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Message:

default constructors

Posted by Jody Brown on December 09, 2000 at 5:30 PM

Harpreet,

Constructors are called whenever a new object is created. If, when you are writing a class, you dont define a constructor, a default one is implied. For example:

Class MyClass() {
// this is my class
}

is interpreted as:

Class MyClass() {
MyClass() {
// this is the implied constructor
}
// this is my class
}

This constructor will be called when the following code is executed:

MyClass testClass = new MyClass();

However, if you declare a constructor that takes a parameter, the default constructor is no longer implied. For example:

Class MyClass() {
MyClass(int i) {
// constructor to do something with i
}
// this is my class
}

so the following code is now illegal:

MyClass testClass = new MyClass();

An int HAS to be passed as a parameter, because MyClass's only constructor requires one.

This is complicated when you use inheritance. In your example, in b's constructor, an implicit call is made to a's default constructor using super();
This is so a subclass can use its parent class's constructor to initalise inherited members.

You cant see this call to super() in your code, but it is there. Because b is attempting to call a's default constructor, which doesnt exist, it results in an error.

What you need to do is add a default constructor to class a, so class a reads along the lines of:

class a
{
a()
{}
a(int i)
{
System.out.println("In Class A");
}
}

Hope this is clear enough. I found this tricky to grasp when I first started using Java.

Good luck with Java,

Regards,

Jody