Inheritance - Absolute Java - page 444

following definition of the no-argument constructor for the class HourlyEmployee
(with super omitted) is equivalent to the version we gave in Display 7.3:

public HourlyEmployee()
{
wageRate = 0;
hours = 0;
}
A derived class object has all the instance variables of the base class. These inherited
instance variables should be initialized, and the base class constructor is the most
convenient place to initialize these inherited instance variables. That is why you should
always include a call to one of the base class constructors when you define a constructor
for a derived class. As already noted, if you do not include a call to a base class constructor
(using super ), then the no-argument constructor of the base class is called automatically.
(If there is no no-argument constructor for the base class, that is an error condition.)

Call to a Base Class Constructor

Within the definition of a constructor for a class, you can use super as a name for a constructor
of the base class. Any invocation of super must be the first action taken by the constructor.
EXAMPLE
public SalariedEmployee(SalariedEmployee originalObject)
{

super (originalObject); //Invocation of base class
//constructor.
salary = originalObject.salary;
}

The this Constructor
When defining a constructor, it is sometimes convenient to be able to call one of the
other constructors in the same class. You can use the keyword this as a method name
to invoke a constructor in the same class. This use of this is similar to the use of super ,
but with this , the call is to a constructor of the same class, not to a constructor for the
base class. For example, consider the following alternate, and equivalent, definition of
the no-argument constructor for the class HourlyEmployee (from Display 7.3 ):

this

public HourlyEmployee()
{

this ("No name", new Date("January", 1, 1000), 0, 0);

}