Reg Arrays

[Vishnu Munnangi]

This a question from one of the mock exams.
Please go through the code...

What will be the result of compiling and running the given program?
Select one correct answer.

1. class Q55
2. {
3. public static void main(String[] args)
4. {
5. int[] a={11,12,13,14};
6. int[] b={0,1,2,3};
7. System.out.println(a[(a=b)[3]]);
8. }
9. }
1. Compile time error at line number 7.
2. Runtime error at line number 7.
3. Program will compile correctly and print 3 when executed.
4. Program will compile correctly and print 14 when executed.
The answer given is 4.
Can any one explain me how 14 is the output.

Thanks in advance.

[Barry Gaunt]

This is a tricky question, and it's hard to explain how it works.

When evaluating a[(a=b)[3]] the assignment to a (that is, the a=b) does not affect the dereferencing of the outer array (the outermost a[]). The expression is being evaluated in the same way as a[b[3]] would be.

It's a good idea to inspect the output of the "javap -c -l -verbose Q55" command . You can see how the stack machine evaluator is processing that statement.

You will not see a question as tricky as that on the SCJP 1.4 examination.
[ October 22, 2004: Message edited by: Barry Gaunt ]

[kapil munjal]

Hi Barry,

Thanks for such a great use of javap.exe.

But I am facing a problem in windows me, that is, I can not see the whole output of the command given by you in your reply.

Do you know any such tool, which can show the whole output???

Thanks
Kaps

[Barry Gaunt]

Kaps, you should be able to do "javap -c -l -verbose Q55 > somefile.txt"

The you can read the file with your favourite text editor.

Or you can "pipe" it into more:
"javap -c -l -verbose Q55 | more"