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

Inheritance

Inheritance in Java is very simple and is handled by the "extends" keyword. As you might expect, subclasses work just like their superclass (inherit all of the functionality) except that they typically add extra funtionality.

If a class does not specifically extend anything than it will be automaticaly extended from the root Object class

Thus, in the following example, we create a subclass of Applet caled myApplet:

public class myApplet extends Applet
  {
  ...new fields and methods...
  }

Further, extension is transitive. That is, if you extend from a class which is an extension of another, then you gain the functionality of your superclass as well as its superclass.

It is also worth noting that not only can you add to the functionality of a superclass, but you can modify existing functionality as well. This is achieved through overriding a method in one's superclass. To override a superclass' method, you would implement your own method with the same signature as your superclass' method. Thus, the Java Virtual machine would use your implementation rather than your superclass'. Of course, you can explicitly use your superclass' methods and fields using the "super" keyword such as:

super.methodname();

The super keyword is particularly important when working with Constructors because subclasses do not inherit the constructors of their superclasses. Thus, in order to utilize the constructor of your superclass, you must explicitly call it using the super keyword such as in the following example:

class myFrame extends Frame
  {
  public myFrame()
    {
    super();
    }
  public myFrame(String title)
    {
    super(title);
    }
   ...the rest of your methods and fields....
  }

Java also provides the "final" keyword which prevent a class from being subclassed, a method from being overridden and a variable from changing its initialized value. However, it is rarely used for regular application code.

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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