Problem with Inheritance

[John2find]

Hi All,

I am learning Java and was trying to learn Class inheritance.

Class Heiarchy is as such:

SuperClassEx
>> feilds >> length ,breadth
>> Methods > area

SubClassEx
>> feilds >height + superClass feilds
>>Method >> Volume

DemoCInheritance class
Contains main and let me see how inheritance work

But when I run this program I am getting error:

C:\Java Tuts\Complete Java Ref\Inheritance>javac DemoInheritance.java
.\SubClassEx.java:8: error: constructor SuperClassEx in class SuperClassEx cannot be applied to given types;
{
^
required: int,int
found: no arguments
reason: actual and formal argument lists differ in length
1 error

My codes are :

class SuperClassEx
{
//Instance Variable 
int length; //length
int breadth; //breadth

//Constructor>>Overloading (Y/N) >> N
SuperClassEx(int p, int q )
{
length=p;
breadth=q;
}

//Methods>>
int area()
{
return length*breadth;
}//Area-method end

}//Class end

class SubClassEx extends SuperClassEx
{
//feilds
int height;

//Constructor
SubClassEx(int p,int q,int r)
{
length=p;
breadth=q;
height=r;
}//Constructor end

//
int volume()
{
return length*breadth*height;
}//volume method end

}

class DemoInheritance
{
public static void main(String[] args)
{
//object decleration
SuperClassEx supObj= new SuperClassEx(2,4);
SubClassEx subObj= new SubClassEx(3,5,6);


//Business logics
System.out.println("Area: "+ supObj.area());
System.out.println("Volume: "+ subObj.volume());

}//main end
}//Class end

Thank you everyone for helping

[gimbal2]

Yes you get an error, because your code is wrong and the error explains crystal clear what is wrong.

Hint: you are missing a usage of the 'super' keyword somewhere.

[John2find]

Yes you get an error, because your code is wrong and the error explains crystal clear what is wrong.

Hint: you are missing a usage of the 'super' keyword somewhere.

Yeah Super key word can be used, but I have followed an example from book >> Complete reference for Java ,Herbert Schidt, which uses same logic as I used , but that works.

I wanted to know why my logic is not working or what is wrong with my code.

The code form the book is below:

// This program uses inheritance to extend Box.
class Box {
double width;
double height;
double depth;
// construct clone of an object
Box(Box ob) { // pass object to constructor
width = ob.width;
height = ob.height;
depth = ob.depth;
}
// constructor used when all dimensions specified
Box(double w, double h, double d) {
width = w;
height = h;
depth = d;
}
// constructor used when no dimensions specified
Box() {
width = -1; // use -1 to indicate
height = -1; // an uninitialized
depth = -1; // box
}
// constructor used when cube is created
Box(double len) {
width = height = depth = len;
}
// compute and return volume
double volume() {
return width * height * depth;
}
}
// Here, Box is extended to include weight.
class BoxWeight extends Box {
double weight; // weight of box

// constructor for BoxWeight
BoxWeight(double w, double h, double d, double m) {
width = w;
height = h;
depth = d;
weight = m;
}
}
class DemoBoxWeight {
public static void main(String args[]) {
BoxWeight mybox1 = new BoxWeight(10, 20, 15, 34.3);
BoxWeight mybox2 = new BoxWeight(2, 3, 4, 0.076);
double vol;
vol = mybox1.volume();
System.out.println("Volume of mybox1 is " + vol);
System.out.println("Weight of mybox1 is " + mybox1.weight);
System.out.println();
vol = mybox2.volume();
System.out.println("Volume of mybox2 is " + vol);
System.out.println("Weight of mybox2 is " + mybox2.weight);
}
}

[PhHein]

That's the difference between a good Java book and a Schildt book ;)
In the book's example the super class has a default constructor which is implicitly called (of course without the book mentioneing it, or you haven't read carefully).

You should explicitly call super(.....) in your subclass' constructor.

[gimbal2]

// constructor used when no dimensions specified
Box() {

That's the difference between your code and this example; the example in the book has a 'default constructor' with no arguments.

edit: ninja'd!

That's the difference between a good Java book and a Schildt book ;)

Do I read that as "Schildt books suck by definition"?

[Tolls]

That's a dreadful example of inheritance...but in any case Schildt has a no argument constructor for Box, which you do not.

Dear god...the more I look at that the more my eyes bleed.
Can I recommend not using that book?

ETA: And how long exactly did I have this page open before replying?
Even a tortoise could have ninja'ed me on that delay.

[John2find]

Thank you guys for replying.

SO do it mean that its mandate to use Super or either a no variable constructor in this case ?
And their is no other reason why I am getting this error ?

[John2find]

Your replies helped me do some research & I came across this statement:

Note: If a constructor does not explicitly invoke a superclass constructor, the Java compiler automatically inserts a call to the no-argument constructor of the superclass. If the super class does not have a no-argument constructor, you will get a compile-time error. Object does have such a constructor, so if Object is the only superclass, there is no problem.

Using the Keyword super (The Java™ Tutorials > Learning the Java Language > Interfaces and Inheritance)

Thank you all for your time and effort.

And my program is working fine with modification in SubClass code:

class SubClassEx extends SuperClassEx
{
//feilds
int height;

//Constructor
SubClassEx(int p,int q,int r)
{
super(p,q);
height=r;
}//Constructor end

//
int volume()
{
return length*breadth*height;
}//volume method end

}

[PhHein]

Do I read that as "Schildt books suck by definition"?

More or less, yes. I haven't seen one that didn't suck.

[gimbal2]

Noted. Boy the list of no-no books keeps growing.

[Tolls]

This is the classic list of mistakes in Schildt's C++ reference.

And from what I've heard the Java one is really no better, though there doesn't seem to have been the same point by point listing of them.

[JosAH]

This is the classic list of mistakes in Schildt's C++ reference.

And from what I've heard the Java one is really no better, though there doesn't seem to have been the same point by point listing of them.

A book by Herbert Schildt is only good in the fire place; those books are a waste of trees.

kind regards,

Jos