Conversion.h

#import <Cocoa/Cocoa.h>

@interface Conversion : NSObject
@end

@interface Conversion (Abstract)
- (NSString *)firstUnitName;
- (NSString *)secondUnitName;

- (double)secondValueForFirstValue:(double)firstValue;
- (double)firstValueForSecondValue:(double)secondValue;
@end

Conversion2.h

#import <Cocoa/Cocoa.h>

@protocol Converting <NSObject>
@required
- (NSString *)firstUnitName;
- (NSString *)secondUnitName;
- (double)secondValueForFirstValue:(double)firstValue;
- (double)firstValueForSecondValue:(double)secondValue;
@end

@interface TemperatureConversion : NSObject <Converting>
@end

@interface OtherConversion : NSObject <Converting>
@end

Conversion2.m

#import "Conversion.h"

@implementation TemperatureConversion

#pragma mark -
#pragma mark Converting methods

- (NSString *)firstUnitName {
    return @"Celsius";
}

- (NSString *)secondUnitName {
    return @"Fahrenheit";
}

- (double)secondValueForFirstValue:(double)celsius {
    return (1.8 * celsius + 32.0);  
}

- (double)firstValueForSecondValue:(double)fahrenheit {
    return ((fahrenheit - 32.0) / 1.8); 
}

@end


@implementation OtherConversion

#pragma mark -
#pragma mark Converting methods

- (NSString *)firstUnitName {
    return @"Foo";
}

- (NSString *)secondUnitName {
    return @"Bar";
}

- (double)secondValueForFirstValue:(double)foo {
    return foo; // TODO: actually convert to something
}

- (double)firstValueForSecondValue:(double)bar {
    return bar; // TODO: actually convert to something
}

@end

TemperatureConversion.h

#import "Conversion.h"

@interface TemperatureConversion : Conversion
@end

TemperatureConversion.m

#import "TemperatureConversion.h"

@implementation TemperatureConversion

- (NSString *)firstUnitName {
    return @"Celsius";
}

- (NSString *)secondUnitName {
    return @"Fahrenheit";
}

- (double)secondValueForFirstValue:(double)celsius {
    return (1.8 * celsius + 32.0);  
}

- (double)firstValueForSecondValue:(double)fahrenheit {
    return ((fahrenheit - 32.0) / 1.8); 
}

@end