On Apr 19, 9:06 pm, Eric <[email protected]> wrote:
> As I've been told, one should not declare parameters to client GWT
> code as interfaces like List<T>.
AFAICT it only applies to things that'll go through GWT-RPC calls
(because you might be sent any implementation of List<T>, GWT has to
generate the code for all known implementation so it can deserialize
them; the more you're explicit the less code it involves: specifying
AbstractList<T> would only generate serialization/deserialization code
for classes that inherit AbstractList<T>, ArrayList<T> only
ArrayList<T> itself (as it's a concrete class) and those that extend
ArrayList<T> and so on)
> The typical way to write a JavaBean
> containing a list is this:
>
> public class Frames {
> private List<Integer> frames = new ArrayList<Integer>();
> public void setFrames(List<Integer> frames) {
> this.frames = frames;
> }
> // ...
>
> }
>
> Google recommends that GWT classes use the specific ArrayList<T>
> instead, because these declarations will force GWT to generate code
> for LinkedList<T> and every other implementation of List.
>
> However, this runs into another problem: encapsulation. The class
> Frames2 is unsafe:
>
> public class Frames2 {
> private ArrayList<Integer> frames = new ArrayList<Integer>();
> public void setFrames(ArrayList<Integer> frames) {
> this.frames = frames;
> }
> public ArrayList<Integer> getFrames() {
> return frames;
> }
>
> }
>
> It suffers from aliasing, as setting the frames instance variable
> allows the setter to interfere with the internals of the Frames2
> instance:
>
> Frames2 frames = new Frames2();
> ArrayList<Integer> myScoreLine = new ArrayList<Integer>();
> myScoreLien.add(9);
> frames.setFrames(myScoreLine);
> myScoreLine.set(0, 30); // CHEAT!
I can't tell the difference with your Frame class above:
Frames frames = new Frames();
// using a linked list just to change the scenario a bit
List<Integer> myScoreLine = new LinkedList<Integer>();
myScoreLine.add(9);
frames.setFrames(myScoreLine);
myScoreLine.set(0, 30); // CHEAT!
And how is it different from this code:
Frames2 frames = new Frames2();
List<Integer> myScoreLine = frames.getFrames();
myScoreLine.add(9);
// ...
myScoreLine.set(0, 30);
> Usually, one copies the lists on both setting and getting:
>
> public class Frames3 {
> private ArrayList<Integer> frames = new ArrayList<Integer>();
> public void setFrames(ArrayList<Integer> frames) {
> this.frames = new ArrayList<Integer>(frames);
> }
> public ArrayList<Integer> getFrames() {
> return new ArrayList<Integer>(frames);
> }
>
> }
>
> This solves the aliasing problem, but is slow. Perhaps I should not
> vend the list but provide access to its members. I probably should
> provide a way of getting all the list's members at once:
>
> public class Frames4 {
> private ArrayList<Integer> frames = new ArrayList<Integer>();
> public void addFrame(int score) {
> frames.add(score);
> }
>
> public Iterator<Intgeer> frames() {
> return frames.iterator();
> }
>
> }
How about:
public class Frames5 {
private ArrayList<Integer> frames = new ArrayList<Integer>();
public List<Integer> getFrames() {
return frames;
}
// no setter
}
and manipulating the list returned by getFrames() directly?
(see code sample above)
And if you want to add a bunch the frames:
frames.getFrames().addAll(myScoreLine);
eventually preceded by a call to frames.getFrames().clear() if you
want to replace the list. Sure this will be a bit inefficient (just as
if you copied the list on a setter), but at least it's made clear from
the caller's POV.
> Will this force GWT to generate code for every Iterator<E> in my
> system?
No, because GWT-RPC looks at fields, and your field is an
ArrayList<T>.
> I probably am programming with too much paranoia, of course.
Probably yes, moreover if you're not developing a library but an
application, where you control how your classes will be used from end
to end (within your team).
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