dan exam doubt 7

[amit taneja]

hi in one question..

class GFC211 {} class GFC212 extends GFC211 {}
class GFC213 extends GFC212 {
static void m(GFC211 x, GFC211 y) {System.out.print("GFC211,GFC211");}
static void m(GFC211 x, GFC212 y) {System.out.print("GFC211,GFC212");}
static void m(GFC212 x, GFC211 y) {System.out.print("GFC212,GFC211");}
static void m(GFC212 x, GFC212 y) {System.out.print("GFC212,GFC212");}
static void m(GFC211 x, GFC213 y) {System.out.print("GFC211,GFC213");}
public static void main(String[] args) {
GFC213 gfc213 = new GFC213(); m(gfc213, gfc213);
}}

What is the result of attempting to compile and run the program?

a. Prints: GFC211,GFC211
b. Prints: GFC211,GFC212
c. Prints: GFC212,GFC211
d. Prints: GFC212,GFC212
e. Prints: GFC211,GFC213
f. Compile-time error
g. Run-time error
h. None of the above


its answer is "f"

and its explanation is
The method invocation expression, m(gfc213, gfc213), is ambiguous; because, no applicable method is more specific than all of the others. Method m(GFC212 x, GFC212 y) is more specific than m(GFC212 x, GFC211 y), because any invocation of m(GFC212 x, GFC212 y) could also be handled by m(GFC212 x, GFC211 y) without causing a compile-time type error. However, some invocations of m(GFC212 x, GFC212 y) can not be handled by m(GFC211 x, GFC213 y), so m(GFC212 x, GFC212 y) is not more specific than m(GFC211 x, GFC213 y). Furthermore, not all invocations of m(GFC211 x, GFC213 y) could be handled by m(GFC212 x, GFC212 y)

...

so is that means that method will be choose that is most specific as compare to all other given types... and that will satisfy "is a" relation ship of every class inheritance hirarchy.?

pls do give comments and make the explnation easy ....

regards,
[ February 12, 2005: Message edited by: amit taneja ]

[Animesh Shrivastava]

Amit,
Read the JLS 15.12.2.2 Choose the Most Specific Method explanation, here they have given a detailed explanation to go about chosing the most specific method.
The main points to be noted in the explanation are:

The informal intuition is that one method declaration is more specific than another if any invocation handled by the first method could be passed on to the other one without a compile-time type error.

If there is exactly one maximally specific method, then it is in fact the most specific method; it is necessarily more specific than any other method that is applicable and accessible

Now here is the answer to ur question:

1) We invoke the method m(gfc213, gfc213)
2) Suppose we take up the method "static void m(GFC211 x, GFC213 y)...."
U can see that invocation handled by m(GFC211 x, GFC213 y) can also be passed to the methods:
:- m(GFC211 x, GFC212 y)
:- m(GFC211 x, GFC211 y)
without giving any compilation errors
3) Suppose we take up the method "static void m(GFC212 x, GFC212 y)..."
Again here we can see that invocation handled by m(GFC212 x, GFC212 y) can also be passed to the methods:
:- m(GFC212 x, GFC211 y)
:- m(GFC212 x, GFC211 y)

Now according to the first point i posted from JLS

The informal intuition is that one method declaration is more specific than another if any invocation handled by the first method could be passed on to the other one without a compile-time type error.

According to the statement above, there are two most specifc methods,but we require only 1, so this results in compilation errors which says ambiguous methods

Now i change ur program a bit:
----------------------------------------------------
class GFC211 {} class GFC212 extends GFC211 {}
class GFC213 extends GFC212 {
static void m(GFC211 x, GFC211 y) {System.out.print("GFC211,GFC211");}
static void m(GFC211 x, GFC212 y) {System.out.print("GFC211,GFC212");}
static void m(GFC211 x, GFC213 y) {System.out.print("GFC211,GFC213");}
public static void main(String[] args) {
GFC213 gfc213 = new GFC213(); m(gfc213, gfc213);
}
--------------------------------------------------------
1) We invoke the method m(gfc213, gfc213)
2) Suppose we take up the method "static void m(GFC211 x, GFC213 y)...."
U can see that invocation handled by m(GFC211 x, GFC213 y) can also be passed to the methods:
:- m(GFC211 x, GFC212 y)
:- m(GFC211 x, GFC211 y)
without giving any compilation errors
3) Suppose we take up the method "static void m(GFC211 x, GFC212 y)..."
U can see that the invocation handled by m(GFC211 x, GFC212 y) cannot be passed to any other method
4) Suppose we take up the method "static void m(GFC211 x, GFC211 y..."
Again here also U can make out that the invocation handled by m(GFC211 x, GFC211 y) cannot be passed to any other method

So, from these points u can figure out that there is only one most specific method i.e. m(gfc213, gfc213) and so no compilation errors

I hope this makes u clear of ur doubt,
Anyone pelase let me know whether i am making sense or not, do tell me if at all i am wrong at any place

Thanks