Encapsulation

At this point we can introduce another bit of jargon you can use to impress or bore your friends - encapsulation. Encapsulation refers to the hiding of items of data and methods within an object. This is achieved by specifying them as private in the definition of the class. In the CowboyHat class, the instance variables owner, type, size, and hatOn were encapsulated. They were accessible only through the methods defined for the class. Therefore, the only way to alter the values they contain is to call a method that does that. Being able to encapsulate members of a class in this way is important for the security and integrity of class objects. You may have a class with data members that can take on only particular values. By hiding the data members and forcing the use of a method to set or change the values, you can ensure that only legal values are set.

I mentioned earlier another major advantage of encapsulation - the ability to hide the implementation of a class. By allowing only limited access to the members of a class, you have the freedom to change the internals of the class without necessitating changes to Java programs that use the class. As long as the external characteristics of the methods that can be called from outside the class remain unchanged, the internal code can be changed in any way that you, the programmer, want.

A particular object, an instance of CowboyHat, incorporates, or encapsulates, the owner, the size of the object, and the status of the hat in the instance variable hatOn. Only the constructor, and the putHatOn(), takeHatOff(), changeOwner(), and getSize() methods can be accessed externally.

Java Program Statements

As you saw in the CowboyHat class example, the code for each method in the class appears between braces, and it consists of program statements. A semicolon terminates each program statement. A statement in Java can spread over several lines if necessary, since the end of each statement is determined by the semicolon, not by the end of a line. Here is a Java program statement:

hatOn = false;

If you wanted to, you could also write this as:

hatOn = false;

You can generally include spaces and tabs, and spread your statements over multiple lines to enhance readability if it is a particularly long statement, but sensible constraints apply. You can’t put a space in the middle of a name for instance. If you write hat On, for example, the compiler will read this as two words.

Operating on Objects

A class object is not just a collection of various items of data. In addition to the parameters that characterize an object, 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. The operations that you specify for objects of a given type will depend on what sort of objects you are talking about, the attributes they contain, and how you intend to use them.

For the CowboyHat class, you may want to have operations that you could refer to as putHatOn and takeHatOff, which would have meanings that are fairly obvious from their names, and do make sense for CowboyHat objects. These operations on a particular CowboyHat object would set the value of hatOn for the object. To determine whether your CowboyHat was on or off, you would just need to look at this value. Conceivably, you might also have an operation changeOwner by which you could set the instance variable recording the current owner’s name to a new value.

Of course, for each type of object you can have any operation that makes sense for you. If you want to have a shootHoleIn operation for Hat objects, that’s no problem. You just have to define what that operation does to an object. You are probably wondering at this point how an operation for a class is defined. As you’ll see in detail a bit later, it boils down to a self-contained block of program code called a method that is identified by the name you give to it. You can pass data items - which can be integers, floating-point numbers, character strings, or class objects - to a method, and these will be processed by the code in the method.

A method may also return a data item as a result. Performing an operation on an object amounts to executing the method that defines that operation for the object. Just so you’ll recognize one when you see it, let’s take a look at an example of a complete class definition. The code for the class CowboyHat we have been talking. This code would be saved in a file with the name CowboyHat.java. The name of a file that contains the definition of a class is always the same as the class name, and the extension will be .java to identify that the file contains Java source code. The code for the class definition appears between the braces that follow the identification for the class.

The code for each of the methods in the class also appears between braces. The class has three instance variables, owner, size, and hatOn, and this last variable is always initialized as false. Each object that is created according to this class specification will have its own independent copy of each of these variables in Java program, so each object will have its own unique values for the owner, the hat size, and whether the hat is on or off. I omitted the type parameter in this version of the class to make the code a little bit shorter. The keyword private, which has been applied to each instance variable, ensures that only code within the methods of the class can access or change the values of these directly.

Methods of a class can also be specified as private. Being able to prevent access to some members of a class from outside is an important facility. It protects the internals of the class from being changed or used incorrectly. Someone using your class in another program can get access only to the bits to which you want them to have access. This means that you can change how the class works internally without affecting other programs that may use it. You can change any of the things inside the class that you have designated as private, and you can even change the code inside any of the public methods, as long as the method name and the number and types of values passed to it or returned from it remain the same.

Our CowboyHat class also has five methods, so you can do five different things with a CowboyHat object. One of these is a special method called a constructor, which creates a CowboyHat object - this is the method with the name, CowboyHat, that is the same as the class name. The items between the parentheses that follow the name of the constructor specify data that is to be passed to the method when it is executed - that is, when a CowboyHat object is created.

What Defines a Class of Objects?

A class definition identifies all the parameters that define an object of that particular class, at least, so far as your needs go. Someone else might define the class differently, with 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 decide what aspects of the objects you include to define that particular class of object, and you 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.

If you were defining 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. The 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 can 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 example, which might be another string of characters such as “Blue”. To specify a class you just decide what set of attributes meet your requirements, 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 the definition of the class Hat would look something like this:

class Hat {

// Stuff defining the class in detail goes here.

// This could specify the name of the hat, the size,

// maybe the color, and whatever else you felt was necessary.

}

The name of the class follows the word class, and the details of the definition appear between the curly braces. I won’t go into the detail of how the class Hat is defined, since you don’t need it at this point. The lines appearing between the braces above are not code; they are actually program comments, because they begin with two successive forward slashes. The compiler will ignore anything on a line that follows two successive forward slashes in your Java programs, so you’ll use this to add explanations to your programs. Generally, the more useful comments you can add to your programs, the better. You will see in next post that there are other ways you can write comments in Java.

Each object of your class will have a particular set of values defined that characterize that particular object. You could have an object of type CowboyHat, which might be defined by values such as “Stetson” for the type of the hat, “White” for the color, and the size as 7. Although shows CowboyHat objects defined by a set of three values that you would not normally expect to change for a given instance, in general the parameter values that define an object are not necessarily fixed. You would expect the type and size attributes for a particular CowboyHat object to stay fixed since hats don’t usually change their size - at least, not unless it’s raining - but you could have other attributes.

You might have a parameter owner, which would record the owner’s name, so the value stored as the attribute owner could be changed when the hat was sold or otherwise transferred to someone else. You might also have a parameter hatOn, for example, which would indicate whether the hat was on or off the owner’s head; the value true would indicate that the owner was indeed wearing the hat, whereas the value false would mean that the hat had been removed and was just lying about somewhere.

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. The notion of a Tree in general is a rather abstract concept - although any tree fits the description, it is more useful to think of more specific types of 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 blueprint - expressed as a piece of program code - that 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. Particular classes of Tree, such as Oak or Ash, have all the characteristics of the most general type, Tree; otherwise, they could not be considered to be such. However, each subclass of Tree, such as Oak, has its own characteristics that differentiate Oak objects from other types of Tree.

Of course, you define a class specification to fit what you want to do in your application context. There are no absolutes here. For my trivial problem, the specification of a Tree class might just consist of its species name 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 involves 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 welldefined way to group things together.

An instance of a class is a technical term for an existing object of that class. Ash is a specification for a type of object, and yourAsh is an object constructed to that specification. So, yourAsh would be an instance of the class Ash. Once you have a class defined, then you can come up with objects, or instances, of that class. This raises the question of what differentiates an object of a given class from an object of another class, an Ash class object, say, from a Redwood object.