Previously we seen methods to create objects in Objective C : Objective C : object creation method explanations 

Objective-C arrays allow you to define type of variables that can hold several data items of the same kind but structure is another user-defined data type available in Objective-C programming which allows you to combine data items of different kinds.  

Objective-C program basically consists ofthe following parts:

§ Preprocessor Commands

§ Interface

§ Implementation

§ Method

§ Variables

§ Statements & Expressions

§ Comments

Let uslook at a simple code that would print the words "Hello World":

#import <Foundation/Foundation.h>
@interface SampleClass:NSObject
    - (void)sampleMethod;
@end
@implementation SampleClass
   - (void)sampleMethod {
            NSLog(@"Hello, World! \n");
}
@end
int main()
{
   NSAutoreleasePool * pool = [[NSAutoreleasePool alloc] init];
   
   /* my first program in Objective-C */
   SampleClass *sampleClass = [[SampleClass alloc]init];
   [sampleClass sampleMethod];
    
   [pool drain];
   return 0;
}

Compile & Execute Objective-C Program:

Now when we compile and run the program, we will get the following result.

2015-07-09 08:28:58.203 demo[11674] Hello, World! 

Let uslook various parts of the above program:

1.  The first line of the program #import<Foundation/Foundation.h> is a preprocessor command, which tellsa Objective-C compiler to include Foundation.h file before going toactual compilation. 

2.  The next line @interface SampleClass:NSObjectshows how to create an interface. It inherits NSObject, which is thebase class of all objects. 

3.   The next line - (void)sampleMethod;shows how to declare a method. 

4.   The next line @end marks the end of aninterface.

5.   The next line @implementation SampleClassshows how to implement the interface SampleClass.

6.   The next line - (void)sampleMethod{}shows the implementation of the sampleMethod.

7.   The next line @end marks the end of animplementation.

8.   The next line int main() is the mainfunction where program execution begins.

9.   The next line /*...*/ will be ignoredby the compiler and it has been put to add additional comments in the program.So such lines are called comments inthe program.

10.  The next line NSLog(...) is anotherfunction available in Objective-C which causes the message "Hello, World!"to be displayed on the screen.

11.  The next line return 0; terminates main()functionand returns the value 0. 

Next, wewill learn about : Objective C - Pointers