question from mughals book

[Raju Champaklal]

  What is the label of the first line that will cause compilation to fail in the following
program?
// Filename: MyClass.java
class MyClass {
public static void main(String[] args) {
MyClass a;
MySubclass b;
a = new MyClass(); // (1)
b = new MySubclass(); // (2)
a = b; // (3)
b = a; // (4)
a = new MySubclass(); // (5)
b = new MyClass(); // (6)
}
}
class MySubclass extends MyClass {}
Select the one correct answer.
(a) (1)
(b) (2)
(c) (3)
(d) (4)
(e) (5)
(f) (6)

  

the answer given is 4...but why not 6?

[Raju Champaklal]

ya maybe the because tthe first error is asked......crazy book :|

[Minhaj Mehmood]

Raju Champaklal wrote:

  What is the label of the first line that will cause compilation to fail in the following
program?
// Filename: MyClass.java
class MyClass {
public static void main(String[] args) {
MyClass a;
MySubclass b;
a = new MyClass(); // (1)
b = new MySubclass(); // (2)
a = b; // (3)
b = a; // (4) you are doing same work here which is done into label (6) as well.
a = new MySubclass(); // (5)
b = new MyClass(); // (6)
}
}
class MySubclass extends MyClass {}
Select the one correct answer.
(a) (1)
(b) (2)
(c) (3)
(d) (4)
(e) (5)
(f) (6)

  

the answer given is 4...but why not 6?

see the question he's is asking about "first line that will cause compilation".
but actually error will rise on both label 4 and 6 on compile time.

[Minhaj Mehmood]

there is a simple real life rule:

A child can be parent, but a parent can not be child!

[Raju Champaklal]

[T Vergilio]

I don't understand.

line 3:

a = b;

now both a and b refer to an object of type MySubclass.

line 4:

b = a;

reference b now refers to the same object which reference a refers to (i.e. the MySubclass instance).

or not?

Would I need a cast, because the compiler doesn't look at the object, it looks at the variable type?

[Ikpefua Jacob-Obinyan]

Chandella Montero wrote:Would I need a cast, because the compiler doesn't look at the object, it looks at the variable type?

Hello Chandella...Yes you got it right, the compiler checks the reference variable type so if you cast the code compiles as follows:

b = (MySubclass)a;

Regards

Ikpefua

[T Vergilio]

Cheers Ikpefua, it's all clear now

[Ikpefua Jacob-Obinyan]

@Chandella the cast is ONLY to resolve the problem in line 11 of the code, because line 13 has the SAME problem(even worse)...Now here is a complete version of the program MODIFIED to make it compile and execute without problems:

class MyClass {  
public static void main(String[] args) {  
MyClass a;  
MySubclass b;  
a = new MyClass();   
b = new MySubclass();   
a = b;   
b = (MySubclass)a;  // must cast here 
a = new MySubclass();  

if (a instanceof MySubclass){
b = (MySubclass)new MyClass(); // compiles but throws an exception
                                   //  at runtime but the instanceof prevents
		                               //  the line of code from executing 		                               
}
}  
}  
class MySubclass extends MyClass {} 

You can try it out yourself and you will see that this program as-it-stands compiles and runs without exceptions

Regards

Ikpefua

[T Vergilio]

That's a good way of preventing line 11 from executing, using the recommended if + instanceof test.

When I first did the exercise, I marked (f) too. Naïve...

[Ikpefua Jacob-Obinyan]

@Chandella...thats Java for you...It took me a long time to understand the concepts of inheritance-polymorphism-reference-variable-casting, you need to practice with a lot of coding, that is write lots of small test programs and write down your findings.

Cheers

Ikpefua.

[T Vergilio]

Thanks Ikpefua, you keep me motivated. I'm working through Mughal's book now and doing all the exercises: the programming ones are quite fun to do, if only the exam were like that (i.e. allowed you to make silly mistakes, then go and correct them, then try to compile again). Oh well, at least I am learning in the process. As you said, it takes time. That's why it's worth something in the end. If it wasn't so hard, do you think we would want it that much?