//实现CarParts#import<Foundation/Foundation.h>@interface Tire : NSObject@end@implementation Tire-(NSString *)description{    return (@"I am a tire.I last a while.");}@end@interface Engine : NSObject@end@implementation Engine-(NSString *)description{    return(@"I am an engine.Vrooom!");}@end@interface Car : NSObject{  Engine *engine;  Tire *tires[4];}-(Engine *)engine;-(void)setEngine:(Engine *)newEngine;-(Tire *)tireAtIndex:(int)index;-(void)setTire:(Tire *)tire atIndex:(int)index;-(void)print;@end@implementation Car-(Engine *)engine{    return(engine);}-(void)setEngine:(Engine *)newEngine{    engine=newEngine;}-(void)setTire:(Tire *)tire atIndex:(int)index{    if(index<0||index>3)    {        NSLog(@"bad index (%d) in setTire:atIndex:",index);        exit(1);    }    tires[index]=tire;}-(Tire *)tireAtIndex:(int)index{    if(index<0||index>3)    {        NSLog(@"bad index (%d) in tireAtIndex:",index);        exit(1);    }return (tires[index]);}-(void)print{    NSLog(@"%@",engine);    NSLog(@"%@",tires[0]);    NSLog(@"%@",tires[1]);    NSLog(@"%@",tires[2]);    NSLog(@"%@",tires[3]);}@endint main(int argc, const char * argv[]){    Car *car=[Car new];    Engine *engine=[Engine new];    [car setEngine:engine];    for(int i=0;i<4;i++)        {            Tire *tire=[Tire new];            [car setTire:tire atIndex:i];        }    [car print];    return 0;}