mutable string initialization problem in Objective C ..

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C

Objective-C

int main (int argc, const char * argv[])
{
    NSAutoreleasePool * pool = [[NSAutoreleasePool alloc] init];
    NSMutableString *str1;
    [str1 setString:@"again"];
    NSLog(@"string is -- %@",str1);   // It prints " string is -- (null) " 
    [str1 release];
    [pool drain];
    return 0;
}

------------------------------------------------------------------------------------

int main (int argc, const char * argv[])
{
    NSAutoreleasePool * pool = [[NSAutoreleasePool alloc] init];
    NSMutableString *str1;
    
    str1 = [NSMutableString stringWithString:@"hello again "]; // By adding this 
   [str1 setString:@"again"];                // Now this statements works properly .
    NSLog(@"string is -- %@",str1);    
    [str1 release];
    [pool drain];
    return 0;
}

I can not understand why if I use setString method first, str1 is not initializing??