Learn Java for Web Development

APPENDIX A: Introduction to Java 387

Constructors

When an object is created using a new operator, the constructors are called to set the initial state
of an object. A constructor declaration is comprised of the accessibility modifier followed by the
parameter list with the following declarations:

   Modifiers other than an accessibility modifier are not permitted in the

constructor header.

   Constructors cannot return a value and, therefore, do not specify a return type,

not even void, in the constructor header.

   The constructor name must be the same as the class name.

When no constructors are specified in a class, then an implicit default constructor, that is, an
implicit constructor without any parameters, is generated for the class by the compiler that
comprises a call to the superclass’s constructor. The compiler inserts this call to a superclass’s
constructor to ensure that the inherited state of the object is initialized. Listing A-10 illustrates a call
to an implicit default constructor.

Listing A-10. Implicit Default Constructor

class ClassA {
int i;
}
class ClassB {

ClassA var1 = new ClassA(); // (1) Call to implicit default constructor.
}

In the listing, the following implicit default constructor is called when a ClassA object is created
in ClassB:

ClassA() { super(); }

In Listing A-11, the class ClassA provides an explicit default constructor at line 4.

Listing A-11. Explicit Default Constructor

1. class ClassA {


2. int i ;


3. // Explicit Default Constructor:


4. ClassA() {


5. i = 1;


6. }


7. 
   


8. }


9. class ClassB {


10. // ...


11. ClassA var1 = new ClassA(); // Call of explicit default constructor.


12. }