some doubts,please explain

[aditi mudholkar]

Doubt 1:

1.class programmer{
2.programmer debug(){return this;}
3.}
4.class SCJP extends programmer{
5.//insert code here
6.}

Question: which inserted at line 5 will compile?

one of the answers to the question is
programmer debug(){return this;}

did not understand how did that happen as there is no overriding or overloading taking place.

Doubt 2:

1.class dog{}
2.class beagle extends dog{}
3.class kennel{
4.public static void main(String []args){
5.beagle b1=new beagle();
6.dog dog1=new dog();
7.dog dog2=b1;
8.//insert code here
9.}}

question:which inserted at line 8 will compile?
the answer to tis question was
beagle b2=(beagle)dog1;
beagle b3=(beagle)dog2;

did not understand this.Please help!!

Doubt 3:
The concept of var-arg is also not clear. i know it should come at the last in the method's argument.But could you please explain with an example.

Thanks.

[Gordon Brown]

Hi Aditi,

doubt 1: why do you say there's no overriding taking place?

doubt 2: When doing a downcast, all the compiler does is check that Dog is in Beagle's inheritance hierarchy.

Gordy

[Remko Strating]

doubt 3:

The following link has some varargs examples
VarArgs Example

[Katrin Perry]

There is a mistake in 1.
1. beagle b2=(beagle)dog1;
2. beagle b3=(beagle)dog2;

1. will cause a ClassCastException. It will compile OK because all the compalier is interested in when assessing casting is that the 2 classes are from the same hierarchy. (beagle and dog are) But at run time the real class of the object referenced by dog1 will be assessed. dog1 points to object of class dog. Can class dog be converted to beagle? NO, because it is trying to convert down to a subclass, rather than to superclass which is allowed.

2. Again no problem at compile time, as above. At run time JVM will see that in fact dog2 point to object of class beagle so the conversion is successfully done.

Regards
Katrin