Open the VariableTest project from the previous chapter.

Click the MyClass.h file inside the Classes folder, and modify the code as follows:

#import <Cocoa/Cocoa.h>

@interface MyClass : NSObject {

@private
    int position;
    NSString *name;

}

@property (assign) int position;
@property (retain) NSString *name;

-(int)changePosition: (int)myPosition;

@end

Choose File

Click the MyClass.m file inside the Classes folder, and modify the code as follows:

#import "MyClass.h"

@implementation MyClass

@synthesize position;
@synthesize name;

- (id)init {
    if ((self = [super init])) {
        // Initialization code here.

    }

    return self;
}

- (void)dealloc {
    // Clean-up code here.
    [name release];

[super dealloc];
}

-(int)changePosition: (int)myPosition
{
    int newPosition;
    newPosition = self.position + myPosition;
    return newPosition;
}

@end

The changePosition method takes its own property (position) and adds it to the myPosition variable. Normally when you need to use an object's property, you must state the object's name, but since the object is manipulating its own property, you can just replace its name with self instead.

Choose File

Click the VariableTestAppDelegate.m file stored inside the Classes folder, and modify the code in the VariableTestAppDelegate.m file as follows:

#import "VariableTestAppDelegate.h"
#import "MyClass.h"

@implementation VariableTestAppDelegate

@synthesize window;

- (void)applicationDidFinishLaunching:(NSNotification *)aNotification {
    MyClass *testObject = [[MyClass alloc] init];
    testObject.position = 45;
    NSLog (@"The object's current position = %i", testObject.position);
    testObject.position = [testObject changePosition:10];
    NSLog (@"The new object position = %i", testObject.position);
    [testObject release];
}

@end

The applicationDidFinishLoading method creates an object called testObject, sets its property (position) to 45, prints out this position property, calls the changePosition method, and assigns the changePosition method's result to the position property, which gets printed again so you can see the result.

Choose File

Click the Build and Run button, or choose Build

Quit the program by clicking the Stop button or choosing Product

Choose Run

2010-09-11 21:30:26.157 VariableTest[4567:a0f] The object's current position = 45
2010-09-11 21:30:26.160 VariableTest[4567:a0f] The new object position = 55

Open the VariableTest project from the previous section.

Choose File

Click Cocoa under Mac OS X in the left pane of the template dialog box.

Click the Objective-C class icon, and click the Next button. A Save As dialog box appears.

Choose the Classes folder in the Group pop-up menu, and type NewClass for your class name. Then click the Save button. The NewClass.h and NewClass.m files appear in your Classes folder.

Click the NewClass.h file, and modify the code as follows:

#import <Cocoa/Cocoa.h>
#import "MyClass.h"

@interface NewClass : MyClass {
@private

}

@end

The two main changes you need to make in the NewClass.h file are to add the #import "MyClass.h" line and define NewClass to inherit from MyClass through the @interface NewClass :MyClass line.

Choose File

Click the VariableTestAppDelegate.m file, and modify the code as follows:

#import "VariableTestAppDelegate.h"
#import "MyClass.h"
#import "NewClass.h"

@implementation VariableTestAppDelegate

@synthesize window;

- (void)applicationDidFinishLaunching:(NSNotification *)aNotification {
    MyClass *testObject = [[MyClass alloc] init];
    testObject.position = 45;
    NSLog (@"The object's current position = %i", testObject.position);

testObject.position = [testObject changePosition:10];
    NSLog (@"The new object position = %i", testObject.position);
    [testObject release];
    NewClass *newObject = [[NewClass alloc] init];
    newObject.position = 32;
    NSLog (@"The object's current position = %i", newObject.position);
    newObject.position = [newObject changePosition:10];
    NSLog (@"The new object position = %i", newObject.position);
    [newObject release];
}

@end

To make the VariableTestAppDelegate.m file capable of accessing the code stored in the NewClass.h file, you must import it using the #import "NewClass.h" line. The last five lines in the applicationDidFinishLaunching method create a newObject, based on NewClass. At this point, the NewClass file does not contain any code since it inherits all its code from the MyClass file.

To verify that NewClass really does inherit its code from the MyClass file, this code then assigns the value 32 into the position property of newObject, which it then prints out. Next, it runs the moveMe method using a value of 10. The result of this moveMe method gets stored in the position property of newObject, which then gets printed again to show how it changed.

Choose File

Click the Build and Run button, or choose Build

Quit the program by clicking the Stop button or choosing Product

Choose Run

2010-09-11 23:03:18.721 VariableTest[4776:a0f] The object's current position = 45
2010-09-11 23:03:18.724 VariableTest[4776:a0f] The new object position = 55
2010-09-11 23:03:18.721 VariableTest[4776:a0f] The object's current position = 32
2010-09-11 23:03:18.724 VariableTest[4776:a0f] The new object position = 42

Open the VariableTest project from the previous section.

Click the NewClass.h file, and modify the code as follows:

#import <Cocoa/Cocoa.h>
#import "MyClass.h"

@interface NewClass : MyClass {
@private

}

-(int)changePosition: (int)myPosition;

@end

All you're doing in this code is declaring a moveMe method that's identical to the moveMe method stored in the MyClass file.

Choose File

Click the NewClass.m file, and modify the code as follows:

#import "NewClass.h"

@implementation NewClass

- (id)init {
    if ((self = [super init])) {
        // Initialization code here.
    }

    return self;
}

- (void)dealloc {
    // Clean-up code here.

    [super dealloc];
}

-(int)changePosition: (int)myPosition
{
    int newPosition;
    newPosition = self.position * myPosition;
    return newPosition;
}

@end

This code redefines how the changePosition method works. In the MyClass file, the changePosition method adds self.position + myPosition. In this moveMe method, it multiples self.position * myPosition.

Choose File

Click the Build and Run button, or choose Build

Quit the program by clicking the Stop button or choosing Product

Choose Run

2010-09-11 23:03:18.721 VariableTest[4776:a0f] The object's current position = 45
2010-09-11 23:03:18.724 VariableTest[4776:a0f] The new object position = 55
2010-09-11 23:03:18.726 VariableTest[4776:a0f] The object's current position = 32
2010-09-11 23:03:18.727 VariableTest[4776:a0f] The new object position = 320