Objective-C 继承和多态

　　学习Java我们知道类有三大特征,封装,继承,多态.而在Objective-C中也有继承的概念,今天就来看看Objective-C中的继承和多态.

　　首先,我们来看看示例代码:

//Animal.h
#import <Foundation/Foundation.h>
 
@interface Animal : NSObject
{
	int food_consumption; //食量
	int count; //数量
	int parturition_days; //生产周期
}

-(int)count;

-(void)setCount:(int)c;

-(int)foodConsumption;

-(void)setFoodConsumption:(int)food_c;

-(int)parturitionDays;

-(void)setParturitionDays:(int)parturition_day;

@end

//Animal.m
#import "Animal.h"

@implementation Animal

-(int)count{
	return count;
}

-(void)setCount:(int)c{
	count = c;
}

-(int)foodConsumption{
	return food_consumption;
}

-(void)setFoodConsumption:(int)food_c{
	food_consumption = food_c;
}

-(int)parturitionDays{
	return parturition_days;
}

-(void)setParturitionDays:(int)parturition_day{
	parturition_days = parturition_day;
}
 
@end

//Panda.h
#import "Animal.h"
 
@interface Panda : Animal

@end

//Panda.m
#import "Panda.h"

@implementation Panda

@end

//Tool.h
#import <Foundation/Foundation.h>
#import "Animal.h"
#import "Panda.h"

@interface Tool : NSObject

+(void) initWithAnimalDictionary:(Animal *)animal andDict:(NSDictionary *)dict;

@end

//Tool.m
#import "Tool.h"

@implementation Tool

+(void) initWithAnimalDictionary:(Animal *)animal andDict:(NSDictionary *)dict{
    NSDictionary *animalDict;
    if(YES == [animal isKindOfClass:[Panda class]]){
        animalDict = [dict objectForKey:@"Panda"];
        
    }else{
        NSLog(@"error class!");
    }
    [animal setCount:[[animalDict objectForKey:@"count"] intValue]];
    [animal setFoodConsumption:[[animalDict objectForKey:@"food_consumption"] intValue]];
    [animal setParturitionDays:[[animalDict objectForKey:@"parturition_days"] intValue]];
}

@end

 data.plist文件

<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE plist PUBLIC "-//Apple//DTD PLIST 1.0//EN" "http://www.apple.com/DTDs/PropertyList-1.0.dtd">
<plist version="1.0">
<dict>
	<key>fodder</key>
	<dict>
		<key>count_num</key>
		<string>1024</string>
	</dict>
	<key>Panda</key>
	<dict>
		<key>food_consumption</key>
		<string>2</string>
		<key>count</key>
		<string>6</string>
		<key>parturition_days</key>
		<string>76</string>
	</dict>
</dict>
</plist>

//main.m
#import <Foundation/Foundation.h>
#import "Tool.h"

int main(int argc, const char * argv[]) {
    @autoreleasepool {
        // insert code here...
        Panda *panda = [[Panda alloc] init];
    
        //读取plist
        NSString *plistPath = [[NSBundle mainBundle] pathForResource:@"data" ofType:@"plist"];
        NSMutableDictionary *data = [[NSMutableDictionary alloc] initWithContentsOfFile:plistPath];
    
        [Tool initWithAnimalDictionary:panda andDict:data];
    
        int fooder_num = 0;
        int surplus = 0;
        int day = 1;
    
        NSDictionary *fooderDict = [data objectForKey:@"fodder"];
        fooder_num = [[fooderDict objectForKey:@"count_num"] intValue];
        surplus = fooder_num;
    
        while(surplus > 0){
            if(0 == (day % [panda parturitionDays])){
                [panda setCount:([panda count] + 1)];
            }
            surplus = fooder_num - ([panda count] * [panda foodConsumption]);
            fooder_num = surplus;
        
            if(surplus){
                NSLog(@"第 %d 天，熊猫:%d 只，饲料余量:%d 。
", day, [panda count],  surplus);
            }
            day++;
        }
    }
    return 0;
}

 (一)继承　　

　　继承是类中的一个重要的特性，它的出现使得我们没必要别写重复的代码，可重用性很高。当然Objective-C中的继承和Java中是一样的，没多大区别.不仅仅Java,C++也有继承特性,但C++支持多继承,而Objective-C不支持多继承.

　　接下来,我们了解几个概念:

　　(1)超类(superclass),是你所继承的类,例如,Panda的超类是Animal,Animal的超类是NSObject.

　　(2)父类(parentclass),是超类的另一种表达方式,例如,Animal是Panda的父类.

　　(3)子类(subclass),是实施继承的类,例如,Panda是Animal的子类.

　　(4)孩子类(childclass),是子类的另一种表达方式,例如,Panda是Animal的孩子类.

　　然后,说说继承的工作机制:

　　(1)方法调度

　　当代码发送消息时,Objective-C的方法调度程序将在当前类中搜索相应的方法.如果调度程序无法在接收消息的对象类中找到相应的方法,它就在该对象的超类中进行查找.

　　(2)实例变量

　　接下来我们看看Objective-C如何访问实例变量.创建一个新类时,其对象首先从自身的超类中继承实例变量,然后(可选)添加它们自己的实例变量.

　　最后,说说重写方法,制作自己全新的子类时,通常需要添加自己的方法.有时,为了在类中引入某个独特的特性,需要添加新方法.还有些时候,可能需要替换或增强由这个新类的某个超类所定义的现有方法.当遇到我们子类特有的方法时,则只需要在子类中重写该方法即可.

(二)多态

　　在Objective-C中,多态就是指,父类指针可以指向子类.

+(void) initWithAnimalDictionary:(Animal *)animal andDict:(NSDictionary *)dict;

 　　上例代码中的从字典中获取数据的类方法就很好地使用了多态.该方法主要是从字典中读取数据,示例中只有Panda一个子类,若是以后拥有更多的类,那么该功能的方法需要为每一个子类写一个方法,而使用多态特性,则只需根据传入的参数进行分别处理即可,减少代码冗余问题.

　　以上,是学习Objective-C时对继承和多态的理解.