Java is an object-oriented language. When you use a programming language that is not object oriented, you must express the solution to every problem essentially in terms of numbers and characters – the basic kinds of data that you can manipulate in the language. In an object-oriented language like Java, things are different. Of course, you still have numbers and characters to work with – these are referred to as the basic data types – but you can define other kinds of entities that are relevant to your particular problem. You solve your problem in terms of the entities or objects that occur in the context of the problem. This not only affects how a program is structured, but also the terms in which the solution to your problem is expressed. If your problem concerns baseball players, your Java program is likely to have BaseballPlayer objects in it; if you are producing a program dealing with fruit production in California, it may well have objects that are Oranges in it. Apart from seeming to be inherently sensible, object-oriented programs are usually easier to understand.
In Java almost everything is an object. If you haven't delved into object-oriented programming before, or maybe because you have, you may feel this is a bit daunting. But fear not. Objects in Java are particularly easy. So easy, in fact, that we are going to start out by understanding some of the ideas behind Java objects right now. In that way you will be on the right track from the outset.
This doesn't mean we are going to jump in with all the precise nitty-gritty of Java that you need for describing and using objects. We are just going to get the concepts straight at this point. We will do this by taking a stroll through the basics using the odd bit of Java code where it helps the ideas along.
So What Are Objects?
Anything can be thought of as an object. Objects are all around you. You can consider Tree to be a particular class of objects: trees in general; although it is a rather abstract class as you would be hard pushed to find an actual occurrence of a totally generic tree. Hence the Oak tree in my yard which I call myOak, the Ash tree in your yard which you call thatDarnedTree, and a generalSherman, the well-known redwood, are actual instances of specific types of tree, subclasses of Tree that in this case happen to be Oak, Ash, and Redwood. Note how we drop into the jargon here – class is a term that describes a specification for a collection of objects with common properties.
A class is a specification, or template – expressed as a piece of program code – which defines what goes to make up a particular sort of object. A subclass is a class that inherits all the properties of the parent class, but that also includes extra specialization. Of course, you will define a class specification to fit what you want to do. There are no absolutes here. For my trivial problem, the specification of a Tree class might just consist of its species and its height. If you are an arboriculturalist, then your problem with trees may require a much more complex class, or more likely a set of classes, that involve a mass of arboreal characteristics.
Every object that your program will use will have a corresponding class definition somewhere for objects of that type. This is true in Java as well as in other object-oriented languages. The basic idea of a class in programming parallels that of classifying things in the real world. It is a convenient and well-defined way to group things together.
What Defines a Class of Objects?
You may have already guessed the answer. A class definition lists all the parameters that you need to define an object of that particular class, at least, so far as your needs go. Someone else might choose a larger or smaller set of parameters to define the same sort of object – it all depends on what you want to do with the class. You will decide what aspects of the objects you need to include to define that particular class of object, and you will choose them depending on the kinds of problems that you want to address using the objects of the class. Let's think about a specific class of objects.
For a class Hat for example, you might use just two parameters in the definition. You could include the type of hat as a string of characters such as "Fedora" or "Baseball cap", and its size as a numeric value. These parameters that define an object of a class are referred to as instance variables or attributes of a class, or class fields. The instance variables can be basic types of data such as numbers, but they could also be other class objects. For example, the name of a Hat object could be of type String – the class String defines objects that are strings of characters.
Of course there are lots of other things you could include to define a Hat if you wanted to, color for instance, which might be another string of characters such as "Blue". To specify a class you just decide what set of attributes suit your needs, and those are what you use. This is called data abstraction in the parlance of the object-oriented aficionado, because you just abstract the attributes you want to use from the myriad possibilities for a typical object.
In Java almost everything is an object. If you haven't delved into object-oriented programming before, or maybe because you have, you may feel this is a bit daunting. But fear not. Objects in Java are particularly easy. So easy, in fact, that we are going to start out by understanding some of the ideas behind Java objects right now. In that way you will be on the right track from the outset.
This doesn't mean we are going to jump in with all the precise nitty-gritty of Java that you need for describing and using objects. We are just going to get the concepts straight at this point. We will do this by taking a stroll through the basics using the odd bit of Java code where it helps the ideas along.
So What Are Objects?
Anything can be thought of as an object. Objects are all around you. You can consider Tree to be a particular class of objects: trees in general; although it is a rather abstract class as you would be hard pushed to find an actual occurrence of a totally generic tree. Hence the Oak tree in my yard which I call myOak, the Ash tree in your yard which you call thatDarnedTree, and a generalSherman, the well-known redwood, are actual instances of specific types of tree, subclasses of Tree that in this case happen to be Oak, Ash, and Redwood. Note how we drop into the jargon here – class is a term that describes a specification for a collection of objects with common properties.
A class is a specification, or template – expressed as a piece of program code – which defines what goes to make up a particular sort of object. A subclass is a class that inherits all the properties of the parent class, but that also includes extra specialization. Of course, you will define a class specification to fit what you want to do. There are no absolutes here. For my trivial problem, the specification of a Tree class might just consist of its species and its height. If you are an arboriculturalist, then your problem with trees may require a much more complex class, or more likely a set of classes, that involve a mass of arboreal characteristics.
Every object that your program will use will have a corresponding class definition somewhere for objects of that type. This is true in Java as well as in other object-oriented languages. The basic idea of a class in programming parallels that of classifying things in the real world. It is a convenient and well-defined way to group things together.
What Defines a Class of Objects?
You may have already guessed the answer. A class definition lists all the parameters that you need to define an object of that particular class, at least, so far as your needs go. Someone else might choose a larger or smaller set of parameters to define the same sort of object – it all depends on what you want to do with the class. You will decide what aspects of the objects you need to include to define that particular class of object, and you will choose them depending on the kinds of problems that you want to address using the objects of the class. Let's think about a specific class of objects.
For a class Hat for example, you might use just two parameters in the definition. You could include the type of hat as a string of characters such as "Fedora" or "Baseball cap", and its size as a numeric value. These parameters that define an object of a class are referred to as instance variables or attributes of a class, or class fields. The instance variables can be basic types of data such as numbers, but they could also be other class objects. For example, the name of a Hat object could be of type String – the class String defines objects that are strings of characters.
Of course there are lots of other things you could include to define a Hat if you wanted to, color for instance, which might be another string of characters such as "Blue". To specify a class you just decide what set of attributes suit your needs, and those are what you use. This is called data abstraction in the parlance of the object-oriented aficionado, because you just abstract the attributes you want to use from the myriad possibilities for a typical object.
A class object is not just a collection of various items of data though. The fundamental difference between a class and the complex data types that you find in some other languages is that a class includes more than just data. A class specifies what you can do with an object of the class – that is, it defines the operations that are possible on objects of the class. Clearly for objects to be of any use in a program, you need to decide what you can do with them. This will depend on what sort of objects you are talking about, the attributes they contain, and how you intend to use them.
