INHERITANCE AND POLYMORPHISM - Introduction to Java Programming: Comprehensive Version - page 418

23 public Person() {
24 System.out.println( "(1) Performs Person's tasks" );
25 }
26 }

(1) Performs Person's tasks
(2) Invoke Employee's overloaded constructor
(3) Performs Employee's tasks
(4) Performs Faculty's tasks

The program produces the preceding output. Why? Let us discuss the reason. In line 3,
new Faculty() invokes Faculty 's no-arg constructor. Since Faculty is a subclass of
Employee , Employee 's no-arg constructor is invoked before any statements in Faculty 's
constructor are executed. Employee 's no-arg constructor invokes Employee 's second con-
structor (line 13). Since Employee is a subclass of Person , Person 's no-arg constructor is
invoked before any statements in Employee 's second constructor are executed. This process
is illustrated in the following figure.

Faculty() { Employee() {

this ( "(2) ..." );

Employee(String s) { Person() {

Performs Faculty's
tasks;

Performs Employee's
tasks;

Performs Employee's
tasks;

Performs Person's
tasks;

} } } }

Caution
If a class is designed to be extended, it is better to provide a no-arg constructor to avoid
programming errors. Consider the following code:

no-arg constructor

1 public class Apple extends Fruit {
2 }
3
4 class Fruit {
5 public Fruit(String name) {
6 System.out.println( "Fruit's constructor is invoked" );
7 }
8 }

Since no constructor is explicitly defined in Apple , Apple 's default no-arg constructor
is defined implicitly. Since Apple is a subclass of Fruit , Apple 's default constructor
automatically invokes Fruit 's no-arg constructor. However, Fruit does not have a
no-arg constructor, because Fruit has an explicit constructor defined. Therefore, the
program cannot be compiled.

Design Guide
If possible, you should provide a no-arg constructor for every class to make the class
easy to extend and to avoid errors.

no-arg constructor

11.3.3 Calling Superclass Methods
The keyword super can also be used to reference a method other than the constructor in the
superclass. The syntax is:

super .method(parameters);