2.1.1 Classes

The previous example defines a class named HelloWeb. Classes are the fundamental building blocks of most object-oriented languages. A class in Java is akin to the C++ concept of a class. Specifically, it's a group of data items (à la a C struct), with associated functions that perform operations on this data. The data items in a class are called fields or variables; the functions are called methods. A class might represent something concrete, like a button on a screen or the information in a spreadsheet, or it could be something more abstract, such as a sorting algorithm or possibly the sense of ennui in your MUD character. A hypothetical spreadsheet class might, for example, have variables that represent the values of its individual cells and methods that perform operations on those cells, such as "clear a row" or "compute values."

Our HelloWeb class is the container for our Java applet. It holds two general types of variables and methods: those we need for our specific applet's tasks and some special predesignated ones we provide to interact with the outside world. The Java run-time environment, in this case a Java-enabled Web browser, periodically calls methods in HelloWeb to pass us information and prod us to perform actions, as depicted in Figure 2.2. Our simple HelloWeb class defines a single method called paint(). The paint() method is called by Java when it's time for our application to draw itself on the screen.

Figure 2.2: Method invocation in the Java environment

You will see that the HelloWeb class derives some of its structure from another class called Applet. This is why we refer to HelloWeb as an applet.

2.1.2 Class Instances and Objects

A class is a blueprint for a part of an application; it lists methods and variables that go into making up that part. Many individual working copies of a given class can exist while an application is active. These individual incarnations are called instances of the class. Two instances of a given class may contain different data, but they always have the same methods.

As an example, consider a Button class. There is only one Button class, but many actual working instances of buttons can be in an application. Furthermore, two Button instances might contain different data, perhaps giving each a different appearance or specifying a different message for each to send when pushed. In this sense, a class can be considered a mold for making the object it represents: something like a cookie cutter stamping out working instances of itself in the memory of the computer. As you'll see later, there's a bit more to it than that--a class can in fact share information among its instances--but this explanation suffices for now.

The term object is very general and in some other contexts is used almost interchangeably with class. Objects are the abstract entities all object-oriented languages refer to in one form or another. We will use object as a generic term for an instance of a class. We might, therefore, refer to an instance of the Button class as a Button, a Button object, or, indiscriminately, as an object.

A Java-enabled Web browser creates an instance of our HelloWeb class when we first use our applet. If we had included the HelloWeb applet tag in our HTML document twice (causing it to appear twice on the screen), the browser would create and manage two separate HelloWeb objects (two separate instances of the HelloWeb class).

2.1.3 Variables and Class Types

In Java, every class defines a new type. A variable can be of this type and then hold instances of that class. A variable could, for example, be of type Button and hold an instance of the Button class, or of type SpreadSheetCell and hold a SpreadSheetCell object, just as it could be any of the more familiar types such as int or float. In this way, by having variables containing complex objects, a class may use other classes as tools within itself. Using classes in this way is called composition. Our examples in this chapter are somewhat unrealistic in that we are building only a single class of our own. However, we will be using many classes as tools within our applet.

You have seen only one variable so far in our simple HelloWeb example. It's found in the declaration of our lonely paint() method:

public void paint( java.awt.Graphics gc ) {...} 

Just like functions in C (and many other languages), a method in Java declares a list of variables that hold its arguments, and it specifies the types of those arguments. Our paint() method takes one argument named (somewhat tersely) gc, which is of type Graphics. When the paint() method is invoked, a Graphics object is assigned to gc, which we use in the body of the method. We'll say more about paint() and the Graphics class in a moment.

But first, a few words about variables. We have loosely referred to variables as holding objects. In reality, variables that have complex types (class types) don't so much contain objects as point to them. Class-type variables are references to objects. A reference is a pointer to, or another name for, an object.

Simply declaring a variable doesn't imply that any storage is allocated for that variable or that an instance of its type even exists anywhere. When a reference-type variable is first declared, if it's not assigned to an instance of a class, it doesn't point to anything. It's assigned the default value of null, meaning "no value." If you try to use a variable with a null value as if it were pointing to a real object, a run-time error (NullPointerException) occurs.

This discussion begs the question as to where to get an instance of a class to assign to a variable in the first place. The answer, as you will see later, is through the use of the new operator. In our first two passes at this example, we are dealing only with objects handed to us prefabricated from somewhere outside of our class. We examine object creation later in the chapter.

2.1.4 Inheritance

Java classes are arranged in a parent-child hierarchy, in which the parent and child are known as the superclass and subclass, respectively. In Java, every class has exactly one superclass (a single parent), but possibly many subclasses. Of course, a class's superclass probably has its own superclass.

The declaration of our class in the previous example uses the keyword extends to specify that HelloWeb is a subclass of the Applet class:

public class HelloWeb extends java.applet.Applet {...}

A subclass may be allowed to inherit some or all of the variables and methods of its superclass. Through inheritance, the subclass can use those members as if it has declared them itself. A subclass can add variables and methods of its own, and it can also override the meaning of inherited variables and methods. When we use a subclass, overridden variables and methods are hidden (replaced) by the subclass's own versions of them. In this way, inheritance provides a powerful mechanism whereby a subclass can refine or extend its superclass.

For example, the hypothetical spreadsheet class might be subclassed to produce a new scientific spreadsheet class with extra mathematical functions and special built-in constants. In this case, the source code for the scientific spreadsheet might declare methods for the added mathematical functions and variables for the special constants, but the new class automatically has all the variables and methods that constitute the normal functionality of a spreadsheet; they are inherited from the parent spreadsheet class. This means the scientific spreadsheet maintains its identity as a spreadsheet, and we can use it anywhere the simpler spreadsheet is used.

Our HelloWeb class is a subclass of the Applet class and inherits many variables and methods not explicitly declared in our source code. These members function in the same way as the ones we add or override.

2.1.5 Applet

The Applet class provides the framework for building applets. It contains methods that support the basic functionality for a Java application that is displayed and controlled by a Java-enabled Web browser or other Java-enabled software.

We override methods in the Applet class in a subclass to implement the behavior of our particular applet. This may sound restrictive, as if we are limited to some predefined set of routines, but that is not the case at all. Keep in mind that the methods we are talking about are means of getting information from the outside world. A realistic application might involve hundreds or even thousands of classes, with legions of methods and variables and multiple threads of execution. The vast majority of these are related to the particulars of our job. The inherited methods of the Applet class, and of other special components, serve as a framework on which to hang code that handles certain types of events and performs special tasks.

The paint() method is an important method of the Applet class; we override it to implement the way our particular applet displays itself on the screen. We don't override any of the other inherited members of Applet because they provide basic functionality and reasonable defaults for this (trivial) example. As HelloWeb grows, we'll delve deeper into the inherited members and override additional methods. Inherited members will allow us to get information from the user and give us more control over what our applet does. We will also add some arbitrary, application-specific methods and variables for the needs of HelloWeb.

If you want to verify for yourself what functionality the Applet class is providing our example, you can try out the world's least interesting applet: the Applet base class itself. Just use the class name java.applet.Applet in your HTML code, instead of HelloWeb:

<applet code=java.applet.Applet width=300 height=200></applet> 

You should get a blank area of screen. We told you it's not very interesting.

2.1.6 Relationships and Finger Pointing

We can correctly refer to HelloWeb as an Applet because subclassing can be thought of as creating an "is a" relationship, in which the subclass is a kind of its superclass. HelloWeb is therefore a kind of Applet. When we refer to a kind of object, we mean any instance of that object's class or any of its subclasses. Later, we will look more closely at the Java class hierarchy and see that Applet is itself a subclass of the Panel class, which is further derived from a class called Container, and so on, as shown in Figure 2.3.

Figure 2.3: Part of the Java class hierarchy

In this sense, an Applet is a kind of Panel, which is, itself, a kind of Container and each of these can ultimately be considered to be a kind of Component. You'll see later that it's from these classes that Applet inherits its basic graphical user interface functionality and the ability to have other graphical components embedded within it.

Component is a subclass of Object, so all of these classes are a kind of Object. As you'll see later, the Object class is at the top of the Java class hierarchy; Object doesn't have a superclass. Every other class in the Java API inherits behavior from Object, which defines a few basic methods, as you'll see in Chapter 7. The terminology here can become a bit muddled. We'll continue to use the word object (lowercase o) in a generic way to refer to an instance of any class; We'll use Object to refer specifically to that class.

2.1.7 Packages

In our previous example, the Applet class is referenced by its fully qualified name java.applet.Applet:

public class HelloWeb extends java.applet.Applet {...} 

The prefix on the class name identifies it as belonging to the java.applet package. Packages provide a means for organizing Java classes. A package is a group of Java classes that are related by purpose or by application. Classes in the same package have special access privileges with respect to one another and may be designed to work together. Package names are hierarchical and are used somewhat like Internet domain and host names, to distinguish groups of classes by organization and application. Classes may be dynamically loaded over networks from arbitrary locations; within this context, packages provide a crude namespace of Java classes.[1]

[1] There are many efforts under way to find a general solution to the problem of locating resources in a globally distributed computing environment. The Uniform Resource Identifier Working Group of the IETF has proposed Uniform Resource Names (URNs). A URN would be a more abstract and persistent identifier that would be resolved to a URL through the use of a name service. We can imagine a day when there will exist a global namespace of trillions of persistent objects forming the infrastructure for all computing resources. Java provides an important evolutionary step in this direction.

java.applet identifies a particular package that contains classes related to applets. java.applet.Applet identifies a specific class, the Applet class, within that package. The java. hierarchy is special. Any package that begins with java. is part of the core Java API and is available on any platform that supports Java. Figure 2.4 illustrates some of the core Java packages, showing a representative class or two from each.

Figure 2.4: The core Java packages

Some notable core packages include: java.lang, which contains fundamental classes needed by the Java language itself; java.awt, which contains classes of the Java Abstract Window Toolkit; and java.net, which contains the networking classes.

A few classes contain methods that are not written in Java, but are instead part of the native Java implementation on a particular platform. Approximately 22 such classes are in the java package hierarchy; these are the only classes that have to be ported to a new platform. They form the basis for all interaction with the operating system. All other classes are built on or around these and are completely platform independent.

2.1.8 The paint() Method

The source for our HelloWeb class defines just one method, paint(), which overrides the paint() method from the Applet class:

public void paint( java.awt.Graphics gc ) { 
    gc.drawString("Hello Web!", 125, 95 ); 
} 

The paint() method is called by Java when it's time for our applet to draw itself on the screen. It takes a single argument, a Graphics object, and doesn't return any type of value (void) to its caller.

Modifiers are keywords placed before classes, variables, and methods to alter their accessibility, behavior, or semantics. paint() is declared as public, which means it can be invoked (called) by methods in classes other than HelloWeb. In this case, it's the Java windowing environment that is calling our paint() method. A method or variable declared as private is inaccessible from outside of its class.

The Graphics object, an instance of the Graphics class, represents a particular graphical drawing area and is also called a graphics context. It contains methods the applet calls to draw in this area, and variables that represent characteristics such as clipping or drawing modes. The particular Graphics object we are passed in the paint() method corresponds to our applet's area of the screen.

The Graphics class provides methods for rendering primitive shapes, images, and text. In HelloWeb, we invoke the drawString() method of our Graphics object to scrawl our message at the specified coordinates. (For a description of the methods available in the Graphics class, see Chapter 16.)

As in C++, a method or variable of an object is accessed in a hierarchical way by appending its name with a "." (dot) to the object that holds it. We invoked the drawString() method of the Graphics object (referenced by our gc variable) in this way:

gc.drawString( "Hello Web!", 125, 95 ); 

You may need to get used to the idea that our application is drawn by a method that is called by an outside agent at arbitrary times. How can we do anything useful with this? How do we control what gets done and when? These answers will be forthcoming. For now, just think about how you would structure applications that draw themselves on command.