Objective-C NSMutableArray not saving objects - objective-c

Question

I'm learning Objective-C (having come from Java) and I'm am attempting to program a small Cocoa app on OSX. I am thinking with my Java hat on and that's probably why this isn't working. Basically I am trying to get an object of type Car added to an NSMutableArray on a button click but although the rest of the code seems to work, nothing is being saved to the array. I can't figure out why - help is appreciated. Here's the code:
#import "AppDelegate.h"
#import "Car.h"
#import "PileInterface.h"
#interface AppDelegate ()
#property (weak) IBOutlet NSWindow *window;
#end
#implementation AppDelegate
PileInterface *myStack;
Car *car;
//initialise Stack
-(void)stack{
myStack = [[PileInterface alloc]init];
//initialise myStack in Pileinterface
[myStack initialise];
}
- (void)applicationDidFinishLaunching:(NSNotification *)aNotification {
//corrected code here to call the stack method above
//initialise myStack by calling stack method
[self stack];
}
- (void)applicationWillTerminate:(NSNotification *)aNotification {
// Insert code here to tear down your application
}
- (IBAction)addBtn:(NSButton *)sender {
car = [[Car alloc]init];
NSString *licence = [_txtField1 stringValue];
NSString *model = [_txtField2 stringValue];
NSString *colour = [_txtField3 stringValue];
if ([myStack isFull] == false) {
// add Car objects to car
//set licence
[car setLicence:licence];
// set model
[car setModel:model];
//set colour
[car setColour:colour];
//add to stack
[myStack push:car];
//append to textarea
NSString *message = licence;
[_txtArea setStringValue:message];
[_txtField2 setIntegerValue:[myStack size]];
} else {
NSString *message2 = #"Rejected. There are more than 4 cars currently for recycling!";
[_txtArea insertText:message2];
}
}
#end
Effectively Car is the object I want to get into myStack (which is a stack inside PileInterface, which also contains the NSMutableArray)
PileInterface implementation code here:
#import "PileInterface.h"
#implementation PileInterface
//arraylist of objects
NSMutableArray *list;
//initialise array
-(void)initialise{
list = [[NSMutableArray alloc]init];
}
-(void) push:(id) o{
[list insertObject:o atIndex:0];
}
-(id) pop{
if ([list count] > 0) {
NSObject *temp =[list objectAtIndex:0];
[list removeObjectAtIndex:0];
return temp;
} else {
return nil;
}
}
-(BOOL) isEmpty{
if ([list count] == 0) {
return true;
} else {
return false;
}
}
-(int) size{
return (int)[list count];
}
-(BOOL) isFull{
if ([list count] > 4) {
return true;
} else {
return false;
}
}
#end
Help Appreciated!