[NTLUG:Discuss] Java/Linux versus school teacher.

[Chris Albertson]

I don't like the way the teacher is approaching the set of questions.
You are exactly right in your analysis below. I'm not really sure why 
your test program decided to return true. I certainly would not trust 
that particular implementation of string comparison.
The teacher really should not be testing in this way though, in my 
opinion. String.equals is a much better, standard, and reliable method 
for comparing strings.
So, x.equals(y) is much better... provided x!=null of course.

I don't know what to do in your instance. Maybe run the test program for 
the teacher and demonstrate the unpredictability of this method? The 
correct answer is not in the list of choices. The "best" answer is 
indeed b though.
As far as java being a crappy language because of it's ability to 
produce inconsistent results... this is a weird case. String is not a 
privative. It's a class in and of itself. Using == on it is just never a 
good idea.

Chris

steve wrote:
> Do we have any Java/Linux guru's in the house?
>
> My son is learning Java in high school - inevitably they use
> Windows machines - but it's Java - which is portable - so it
> doesn't matter a whole lot.  So my kid can use Linux to do
> his homework without problems.
>
> ...mostly...
>
> The other day, they had a Java Exam. The first two questions
> go like this:
>
>    Assume x and y are String variables with x="Hello" and y=null.
>
>    1) The result of (x==y) is
>        a) true
>        b) false
>        c) a syntax error
>        d) a run-time error
>        e) x being set to the value null.
>
> I think we can all agree that the answer is (b)false.
>
>    2) It the operation y="Hello"; is performed then the result
>       of (x==y) is:
>        a) true
>        b) false
>        c) x and y becoming aliases
>        d) x being set to the value null
>        e) a run-time error
>
> Question 2 is a problem.  My son said "(a) true" but the
> teacher says that the answer is "(b)false" - and whilst
> I'm not a Java expert (I'm a C++ person) I can see why.  Even
> though both x and y are set to "Hello", they don't refer to
> the same String object and the '==' operator doesn't compare
> the contents of the objects - it only tests whether the same
> object is being referenced on both sides.  (At least according
> to "Java2 for Dummies" and my C++ intuition).
>
> When my son makes mistakes like that, I encourage him to write
> a little program to convince himself.  The trouble is that when
> we tested it by writing a little Java program and running it
> under Linux:
>
>      String x="Hello";
>      String y="Hello";
>      if ( x==y )
>        println ( "true" ) ;
>      else
>        println ( "false" ) ;
>
> The answer comes out "true"...oh-oh!
>
> This suggests that some kind of optimisation is going on inside
> the Java compiler and the two instances of the "Hello" string are
> being combined into one single object or something.
>
> Clearly then the teacher is wrong and the correct answer is "either
> true or false depending on the implementation"...but I'm having
> a hard time believing that Java is that ill-specified.  Is it in
> fact the case that Java under Windows would also have reported
> 'true'?
>
> So should I complain to the teacher?  Stop using Linux for this
> stuff? (over my cold, dead body!)...Or is there something subtle
> that I'm missing here?
>
>
> Thanks!
>
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