Can explain the relationship between classes and objects
Writing an OOP program is essentially writing instructions that the computer will use to,
- create the virtual world of the object network, and
- provide it the inputs to produce the outcome you want.
A class contains instructions for creating a specific kind of objects. It turns out sometimes multiple objects keep the same type of data and have the same behavior because they are of the same kind. Instructions for creating a 'kind' (or ‘class’) of objects can be done once and those same instructions can be used to objects of that kind. We call such instructions a Class.
Classes and objects in an example scenario
Consider the example of writing an OOP program to calculate the average age of Adam, Beth, Charlie, and Daisy.
Instructions for creating objects Adam, Beth, Charlie, and Daisy will be very similar because they are all of the same kind: they all represent ‘persons’ with the same interface, the same kind of data (i.e. name, dateOfBirth, etc.), and the same kind of behavior (i.e. getAge(Date), getName(), etc.). Therefore, you can have a class called Person containing instructions on how to create Person objects and use that class to instantiate objects Adam, Beth, Charlie, and Daisy.
Similarly, you need classes AgeList, Calculator, and Main classes to instantiate one each of AgeList, Calculator, and Main objects.
| Class | Objects |
|---|---|
Person | objects representing Adam, Beth, Charlie, Daisy |
AgeList | an object to represent the age list |
Calculator | an object to do the calculations |
Main | an object to represent you (i.e., the one who manages the whole operation) |
Exercises
Identify Classes and Objects
Classes for CityConnect app
Can explain class-level members
While all objects of a class have the same attributes, each object has its own copy of the attribute value.
All Person objects have the name attribute but the value of that attribute varies between Person objects.
However, some attributes are not suitable to be maintained by individual objects. Instead, they should be maintained centrally, shared by all objects of the class. They are like ‘global variables’ but attached to a specific class. Such variables whose value is shared by all instances of a class are called class-level attributes.
The attribute totalPersons should be maintained centrally and shared by all Person objects rather than copied at each Person object.
Similarly, when a normal method is being called, a message is being sent to the receiving object and the result may depend on the receiving object.
Sending the getName() message to the Adam object results in the response "Adam" while sending the same message to the Beth object results in the response "Beth".
However, there can be methods related to a specific class but not suitable for sending messages to a specific object of that class. Such methods that are called using the class instead of a specific instance are called class-level methods.
The method getTotalPersons() is not suitable to send to a specific Person object because a specific object of the Person class should not have to know about the total number of Person objects.
Class-level attributes and methods are collectively called class-level members (also called static members sometimes because some programming languages use the keyword static to identify class-level members). They are to be accessed using the class name rather than an instance of the class.
Exercises
Can explain the meaning of enumerations
An Enumeration is a fixed set of values that can be considered as a data type. An enumeration is often useful when using a regular data type such as int or String would allow invalid values to be assigned to a variable.
Suppose you want a variable called priority to store the priority of something. There are only three priority levels: high, medium, and low. You can declare the variable priority as of type int and use only values 2, 1, and 0 to indicate the three priority levels. However, this opens the possibility of an invalid value such as 9 being assigned to it. But if you define an enumeration type called Priority that has three values HIGH, MEDIUM and LOW only, a variable of type Priority will never be assigned an invalid value because the compiler is able to catch such an error.
Priority: HIGH, MEDIUM, LOW