Objects and Classes
Objects and classes are fundamental concepts in object-oriented programming (OOP). Here’s a brief overview:
Classes:
- Classes are blueprints or templates for creating objects. They define the properties (attributes) and behaviors (methods) that objects of that class will have.
- They encapsulate data for the object and provide methods to operate on that data.
- Classes allow for abstraction, inheritance, and polymorphism, which are core principles of OOP.
- In many programming languages like Python, Java, and C++, classes are used to create objects.
Objects:
- Objects are instances of classes. They are concrete entities created based on the class blueprint.
- Each object has its own set of attributes and methods defined by its class.
- Objects can interact with each other by invoking methods or accessing attributes of other objects.
For example, consider a class Car:
class Car:
def __init__(self, make, model, year):
self.make = make
self.model = model
self.year = year
def drive(self):
print(f"{self.make} {self.model} is driving.")
Now, you can create objects (instances) of this class:
car1 = Car("Toyota", "Camry", 2022)
car2 = Car("Honda", "Civic", 2023)
Here, car1 and car2 are objects of the Car class. They each have attributes make, model, and year, and they can invoke the drive() method.
Objects and classes provide a way to organize and structure code, making it easier to manage and maintain, especially in larger software projects.
Object-oriented programming is modeled on how, in the real world, objects are often made up of many kinds of smaller objects. This capability of combining objects, however, is only one very general aspect of
object-oriented programming. Object-oriented programming provides several other concepts and features to make creating and using objects easier and more flexible, and the most important of these features is classes.
When you write a program in an object-oriented language, you don’t define actual objects. You define classes of objects, where a class is a template for multiple objects with similar features. Classes embody all the
features of a particular set of objects. For example, you might have a Tree class that describes the features of all trees (has leaves and roots, grows, creates chlorophyll). The Tree class serves as an abstract model for
the concept of a tree-to reach out and grab, or interact with, or cut down a tree you have to have a concrete instance of that tree. Of course, once you have a tree class, you can create lots of different instances of that
tree, and each different tree instance can have different features (short, tall, bushy, drops leaves in autumn).
| New Term |
|---|
|
A class is a generic template for a set of objects with similar features.
|
An instance of a class is another word for an actual object. If class is the general (generic) representation of an object, an instance is its concrete representation. So what, precisely, is the difference between an instance
and an object? Nothing, really. Object is the more general term, but both instances and objects are the concrete representation of a class. In fact, the terms instance and object are often used interchangeably in OOP
lingo. An instance of a tree and a tree object are both the same thing.
| New Term |
|---|
|
An instance is the specific concrete representation of a class. Instances and objects are the same thing.
|
What about an example closer to the sort of things you might want to do in Java programming? You might create a class for the user interface element called a button. The Button class defines the features of a button (its
label, its size, its appearance) and how it behaves. (Does it need a single-click or a double-click to activate it? Does it change color when it’s clicked? What does it do when it’s activated?) After you define the Button
class, you can then easily create instances of that button-that is, button objects-that all take on the basic features of the button as defined by the class, but may have different appearances and behavior based on what you
want that particular button to do. By creating a Button class, you don’t have to keep rewriting the code for each individual button you want to use in your program, and you can reuse the Button class to create different
kinds of buttons as you need them in this program and in other programs
| Tips |
|---|
|
If you’re used to programming in C, you can think of a class as sort of creating a new composite data type by using struct and typedef. Classes, however, can
provide much more than just a collection of data, as you’ll discover in the rest of today’s lesson.
|
When you write a Java program, you design and construct a set of classes. Then when your program runs, instances of those classes are created and discarded as needed. Your task, as a Java programmer, is to create
the right set of classes to accomplish what your program needs to accomplish.
Fortunately, you don’t have to start from the very beginning: The Java environment comes with a standard set of classes (called a class library) that implement a lot of the basic behavior you need-not only for basic
programming tasks (classes to provide basic math functions, arrays, strings, and so on), but also for graphics and networking behavior. In many cases, the Java class libraries may be enough so that all you have to do in
your Java program is create a single class that uses the standard class libraries. For complicated Java programs, you may have to create a whole set of classes with defined interactions between them.
| New Term |
|---|
|
A class library is a collection of classes intended to be reused repeatedly in different programs. The standard Java class libraries contain quite a few classes for
accomplishing basic programming tasks in Java.
|
