Objective-C category compared to features in other languages

Question

I am trying to draw parallels between Objective-C Category and features in Java and Ruby. Please see if these conclusions are accurate -

Objective-C Category adds new methods to an existing class even when you do not have access to source code of the class. Java does not have anything similar but in Ruby you can "open" any class, including core language class like String.

class String 
  def my_new_method
  "new test method"
 end
end

"some string".my_new_method
=> "new test method"

Similarly in Objective-C - you can "re-open" the NSString class using category.

#import "NSString+ForTest.h"

@implementation NSString (ForTest)

-(NSString*) myNewMethod {
  return @"new test method";
}

@end

However, one difference I see is that, in Ruby, once a class is opened and modified within a runtime, ALL subsequent uses of that class are affected, in other words it is a system wide change, while in Objective-C only code that imports this header file (and any subclasses) are affected. Are there any other differences?

Answer 1

However, one difference I see is that, in Ruby, once a class is opened and modified within a runtime, ALL subsequent uses of that class are affected, in other words it is a system wide change, while in Objective-C only code that imports this header file (and any subclasses) are affected

Nope—in Objective-C, all classes in a program (or library) compiled with the class extension are affected, even if your code never imports the relevant header file, so the feature is very similar to that of Ruby.