instanceOf explanation needed

[Suhas Wadadekar]

Pls explain the following:

interface Face(){}
class Bar implements Face{}
class Foo extends Bar{}

instance instanceOf what result my comments
Foo[] Foo, Bar, Face, Object false OK
Foo[1] Foo, Bar, Face, Object true How??

[ram gaurav]

Sir please write question in more detail as we does not understand that what are you trying to say exactly , i am not hurting you but please explain your question again.

Thanks
Regards
Gaurav

[Suhas Wadadekar]

Pls explain the following: This is from K & B book Chapter 3

interface Face(){}
class Bar implements Face{}
class Foo extends Bar{} //code ends here


Case 1:
instance: Foo[]
Foo[] instanceOf Foo(or Bar or Face or Object)
result: false
my comments: OK

Case 2:
instance: Foo[1]
Foo[1] instanceOf Foo(or Bar or Face or Object)
result: true
my comments: How? What difference does the number 1 make?

[Rajah Nagur]

Originally posted by Suhas Wadadekar:
Pls explain the following: This is from K & B book Chapter 3

interface Face(){}
class Bar implements Face{}
class Foo extends Bar{} //code ends here


Case 1:
instance: Foo[]
Foo[] instanceOf Foo(or Bar or Face or Object)
result: false
my comments: OK

Case 2:
instance: Foo[1]
Foo[1] instanceOf Foo(or Bar or Face or Object)
result: true
my comments: How? What difference does the number 1 make?

Foo[] is of type Array. It is an array of Foo Objects. The array of Foo Objects is not an instance of Foo/Bar/Face etc. So in the first case it prints false.

Foo[1] means the value inside the Foo array. Since this can hold only Objects of Foo type, Foo[1] will give an Object of Foo type. And Foo is an instance of Foo/Bar/Face etc. So this results in true.

Hope this helps

[Gowher Naik]

hi
i have facing same problem but u wrote that u understood case 1,plz can u explain case 1 by using example.

[Gowher Naik]

ok
what i understood by this line "Foo[] is of type Array. It is an array of Foo Objects. The array of Foo Objects is not an instance of Foo/Bar/Face etc. So in the first case it prints false."



Foo[] f=new Foo[]{new Foo()};
System.out.println(f instanceof Foo);//but this is compile time error

[Suhas Wadadekar]

thanks Rajah....that did explain the things....i don't know why...but the 3 rd chapter was a disaster for me.....

and gowher..... rajah provided the explanation to ur doubt too...do u still need any more explanation?

1. a Foo instance will pass the instanceOf test of Foo, Bar, Face, Object....
2. but Foo[] is an array type...it refers to an array and not to any instance of Foo, Bar, Face, Obeject
3. Hence Foo[] fails the instanceOf test

While in second case Foo[1] represents a Foo object
(just like: int[] array = new int[2]; where array[0] refers to an int value stored in the arry)
Hence it passes the instanceOf test....

[Aum Tao]

I thought, I would bring to your notice a minor error in the above discussion.

There is no instanceOf operator in Java. Its instanceof.

Many a times, students overlook this fact.

[Gowher Naik]

Hi Suhas
Does Foo[] fails the instanceOf test means this

Foo[] f=new Foo[]{new Foo()};
System.out.println(f instanceof Foo);//compile time error
Is i am Right?

[Suhas Wadadekar]

yes...the code does not compile because they are inconvertible types

Thanks Aum Tao for pointing out my negligence about the keyword instanceof
[ July 11, 2006: Message edited by: Suhas Wadadekar ]

[Amit Batra]

Yup as stated 'System.out.println(f instanceof Foo);' would print false for the above stated reasons. but 'System.out.println(f[0] instanceof Foo);' will compile and result in true.

[Aum Tao]

Yup as stated 'System.out.println(f instanceof Foo);' would print false for the above stated reasons.

It will result in a compile time error.