Re: Usefulness of "final" (Was: Re: Inserting In a List)
On Wed, 03 Apr 2013 16:44:45 -0700, markspace wrote:
On 4/3/2013 4:25 PM, Joerg Meier wrote:
Yours would be immutable with or without the final keyword.
No no no. This is my point. The final keyword has special semantics
associated with it in that particular case. It works a bit like the
volatile keyword: all writes to that point are made visible. In the
case of a private final field, the writes are made visible to ALL
THREADS in the system. THAT is what makes instances of the class
Stooges immutable.
Now I'm to being confused by you. final prevents any writes other than the
initial one. That initial write is not synchronized to other threads. If
you let an object instance get out that isn't fully constructed, then you
will get the usual synchornization issues, final or not. Don't believe me ?
Guess what this will print out:
public class FinalTest {
public static class Test {
public final String bla;
public Test() {
new Thread(new Runnable() {
public void run() {
try {
Thread.sleep(200);
} catch (final InterruptedException e) {}
System.out.println(bla);
try {
Thread.sleep(1000);
} catch (final InterruptedException e) {}
System.out.println(bla);
}
}).start();
try {
Thread.sleep(1000);
} catch (final InterruptedException e) {}
bla = "1234";
}
}
public static void main(final String[] args) {
final Test test = new Test();
}
}
If I misunderstood, and you believe that structural changes to the
ArrayList would be visible to all threads, then you are still wrong, but
you're going to have to write this test case as I restrict myself to one
test case per post ;)
That's why no synchronization is needed. Which is huge, conceptually.
I'll stop there because I think this may be the whole point of
misunderstanding here.
The mere use of final does not remove the need for synchronization. Nor
does the mere lack of it dictate a need. The reason synchronization is not
needed with proper immutability is an effect from what final does - because
it can only be assigned once, once it is you can then let everyone play
with it, because you dont NEED to worry about writes - there will be none.
Liebe Gruesse,
Joerg
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