Articles : JavaBoutique's Introduction to Java : Q and A :

What is the "super" class

Q

What is the "super" class and how is it used?

A

I've seen it used two different ways:

super("text here to add to a title bar of a dialog window") instead of using showMessageDialog method and super.init( )

I haven't found an understandable explanation of this in any Java books.

The super keyword is used by an object to reference the methods and fields of a class that it has extended. It is an essential part of implementing inheritance and polymorphism. Most commonly, it is used in the constructor of an object in order to pass construction details up the inheritance chain.

Sounds pretty gnarly, but it really is not.

Consider an Employee class. An Employee class could have a simple constructor that took a string and an int and assigned them to variables:

       Employee (String n, int i) {
         String name = n;
         int employee_id = i;
       }

Now suppose we were to "extend" the Employee object in order to create a Programmer.

      class Programmer extends Employee {

The Programmer object would be the same as the Employee object (since it extends the Employee object) except that it would have another property.....let's say...a language_preference.

Once the Programmer declares that it extends Employee, it gets all of the properties and methods declared by Employee. Great, less typing!

Rather than redefine everything that Employee has already defined, we use the super keyword to access it. For example, to access the constructor of the Employee class, we can do the following:

class Programmer extends Employee {
      Programmer (String n, int i, String l) {
         super (n,i);
         String language_prefernce = l;
      }
}

Suppose Employee defined a method pay(). Then suppose that the Programmer class overrode the Employee pay() method by implementing its own pay() method. How would you get to the Employee pay() method from Programmer?

If you called Programmer.pay(), you would get Programmer.pay() not Employee.pay().

Well, to get to Employee.pay() you need to use super.pay().

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