用继承实现XYPoint和Circle两个类

 #import <Foundation/Foundation.h>
 @protocol show
 @required
 -(void)printOn;
 @end

 @interface XYPoint : NSObject<show>{
     int x;
     int y;
 }
 @property (nonatomic,assign)int x,y;
 -(id)setX:(int)_x andY:(int)_y;

 @end

 #import "XYPoint.h"

 @implementation XYPoint
 @synthesize x,y;

 -(id)setX:(int)_x andY:(int)_y{
     self.x=_x;
     self.y=_y;
     return self;
 }
 -(void)printOn{
     NSLog(@"x = %d ,y = %d",x,y);
 }
 @end

 #import <Foundation/Foundation.h>
 #import "XYPoint.h"
 @interface Circle : XYPoint<show>{
     int radius;
 }
 @property (nonatomic,assign)int radius;
 -(id)initWithX:(int)_x andY:(int)_y andRadius:(int)_r;
 -(double)area;
 -(double)perimeter;
 @end

 #import "Circle.h"

 @implementation Circle
 @synthesize radius;
 -(id)initWithX:(int)_x andY:(int)_y andRadius:(int)_r;
 {
     self.x=_x;
     self.y=_y;
     self.radius=_r;
     return self;
 }

 -(double)area{
     return 3.14*radius*radius;
 }

 -(double)perimeter{
     return *,*radius;
 }

 -(void)printOn{
     NSLog(@"x = %d, y = %d ,radius = %d",x,y,radius);

 }
 @end