constructors are made public.
  • A class can define more than one constructor. If no constructor is defined for a class, then a "default constructor" is used, which simply sets all the fields to their default values.
• Methods
  • A method is a set of instructions that are typically focused on manipulating or working with the fields and properties of an object.
  • Methods are identified by a name (called the Method Name) and a parameter list; together the method name and parameter list make up what is called the Method Signature.
  • If a method has a return type of void, then it is known as a Subroutine; subroutines do not return any information to the code that called it.
  • If a method has any return type except void, then it is called a Function; functions can only return one item, and must do so using a return statement.