[Tiger79]

Hi,
I am struggling to retrieve what kind of Class a specific Layer is as used in the LayerManager API...
You can add Sprites to a LayerManager which become instances of Layer... Let's say I've got 3 custom Sprite derivates, classes Apple, Pear and Banana...
Lets say I add 3 Apples, 2, Pears and one Banana to the LayerManager...
How do I recognize a Pear object afterwards ? Lets say I want to remove only the Pear elements, how can that be achieved ?
Thanks in advance

[grahamhughes]

Tut... you know this! Sprites don't become instances of Layer... they already are (Sprite extends Layer). You can tell what specific type a Layer is, by using instanceof.

PHP Code:

Layer layer = layerManager.getLayerAt(n);
if (layer instanceof Sprite) {
    Sprite sprite = (Sprite)layer;
} 


Same deal with any subclass of Sprite you create.

PHP Code:

// remove Pears (Pear extends Sprite)
for (int i = 0; i < layerManager.getSize(); ) {
    Layer layer = layerManager.getLayerAt(i);
    if (layer instanceof Pear) {
        layerManager.remove(layer);
    } else {
        i++;
    }
} 


Or... is this what you're doing and it doesn't work? :)

Cheers,
Graham.

[Tiger79]

hehe,
thx Graham, it seems to work...
I was trying something a little too complex, my algorithm had one extra step in which instead of checking the retreived Layer to the needed class I first extracted the Object from the Layer and then did an instance of check, which actually didnt work :(

btw great if-else loop, already taking into account that if a Layer is removed the indexes within the layermanager decrease by one and thus no i++ ;)
another way (which I used till now) was the opposite, starting out at the end of the Layers stack (highest index) and working downwards...

anyways thx graham, as always u have been of great help :D

"Tut... you know this!".... ehhrmm... yeah I knew that, was just a momentary lapse of reason :P or the lack of that :D

[grahamhughes]

Originally Posted by Tiger79

another way (which I used till now) was the opposite, starting out at the end of the Layers stack (highest index) and working downwards...

Like this:

PHP Code:

// remove Pears (Pear extends Sprite), Tiger69-style
for (int i = layerManager.getSize() - 1; i >= 0; i--) {
    Layer layer = layerManager.getLayerAt(i);
    if (layer instanceof Pear) {
        layerManager.remove(layer);
    }
} 


Sure, this is better, since you avoid a (potentially expensive) method call on every iteration. For some reason, I have an "issue" with loops that go backwards. They seem to confuse me somehow. :)

Cheers,
Graham.

[Tiger79]

hehe well if we have to start tweaking and optimizing I would declare Layer layer outside the loop,so that it gets reutilized instead of creating a new Object every time :

PHP Code:

Layer layer;
for (int i = layerManager.getSize() - 1; i >= 0; i--) 
{
    layer = layerManager.getLayerAt(i);
    if (layer instanceof Pear) 
    {
        layerManager.remove(layer);
    }
} 


or doesnt that make any sense ?
U know how Pears are, they are pretty complex fruits ;) they come in different sizes, colors, countries, wights and so forth, and not to forget they come in bunch :P So like this I might be saving some memory by not instanciating a Layer object every time :P

hehe btw I dont know what u were thinking about when u typed this

Tiger69-style

hehe

[grahamhughes]

Originally Posted by Tiger79

or doesnt that make any sense ?

Again, tut! :) That's just a declaration, it doesn't allocate memory or run any code.

What it does do, is tell the compiler to allocate a slot in the stack-frame for the variable. Since that slot only needs to be reserved for the scope of the variable (it's enclosing {...}), nesting the declaration as deep as possible keeps the scope small, and allows the compiler to re-use the stack-frame slot for another local variable later.

PHP Code:

// this requires three slots, one for each i, j and o
public static void a() {
    int i;
    Integer j;
    Object o;
    for (i = 0; i < 10; i++) {
        j = new Integer(i);
        myVector.addElement(j);
    }
    if (System.currentTimeMillis() > 0) {
        o = new Object();
        myVector.addElement(o);
    }
}

// this only requires two slots
public static void b() {
    for (int i = 0; i < 10; i++) {
        // two slots in here, for i and j
        Integer j = new Integer(i);
        myVector.addElement(j);
    }
    if (System.currentTimeMillis() > 0) {
        // one slot here, for o
        Object o = new Object();
        myVector.addElement(o);
    }
} 


Re-using slots keeps the frame size down, so saves some memory. But not much. More usefully, the first four frame slots are accessed by shorter byte-code instructions, so you reduce the code size, and potentially increase performance. In an instance method, slot zero is always "this".

More significantly, you could also be preventing an object from becoming garbage, by keeping a reference to it in a variable you no longer want. By letting the variable fall out of scope, you increase the chances of the garbage collector will notice that the object is unreferenced, and release the memory.

So, as a rule, keep the declaration as close to the use as possible.

Originally Posted by Tiger79

I dont know what u were thinking about when u typed this

Someone called you that in another post... you were amused, but I think you secretly like it... :P

[Tiger79]

hhmmm,
ok, thanks for the insight, that about the slots I didn't know actually...
well thats another lesson, still enough to learn, and it's good to have people in this forum which can give such extended information regarding not only the devices but Java specifically as a language...