学步园

Ruby代码

1. class IntelligentLife   
2.   @@home_planet = nil 
   
3.    
4.   def self.home_planet 
   
5.     @@home_planet 
6.   end 
7.   def self.home_planet=(x) 
   
8.     @@home_planet = x 
9.   end 
10.   #... 
11. end 
12. class Terran < IntelligentLife 
    
13.   @@home_planet = "Earth" 
    
14. end 
15.  
16. class Martian < IntelligentLife 
    
17.   @@home_planet = "Mars" 
    
18. end 
19.  
20.  
21. p IntelligentLife.home_planet 
22. p Terran.home_planet 
23. p Martian.home_planet 

class IntelligentLife  
  @@home_planet = nil
  
  def self.home_planet
    @@home_planet
  end
  def self.home_planet=(x)
    @@home_planet = x
  end
  #...
end
class Terran < IntelligentLife
  @@home_planet = "Earth"
end

class Martian < IntelligentLife
  @@home_planet = "Mars"
end


p IntelligentLife.home_planet
p Terran.home_planet
p Martian.home_planet

可以看到结果是相同的，都是"Mars".这是因为父类的类变量是被整个继承体系所共享的.

在这里我们如果想要得到我们所需要的结果，我们就要使用类实例变量，因为类实例变量是严格的per-class。而不是被整个继承体系所共享。

Java代码

1. class IntelligentLife   
2.   @home_planet = nil 
3.    
4. class << self 
5.     attr_accessor :home_planet 
6.   end 
7. end 
8. class Terran < IntelligentLife 
   
9.   @home_planet = "Earth" 
   
10. end 
11.  
12. class Martian < IntelligentLife 
    
13.   @home_planet = "Mars" 
    
14. end 
15.  
16.  
17. p IntelligentLife.home_planet 
18. p Terran.home_planet 
19. p Martian.home_planet 

class IntelligentLife  
  @home_planet = nil
  
 class << self
    attr_accessor :home_planet
  end
end
class Terran < IntelligentLife
  @home_planet = "Earth"
end

class Martian < IntelligentLife
  @home_planet = "Mars"
end


p IntelligentLife.home_planet
p Terran.home_planet
p Martian.home_planet