Java Reference
In-Depth Information
The output proves that the constructor of the subclass invokes the base class's no-arg constructor.
The Java compiler changed
ClassB
's constructor to the following:
public ClassB(String title) {
super();
System.out.println("Class B constructor ");
}
The keyword
super
represents an instance of the direct superclass of the current object. Since
super
is called from an instance of subclass,
super
represents an instance of
ClassA
. You can explicitly
call the parent's constructor from a subclass's constructor by using the
super
keyword, but
super
must be the first statement in the constructor. Using the
super
keyword is handy if you want another
constructor in the superclass to be invoked.
public ClassB(String title) {
super(title);
System.out.println("Class B constructor ");
}
Polymorphism
Polymorphism
is one of the most important principles of OO programming and as such is the heart
of OOP. Polymorphism refers to how an object in Java can take on many forms. To understand
how polymorphism works in Java, let's look at an example of a class that extends another class
(see Figure
A-2
).
Figure A-2.
Extending a class
