Object-oriented programming (OOP) is one of the biggest programming ideas of recent years, and you might worry that you must spend years learning all about object-oriented programming methodologies and how they can make your life easier than The Old Way of programming. It all comes down to organizing your programs in ways that echo how things are put together in the real world.
Today you'll get an overview of object-oriented programming concepts in Java and how they relate to how you structure your own programs:
What classes and objects
are and how they relate to each other?
The two main parts of a class or object: its behaviors and its attributes
Class inheritance and how inheritance affects the way you design your programs
Some information about packages and interfaces
If you're already familiar with object-oriented programming, much of today's lesson will be old hat to you. You may want to skim it and go to a movie today instead. Tomorrow, you'll get into more specific details.
Thinking in Objects: An Analogy
Consider, if you will, Legos. Legos, for those who do not spend much time with children, are small plastic building blocks in various colors and sizes. They have small round bits on one side that fit into small round holes on other Legos so that they fit together snugly to create larger shapes. With different Lego parts (Lego wheels, Lego engines, Lego hinges, Lego pulleys), you can put together castles, automobiles, giant robots that swallow cities, or just about anything else you can imagine. Each Lego part is a small object that fits together with other small objects in predefined ways to create other larger objects. That is roughly how object-oriented programming works: putting together smaller elements to build larger ones.
Here's another example. You can walk into a computer store and, with a little background and often some help, assemble an entire pc computer system from various components: a motherboard, a CPU chip, a video card, a hard disk, a keyboard, and so on. Ideally, when you finish assembling all the various self-contained units, you have a system in which all the units work together to create a larger system with which you can solve the problems you bought the computer for in the first place.
Internally, each of those components may be vastly complicated and engineered by different companies with different methods of design. But you don't need to know how the component works, what every chip on the board does, or how, when you press the A key, an A gets sent to your computer. As the assembler of the overall system, each component you use is a self-contained unit, and all you are interested in is how the units interact with each other. Will this video card fit into the slots on the motherboard, and will this monitor work with this video card? Will each particular component speak the right commands to the other components it interacts with so that each part of the computer is understood by every other part? Once you know what the interactions are between the components and can match the interactions, putting together the overall system is easy.
What does this have to do with programming?
Everything. Object-oriented programming works in exactly this same way. Using object-oriented programming, your overall program is made up of lots of different self-contained components (objects), each of which has a specific role in the program and all of which can talk to each other in predefined ways.
Objects and Classes
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), while still behaving like and being immediately recognizable as a tree.
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.
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.
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.
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.
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