finalize() doubt

[Chaitanya Lele]

Hi all

I have this question from A Programmer's Guide to SCJP Certification by Khalid Mughal et al Q# 9.12

What scenario can be the result of compiling and executing the following code -

  
class MyClass
{
    private String str;
    MyClass(String str)
    {
        this.str = str;
    }
    public void finalize() throws Throwable
    {
        System.out.println(str);
        super.finalize();      
    }
    public void concat(String s)
    {
        this.str.concat(s);
    }
    public static void main(String[] args)
    {
        new MyClass("Hello").concat(" World");
        System.gc();
    }    
}
  

Select the 2 correct answers -
a - The program may print AB
b - The program may print BA
c - The program may print A
d - The program may print B
e - The program may print nothing

Their answer : c & e
My answer : a & e

Option e is definitely correct as the GC may not run at all.

However how the anonymous MyClass object is GCed before the concat() method is invoked is beyond me. Thus i think that option A is more plausible that option C

Also Option D is false since the anonymous String object " World" invokes the finalize() method of String class and not of the MyClass class. Correct me if I am wrong.

Thanks

[Ryan Beckett]

It may print 'Hello' and it may print nothing. To settle the latter, System.gc(); suggests that the JVM run the garbage collector but doesn't guarantee it will, so finalize() may never execute. And, the code wouldn't print 'Hello World' because the statement on line 16 doesn't reassign the result to the instance variable str. If str were a StringBuilder or StringBuffer it would concatenate.

Hope that helps.

[victor kamat]

Keep in mind that after the concat() executes the value of str is still "Hello" and not "Hello World".

So a & e are correct.

If in concat()
str = this.str.concat(s)

then c & e would be correct.

[Ruben Soto]

victor kamat wrote:Keep in mind that after the concat() executes the value of str is still "Hello" and not "Hello World".

So a & e are correct.

If in concat()
str = this.str.concat(s)

then c & e would be correct.

You got it right, like Ryan did, but I think you meant to say that c & e are correct now, and if concat() were changed then a & e would be correct.

[Chaitanya Lele]

Thanks all

Dumb error on my part
Forgot that Strings are immutable...

Got to avoid those kind of errors in the SCJP on Monday