Articles : JavaBoutique's Introduction to Java :
Java Classes :

Creating and Using Classes by Example

So what does a class look like? Well, let's make one. However, since we have not yet gone into the depths of Java, our class will be very small. We will create an Announcer class that is solely responsible for announcing some phrase. Consider the following code (we will explain it in just a minute so don't get too caught up on the syntax or keywords):

public class Announcer
  {
  private String _announcement;

  public Announcer()
    {
    _announcement = "Hello Cyberspace";
    }

  public Announcer(String s)
    {
    _announcement = s;
    }

  public void printAnnouncement()
    {
    System.out.println(_announcement);
    }

  public void setAnnouncement(String s)
    {
    _announcement = s;
    }

  public String getAnnouncement()
    {
    return _announcement;
    }
  }

Okay, so believe it or not, in 23 lines of code we have demonstrated the creation of an object, the creation of a public API, encapsulation, and polymorphism (we'll show an example of inheritance later). See how easy Java makes object-oriented programming!

Okay, let's take this code apart and see how we achieved out object-oriented goal.

Access Specifiers

So the first thing you see in this class is the word "public". A couple of lines down, you also see the word "private". What do these keywords mean?

Well, the words private, public, and protected (which we did not use in the example above), are used to define scope.

The scope of a class, method or property defines who is allowed access to it. A "public" scope for example, means that anyone is allowed access. A "private" scope on the other hand means that only the object itself has access. A "protected" scope means that only objects derived from the class can have access. Finally, if you do not define a scope at all (usually called "friendly"), then all classes in the same package will have access.

Thus, in our example above, any object in the object space has access to the Announcer class and can use it in order to create Announcer objects. Everyone also has access to use the printAnnouncement() method in order to tell the Announcer object to print out the message. However, only an Announcer object itself has access to the actual content of the announcement that is stored in the _announcement property. Of course, outside objects can request that the announcement be changed using the get and set API provided by the Announcer class as well.

The benefit of scope or course, is that it "enforces" encapsulation. Programmers are strongly pushed to hide their objets' data and provide an API such as setAnnouncement().

Essentially, the "private" keywords constructs the walls of the black box that are so crucial for object oriented design.

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Selena Sol contributes to the JavaBoutique's Introduction to Java. Selena curently works for Barclays Capital in London, one of the leading global investment banks in Europe and has worked as a software developer for the National Center for Human Genome research, Microline Software, Neuron Data, and Electric Eye in Singapore. Selena is perhaps best-known for creating the Public Domain Web Script Archive (Extropia) and writing several books on Web Programming (Perl, CGI, Java).
Email: selena@extropia.com