I'm new to Objective-C, though I have a very good hand in Android. I'm trying to make a call to a method but it gives me 'first use in function'. I know I'm making a silly mistake but experts could figure it out easily.

RootViewController.h

#import <UIKit/UIKit.h>
#import "ContentViewController.h"

@interface RootViewController : UITableViewController {
    ContentViewController *contentViewController;
}

@property (nonatomic, retain) ContentViewController *contentViewController;

- (NSString*)getContentFileName:(NSString*)title; //<--- This function declartion

@end

RootViewController.m

#import "RootViewController.h"
#import "HAWATAppDelegate.h"
#import "ContentViewController.h"

@implementation RootViewController
@synthesize contentViewController;

...
more methods
...

#pragma mark -
#pragma mark Table view delegate

- (void)tableView:(UITableView *)tableView didSelectRowAtIndexPath:(NSIndexPath *)indexPath {
    HAWATAppDelegate *appDelegate = (HAWATAppDelegate *)[[UIApplication sharedApplication] delegate];
    NSString *title = (NSString *) [appDelegate.titles objectAtIndex:indexPath.row];

    NSString *fileName = getContentFileName:title; //<--- Here is the error

    ...
}

- (NSString*) getContentFileName:(NSString*)title {
    return [title lowercaseString];
}

@end

There must be a simple thing I'm missing. Please let me know. Thanks in advance.