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

[steve]

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!