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

[steve]

Patrick R. Michaud wrote:
> steve wrote:
>> Do we have any Java/Linux guru's in the house?
>> [...]
>>    Assume x and y are String variables with x="Hello" and y=null.
>> [...]
>>    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.  
> 
> Actually, the answer in this case is "(a) true".  IIRC,
> Strings in Java are immutable, and so the compiler optimizes
> the compiled code such that any repeated constants are
> folded to always refer to the same String object.  Thus, with
> 
>>      String x="Hello";
>>      String y="Hello";
> 
> The compiler generally optimizes this so that x and y both
> refer to the same String object.

OK - that's what I suspected - and it is indeed what happens
when you run it under Linux (compiling with 'javac' and running
with 'java' from the command line).  But if it's like C++, that
optimisation cannot be relied upon - so you actually have no
idea whether the answer is true or false...it's implementation
dependent.

> A useful test might be to try something like:
> 
>     String x = "Hello";
>     String y = "He" + "llo";
> 
> Here, the value of y ends up being the concatenation
> of two Strings, which is not likely to be the same object
> as x (unless Java is checking for reused String values at
> runtime).  In this case, the expression "x==y" should
> return false.

It does.

> At any rate, I think that your son has a case to be
> made here.  I'm sure that the teacher was intending to
> test the knowledge that in Java one cannot rely on the
> == to test two Strings for equal values, but that's not
> what the question is actually testing.  In Java, when
> x and y are Objects the expression x==y means "are 'x'
> and 'y' the same object", and with Strings the compiler
> is free to re-use existing String objects that result
> in the same value.  

Yep.

> So, the answer "(a) true" is equally as wrong as "(b) false",
> in that it's completely wrong to assume that x==y will
> give a repeatable answer based on the values of x and y
> (because x==y doesn't check for value equality).
> 
> The answer I would've given is
> 
>    (f) None of the above.  The expression x==y tests
>        whether x and y refer to a common String, not 
>        that two Strings have identicial values.  In order
>        to test for identical values, one uses x.equals(y) .

Tricky to do 'write in' answers on one of those online
multiple choice things!  You have 5 radio buttons...click
one!