Bad OO Design in Objective C

BadDesign1.m

#import <Foundation/Foundation.h>

@interface BankAccount : NSObject

  @property NSString* accountNumber;
  @property double accountBalance;
  extern const double interestRate;

-(double) calculateInterest;
-(double) save:(double) amount;
-(double) withdraw:(double) amount;

@end

@implementation BankAccount

const double interestRate = 5.0;

-(double) calculateInterest {
    return (interestRate * _accountBalance) / 100;
}

-(double) save:(double) amount {
    _accountBalance += amount;
    return _accountBalance;
}

-(double) withdraw:(double) amount {
    _accountBalance -= amount;
    return _accountBalance;
}

@end

@interface InterestCalculator : NSObject

@end

@implementation InterestCalculator

@end

int main(int argc, const char * argv[]) {
    @autoreleasepool {
        BankAccount* account = [BankAccount new];
        double balance = [account save:10];
        NSLog(@"%.2f", balance);
        double interest = [account calculateInterest];
        NSLog(@"%.2f", interest);
    }
    return 0;
}

BadDesign2.m

#import <Foundation/Foundation.h>

@interface Shape : NSObject

extern const int CIRCLE;
extern const int RECTANGLE;

-(double) area;
-(void) createCircle:(double) radius;
-(void) createRectangleWithWidth:(double) width Length:(double) length;

@end

@implementation Shape

const int CIRCLE = 1;
const int RECTANGLE = 2;
int TYPE = 0;

double _radius;
double _width;
double _length;

-(double) area {
    switch(TYPE) {
      case CIRCLE:
        return M_PI * _radius;
      case RECTANGLE:
        return _width * _length;
      default:
        return 0;
    }
}

-(void) createCircle:(double)radius {
    _radius = radius;
    TYPE = CIRCLE;
}

-(void) createRectangleWithWidth:(double)width Length:(double)length {
    _width = width;
    _length = length;
    TYPE = RECTANGLE;
}

@end

int main(int argc, const char * argv[]) {
    @autoreleasepool {
        Shape* shape = [Shape new];
        [shape createRectangleWithWidth:5 Length:4];
        double area = [shape area];
        NSLog(@"%.2f", area);
    }
    return 0;
}

BadDesign3.m

#import <Foundation/Foundation.h>

@protocol Shape <NSObject>

-(double) area;

@end

@interface Rectangle : NSObject <Shape>

@property double width;
@property double length;

- (id) initWithWidth:(double) width AndLength:(double) length;

@end

@implementation Rectangle

-(id) initWithWidth:(double)width AndLength:(double)length {
    self = [super init];
    
    if( self = [super init] ){
        _width = width;
        _length = length;
    }
    
    return self;
}

-(double) area {
    return _width * _length;
}

@end


@interface Square : Rectangle

@property double edge;

-(id) initWithEdge:(double)edge;

@end


@implementation Square

-(id) initWithEdge:(double)edge {
    self = [super initWithWidth:edge AndLength:edge];
    
    return self;
}

@end

int main(int argc, const char * argv[]) {
    @autoreleasepool {
        Square* square = [[Square alloc] initWithEdge:5];
        double area = [square area];
        NSLog(@"%.2f", area);
    }
    return 0;
}

BadDesign4.m

#import <Foundation/Foundation.h>

@protocol Vehicle <NSObject>

-(void) fly;
-(void) drive;

@end

@interface Plane : NSObject <Vehicle>
@end

@implementation Plane

-(void) fly {
    NSLog(@"Look ma I am flying");
}

-(void) drive {
    [NSException raise:@"Exception" format:@"Uh oh. Better not do that"];
}

@end

@interface Car : NSObject <Vehicle>
@end

@implementation Car

-(void) fly {
    [NSException raise:@"Exception" format:@"Can not fly"];
}

-(void) drive {
    NSLog(@"Vroom vroom");
}

@end


int main(int argc, const char * argv[]) {
    @autoreleasepool {
        Car* car = [Car new];
        [car drive];
        [car fly];
    }
    return 0;
}

BadDesign5.m

#import <Foundation/Foundation.h>

@protocol Outputter <NSObject>

-(void) print:(NSString*) output;

@end

@interface ConsoleOutputter : NSObject <Outputter>
@end

@implementation ConsoleOutputter

-(void) print:(NSString *)output {
    printf("%s", [output UTF8String]);
}

@end

@interface MockOutputter : NSObject <Outputter>
@end

@implementation MockOutputter

-(void) print:(NSString *)output {
    NSLog(@"%@", output);
}

@end

@interface CommandInvoker : NSObject
-(void) invoke:(ConsoleOutputter*) outputter;
@end

@implementation CommandInvoker

-(void) invoke:(ConsoleOutputter* )outputter {
    [outputter print:@"Hello World"];
}

@end

int main(int argc, const char * argv[]) {
    @autoreleasepool {
        MockOutputter* outputter = [MockOutputter new];
        CommandInvoker* invoker = [CommandInvoker new];
        [invoker invoke:outputter];
    }
    return 0;
}