hank1
asked on
Java exam question, casting
Here's a test question found on http://www.geocities.com/skmajji/Main.html
The anwer is a LIE! :-) Why is the correct answer D. Did the
program set the base clase object reference to the extended class?
Thanks
: class Test
2: {
3: static void show()
4: {
5: System.out.println("Show method in Test class");
6: }
7: }
8:
9: public class Q2 extends Test
10: {
11: static void show()
12: {
13: System.out.println("Show method in Q2 class");
14: }
15: public static void main(String[] args)
16: {
17: Test t = new Test();
18: t.show();
19: Q2 q = new Q2();
20: q.show();
21:
22: t = q;
23: t.show();
24:
25: q = t;
26: q.show();
27: }
28: }
A) prints "Show method in Test class"
"Show method in Q2 class"
"Show method in Q2 class"
"Show method in Q2 class"
B) prints "Show method in Test class"
"Show method in Q2 class"
"Show method in Test class"
"Show method in Test class"
C) prints "Show method in Test class"
"Show method in Q2 class"
"Show method in Test class"
"Show method in Q2 class"
D) Compilation error.
The anwer is a LIE! :-) Why is the correct answer D. Did the
program set the base clase object reference to the extended class?
Thanks
: class Test
2: {
3: static void show()
4: {
5: System.out.println("Show method in Test class");
6: }
7: }
8:
9: public class Q2 extends Test
10: {
11: static void show()
12: {
13: System.out.println("Show method in Q2 class");
14: }
15: public static void main(String[] args)
16: {
17: Test t = new Test();
18: t.show();
19: Q2 q = new Q2();
20: q.show();
21:
22: t = q;
23: t.show();
24:
25: q = t;
26: q.show();
27: }
28: }
A) prints "Show method in Test class"
"Show method in Q2 class"
"Show method in Q2 class"
"Show method in Q2 class"
B) prints "Show method in Test class"
"Show method in Q2 class"
"Show method in Test class"
"Show method in Test class"
C) prints "Show method in Test class"
"Show method in Q2 class"
"Show method in Test class"
"Show method in Q2 class"
D) Compilation error.
>> q = t;
Incompatible types
found : Test
required: Q2
>> Why is the correct answer D
It's not because a Mercedes is a Car, that every Car is a Mercedes
Mercedes = Q2, Car = Test
Incompatible types
found : Test
required: Q2
>> Why is the correct answer D
It's not because a Mercedes is a Car, that every Car is a Mercedes
Mercedes = Q2, Car = Test
>>The anwer is a LIE! :-)
Says who?! Have a look in any IDE like Eclipse and it will tell you all you need to know.
Try to run it and then change q = t; to q = (Q2) t; and try again
Says who?! Have a look in any IDE like Eclipse and it will tell you all you need to know.
Try to run it and then change q = t; to q = (Q2) t; and try again
ASKER
I guess I assume when you did this
t = q;
they are now the same thing
later when
q = t
isn't that just saying Apples = Apples?
t = q;
they are now the same thing
later when
q = t
isn't that just saying Apples = Apples?
ASKER
if (t instanceof Test) System.out.println("T is a TEST object");
if (t instanceof Q2) System.out.println("T is a Q2 object");
t = q;
if (t instanceof Test) System.out.println("after T is a TEST object");
if (t instanceof Q2) System.out.println("after T is a Q2 object");
AFter the assignment t is a q2.
and this is true
if (t == q) System.out.println("Object
javac error: (buy why ... they are the same?
Q2.java:32: incompatible types
found : Test
required: Q2
q = t;
^
1 error
if (t instanceof Q2) System.out.println("T is a Q2 object");
t = q;
if (t instanceof Test) System.out.println("after T is a TEST object");
if (t instanceof Q2) System.out.println("after T is a Q2 object");
AFter the assignment t is a q2.
and this is true
if (t == q) System.out.println("Object
javac error: (buy why ... they are the same?
Q2.java:32: incompatible types
found : Test
required: Q2
q = t;
^
1 error
They aren't the same...
Image "Test" is called "Humans" and "Q2" is called "Men"
Men are humans, so you can set a Human variable to contain a Man
Not all Humans are Men though, so you cannot store a Human object in a Man variable...
What if it were an instance of a woman? Things would go horribly wrong (seen the Fly?) ;-)
Hope this goes some way to explain it...
Everyone else deserves the points for this Q more than me, as I am just repeating what they have said...
Tim
Image "Test" is called "Humans" and "Q2" is called "Men"
Men are humans, so you can set a Human variable to contain a Man
Not all Humans are Men though, so you cannot store a Human object in a Man variable...
What if it were an instance of a woman? Things would go horribly wrong (seen the Fly?) ;-)
Hope this goes some way to explain it...
Everyone else deserves the points for this Q more than me, as I am just repeating what they have said...
Tim
>> Image "Test" is called "Humans" and "Q2" is called "Men"
IMAGINE "Test" is called "Humans" and "Q2" is called "Men"
IMAGINE "Test" is called "Humans" and "Q2" is called "Men"
ASKER
OK a different question which will help pound this past the rock here
in my skull.
Why is this true?
if (t == q) System.out.println("Object
in my skull.
Why is this true?
if (t == q) System.out.println("Object
that just means that both variables contain exactly the same object...
so assuming you have done:
t = q ;
then both t and q contain the same object (an instance of Q2)
so assuming you have done:
t = q ;
then both t and q contain the same object (an instance of Q2)
It's like you can do:
Object o = new String( "Woot!" ) ;
but you cannot do:
String o = new Object() ;
as String extends Object, so is considered more specific and so cannot contain the more general "Object" instance...
that's probably a good way of thining of it...
Imagine you have:
class Chest
{
// get the weight of this object...
public int getMass() ;
}
class Torso extends Chest
{
// return the number of limbs
public int numberofLimbs() ;
}
class EntireBody extends Torso
{
// get the eyecolor
public Color getEyeColor() ;
}
If you have:
Chest c = new Chest() ;
that's fine...
and
EntireBody b = new EntireBody() ;
is fine too...
and you can do:
c = b ;
as c is more general (and you can call getMass() on an EntireBody class)
but you can't do:
b = c ;
as c is a Chest object, so "getEyeColor()" makes no sense (and cannot be called) on b.
Hmmm...actually...that's kinda badly put too ;-)
hope it makes a bit of sense ;-)
Tim
Object o = new String( "Woot!" ) ;
but you cannot do:
String o = new Object() ;
as String extends Object, so is considered more specific and so cannot contain the more general "Object" instance...
that's probably a good way of thining of it...
Imagine you have:
class Chest
{
// get the weight of this object...
public int getMass() ;
}
class Torso extends Chest
{
// return the number of limbs
public int numberofLimbs() ;
}
class EntireBody extends Torso
{
// get the eyecolor
public Color getEyeColor() ;
}
If you have:
Chest c = new Chest() ;
that's fine...
and
EntireBody b = new EntireBody() ;
is fine too...
and you can do:
c = b ;
as c is more general (and you can call getMass() on an EntireBody class)
but you can't do:
b = c ;
as c is a Chest object, so "getEyeColor()" makes no sense (and cannot be called) on b.
Hmmm...actually...that's kinda badly put too ;-)
hope it makes a bit of sense ;-)
Tim
ASKER
So why the beef with ( since previously they did a t = q )
(ps - would this also schedule the now defunct t object
for garbage collection?)
q = t; ? They are the same. Right?
The compiler want
q = (Q2)t; Why?
ps - thanks for suffering a knuckle head. I still don't get this. :-)
(ps - would this also schedule the now defunct t object
for garbage collection?)
q = t; ? They are the same. Right?
The compiler want
q = (Q2)t; Why?
ps - thanks for suffering a knuckle head. I still don't get this. :-)
ASKER CERTIFIED SOLUTION
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>> ps - thanks for suffering a knuckle head. I still don't get this. :-)
No worries... it's kinda hard to explain ;-) but it will click, believe me :-D
No worries... it's kinda hard to explain ;-) but it will click, believe me :-D
If it is, I think the compilation error you get will explain it ;°)