Getters and Setters
I was chatting to Simon the other day and the question of, should you have getters and setters in your Ruby classes, or should you just let other classes access the attributes directly, came up. We were talking about ActiveRecord, which dynamically creates them for any columns in the table the class is representing.
The short answer is, if you want other classes to access that data, yes, you need getters and/or setters, but for different reasons that you might think if you come from a Java world. Getters and setters are one of my pet peeves in Java. I hate it when I see them thrown in by default for every field in the class, reducing it to nothing more than a C struct.
The good reason for not allowing direct access to a field in Java is data encapsulation. By making it private you can decide if it’s read-only or read/write (or neither). But I think this is a poor reason, in so much that other objects shouldn’t be pulling data out, but asking it to do things. Rather than starting from a view point of what data does this class provide, you should think of what are its responsibilities.
But that is tangential to what I want to talk about, why are getters and setters so prevalent in Java? I think the first reason is the early OR mapping frameworks. They typically required you to inherit from a class, or broke OO (no inheritance in EJBs?!?), so you treated the domain object as struct, just to ferry data from Java to the database. Then you’d put logic in other objects and you’re just doing procedural programming. I think the situation is better now that you can work with POJOs (e.g. Hibernate) but you still see tutorials following the old pattern.
The second reason is the madness that is field access in Java. Getters and setters can protect you from this. Java binds field access at compile time, i.e. to a variable’s static type, e.g.:
public class A {
int num = 0;
}
public class B extends A {
int num = 2;
}
public class Test {
public static void main(String[] args) throws Exception {
A a = new B();
System.out.println(a.num);
B b = new B();
System.out.println(b.num);
}
}
Would output:
0
2
when you’d hope it would do something sensible like:
2
2
i.e. you shadow fields rather than override them, so you can get weird bugs if the static type is different from the runtime type. The variables can even be different types! Methods are determined by your runtime type so we can make sure we’re always accessing the right variable, e.g.:
public class A {
int num = 0;
public int getNum() { return this.num; }
}
public class B extends A {
int num = 2;
public int getNum() { return this.num; }
}
public class Test {
public static void main(String[] args) throws Exception {
A a = new B();
System.out.println(a.getNum());
B b = new B();
System.out.println(b.getNum());
}
}
Does output:
2
2
so unless the field is defined in your class, you really shouldn’t access it directly. This means to be safe, all fields should be private and we need protected scope getters and setters if we want that field available for subclasses, what a pain, but helps explain why they are so prevalent.
Back to Ruby, how does it deal with this? If you look back at the top I said if you want to access the data you need getters and setters. This is because attributes (Ruby’s name for fields) are private to an object, and everything you ask from an object has to be a method, basically you couldn’t get to that attribute directly if you wanted to! Ruby also doesn’t have the problem of field shadowing because you don’t declare variables, so when you reference it, you’re referencing the only variable with that name.
So what do the getters and setters look like?
class Test
def someField
@someField
end
def someField=(newValue)
@someField = newValue
end
end
aObject = Test.new
test.someField = "blah"
test.someField >> "blah"
i.e. our getters and setters look like we’re accessing a field directly but they are actually method calls. Without these, if we tried to access a field, Ruby would complain that no such method exists.
Since this is such a common pattern Ruby has some shortcuts:
read-only:
class Test
attr_reader :someField
end
write only:
class Test
attr_writer :someField
end
read/write:
class Test
attr_accessor :someField
end
All these methods can take multiple arguments if you’re defining multiple fields.
A good summary of all this can be found here:
http://www.rubycentral.com/book/tut_classes.html#S2
But Ruby’s not perfect. A subclass can access attributes from its parent. which means you’re dependant on its implementation, but a code review can stop that.
The bigger problem is clashes with mixins. If a mixin uses an instance variable you can get collisions. A mixin has no state, that instance variable it refers to belongs to the object that included the mixin, so if it uses a variable with the same name you can get some weird behaviour. Because there is no static typing, the object’s methods and the mixin’s method can change what is held in that attribute and you won’t see that error until runtime where you’ll probably see a missing method error. Typical workarounds are prefixing variables with some sort of namespace to avoid collisions, or using a module level hash to store the mixin’s state outside of the object.
All in all I prefer the Ruby way of getters and setters. On the surface it looks the same but by preventing direct access to attributes and using virtual attributes (not covered here), you classes are less likely to become structs and more like genuine objects.
