10 Things Every Java Programmer Should Know About Ruby

Jim Weirich

Adapted S6/S9 Version 1 from Original Slide Deck

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Who Am I?

First Things First

I used to teach an after hours course in C programming for employees of a large manufacturing company. It was pretty easy to tell what programming languages the students had used previously just by looking at the style of C code they produced. It is certainly true that “You can write FORTRAN in any language”.

Java programmers investigating Ruby will find a language that look similar in many ways. There are classes and modules, namespaces and scopes, instance variables and methods. A Java programmer will feel quite at home in this Object Oriented language.

So the temptation will be to continue to program in a Java-style. Sure, there are some things that are different (the lack of type declarations will probably be the first thing that strikes them). But nothing that can’t be worked around with a little effort … and they will miss a golden opportunity.

Sapir-Whorf Hypothesis

The Sapir-Whorf Hypothesis theorizes that thoughts and behavior are determined (or are at least partially influenced) by language. […] To this day it has not been completely disputed or defended, but has continued to intrigue researchers around the world.


A language that doesn’t affect the way you think about programming is not worth knowing – Alan Perlis

The Ruby Way

This is not yet another “Ruby is better than Java” article. Nor is it intended to bash Java or Java programmers. Rather, it is an attempt to aid Java programmers who are investigating Ruby by helping them quickly get over the “Writing Java in Ruby” syndrome and to discover the Ruby Way.

Now, on to our 10 Things…

Item #10 - Learn Ruby Conventions

Some of the conventions are enforced by the language, others are merely standards used by the community.

Item #9 - Everything is an Object

Everything in Ruby that can be bound to a variable name is a full-fledged object.

This has interesting consequences

Classes are Objects!

This make factories trivial

Since Classes create instances of themselves, they are the ultimate factory object.

def create_from_factory(factory)

obj = create_from_factory(Array)

No Primitives!

Even integers are full fledged objects.

0.zero?    # => true
1.zero?    # => false
1.abs      # => 1
-1.abs     # => 1
1.methods  # => list of methods for object 1
2.+(3)     # => 5  (same as 2+3)
10.class   # => Fixnum
           # => Bignum

nil is an Object!

a = nil
a.nil?     # => true
a.methods  # => list of methods
a.abs      # => NoMethodError

Things that are not Objects

More Things that are not Objects

def with_block

with_block {
  # Never converted
def with_proc(&block)

with_proc {
  # Converted internally

Item #8 - (Almost) Everything is a Message

All computation in Ruby happens through:

Yes, all of these are messages…

Send :index (with argument “x”)
Send :length (with no argument)
Send :run_status_reports (to self)
1 + 2
Send :+ (with argument 2) to the object 1
Send :[] (with argument i) to the array

Messages, not Function Calls

Java Programmers tend to think of obj.method() as looking up a member function in a table and calling it.

Ruby programmers tend to think of obj.method as sending a message to an object.

What’s the Difference?

The difference is subtle, but important!

What Kind of Differences?

Consider the following class. It defines an object that is able to record all the messages ever sent to it, and then playback those messages to another object.

class VCR
  def initialize
    @messages = []
  def method_missing(method, *args, &block)
    @messages << [method, args, block]
  def play_back_to(obj)
    @messages.each do |method, args, block|
      obj.send(method, *args, &block)

Playing Back Data

Example Code

require 'src/vcr'

vcr = VCR.new
vcr.sub!(/Java/) { "Ruby" }
vcr[11,5] = "Universe"
vcr << "!"

string = "Hello Java World"
puts string

puts string


Hello Java World
HELLO RUBY Universe!

Message Opportunities…

Remote Proxies
Automatically forward any message to a remote object.
Auto Loaders
Stand in for an object until it gets its first message. Then load it and act like a regular proxy. Great for autoloading database backed objects.
Intercept the messages you want and pass the rest through.
Mock Objects
Just write the methods that need to be mocked. Proxy or ignore the others as needed.
Generate XML/HTML/Whatever based on the methods called on the builder

Item #7 - Ruby is Way More Dynamic Than You Expect

One of the big attractions of Java over C++ was the dynamic features of the language. You could easily load classes at run time, query objects about their classes and methods, and even call methods discovered at runtime.

Dynamic Beyond Java

Ruby takes dynamic behavior several steps beyond Java.

  • method_missing
  • Easy Reflection
  • Open Classes
  • Singleton Objects
  • Definition Hooks
  • Code Evalutation

Easy Reflection: Create Object

public static Object create(Class c, String value)
  throws Exception
  Constructor ctor = c.getConstructor(
    new Class[] { String.class } );
  return ctor.newInstance( new Object[] { "Hello" } );

public static void main (String args[])
  throws Exception
  Greeting g =(Greeting) create(Greeting.class, "Hello");

Easy Reflection

The Ruby Version

def create(klass, value)

g = create(Greeting, "Hello")

Open Classes

Methods can be added to classes at any point … even built in classes.

class Integer
  def even?
    (self % 2) == 0

p (1..10).select { |n| n.even? }
# => [2, 4, 6, 8, 10]

Caution is advised, but this feature can be very useful.

Singleton Methods

Singleton methods are defined on individual objects, not classes.

class Dog

rover = Dog.new
fido = Dog.new

def rover.speak
  puts "Red Rover"

rover.speak  # => "Red Rover"
fido.speak   # => NoMethodError


Hooks allow the user to gain control at interesting moments during the execution of a program.

class MyClass
  def MyClass.method_added(name)
    puts "Adding Method #{name}"
  def new_method
    # Yada yada yada


Adding Method new_method

Code Eval

class Module
  def trace_attr(sym)
    self.module_eval %{
      def #{sym}
        printf "Accessing %s with value %s\n",
          "#{sym}", @#{sym}.inspect
class Dog
  trace_attr :name
  def initialize(string)
    @name = string
Dog.new("Fido").name  # => Accessing name with value "Fido"

Item #6 - Objects are Strongly Typed - Not Statically Typed

What is a Type?

A type is
a set of values
a set of operations

C Code (Weak)

#include <stdio.h>
extern float two();
int main() {
  float x = 1.5 + two();
  printf("%f\n", x);
  printf("%d\n", x);
  return 0;

Java Code (Strong)

public class Main {
  public static
    void main (String args[]) {
    double x = 1.5 + Two.two();
int two() { return 2; }
public class Two {
  public static int two() {
    return 2;





Ruby Code (?)

require 'two'

x = 1.5 + two
puts x
printf "%d", x
def two



So what makes a language type safe?

  • Compiler knowledge of the variable types?
  • Declaring all variables?
  • Compiler catching all type errors?


  • Catching all inappropriate operations on a type, either at
    • compile time, or
    • run time

Ruby Code

def factorial(n)
  result = 1
  (2..n).each do |i|
    result *= i

puts factorial(20)
puts factorial(21)

Java Code

public class Fact {
  static long factorial(long n) {
    long result = 1;
    for (long i=2; i<=n; i++) 
      result *= i;
    return result;
  public static
    void main (String args[]) {





Lanaguage Typing Systems

Java is

  • Strongly,
  • Statically,
  • Manifestly


Ruby is

  • Strongly,
  • Dynamically,
  • Implicitly



I’ve been a statically typed bigot for quite a few years. I learned my lesson the hard way while using C. Too many systems crashed in the field due to silly typing errors. […]

Four years ago I got involved with Extreme Programming. […] I can’t imagine not having a comprehensive suite of unit tests to back up my development. […]

About two years ago I noticed something. I was depending less and less on the type system for safety. My unit tests were preventing me from making type errors. […]

So I tried writing some applications in Python, and then Ruby. I was not entirely surprised when I found that type issues simply never arose.

Bob Martin

Item #5 - Don’t Worry About Interfaces

Ruby Uses Duck Typing

  • If it walks like a duck,
  • And talks like a duck,
    • Then we can treat it like a duck.
    • (who cares what it really is)
class Duck
  def talk() puts "Quack" end
class DuckLikeObject
  def talk() puts "Kwak" end
flock = [
  DuckLikeObject.new ] 
flock.each do |d| d.talk end

No need to inherit from a common interface.

Item #4 - Mix it up with Mix-Ins

Although Ruby does not have interfaces, it does have mix-ins defined by modules.

A module…

Tedious comparison operators

Although all the logic is in the less-than method, all the other comparisons must still be defined.

class Pair
  attr_accessor :first, :second
  # ...
  def <(other)
    (first < other.first) ||
    (first == other.first && second < other.second)
  def >(other)
    other < self
  # Other methods defined in terms of less than:
  #     <=, >=, ==

Reuse the Mix-in

A mix-in allows the commonality to be factored out.

module ComparableUsingLess
  def >(other)
    other < self
  # Other methods defined in terms of less than:
  #     <=, >=, ==
class Pair
  include ComparableUsingLess
  attr_accessor :first, :second
  # ...
  def <(other)
    (first < other.first) ||
      (first == other.first && second < other.second)

Item #3 - Embrace Closures


[1,2,3].each do |item| puts item end

Resource Management

file_contents = open(file_name) { |f| f.read }


widget.on_button_press { puts "Got Button Press" }

Item #2 - ri is Your Friend, irb is Your Other Friend

Ruby Information. Man pages for standard Ruby objects.
Interactive Ruby. Console based interactive Ruby interpreter.
$ ri Array
---------------------------------------------------------- Module: Array
     Arrays are ordered, integer-indexed collections of any object.
     Array indexing starts at 0, as in C or Java. A negative index is
     assumed to be relative to the end of the array---that is, an index
     of -1 indicates the last element of the array, -2 is the next to
     last element in the array, and so on.
     Enumerable(all?, any?, collect, detect, each_with_index, entries,
     find, find_all, grep, include?, inject, map, max, member?, min,
     partition, reject, select, sort, sort_by, to_a, zip)
Class methods:
     [], new
Instance methods:
     &, *, +, -, <<, <=>, ==, [], []=, assoc, at, clear, collect,
     collect!, compact, compact!, concat, delete, delete_at, delete_if,
     each, each_index, empty?, eql?, fetch, fill, first, flatten,
     flatten!, frozen?, hash, include?, index, indexes, indices, insert,
     inspect, join, last, length, map, map!, nitems, pack, pop, push,
     rassoc, reject, reject!, replace, reverse, reverse!, reverse_each,
     rindex, select, shift, slice, slice!, sort, sort!, to_a, to_ary,
     to_s, transpose, uniq, uniq!, unshift, values_at, zip, |

# Another RI Example

Ask about the instance method last

$ ri Array#last
------------------------------------------------------------- Array#last
     array.last     =>  obj or nil
     array.last(n)  =>  an_array
     Returns the last element(s) of _self_. If the array is empty, the
     first form returns +nil+.

        [ "w", "x", "y", "z" ].last   #=> "z"

IRB Sample

Add 1+2, then find the methods defined in Proc

$ irb --simple-prompt
>> 1 + 2
=> 3
>> Proc.instance_methods(false)
=> ["[]", "==", "dup", "call", "binding", "to_s", 
    "clone", "to_proc", "arity"]

Item #1 - Stop Writing So Much Code!

Coworker Quote (paraphrased)

“I decided to try out Ruby to solve my problem. So I wrote a little code and all of a sudden I discovered that I was done.”


Item #Zero - Ruby Puts the Fun Back In Programming


Some More Things You Should Know


(10) Learn Ruby Conventions
(9) Everything is an Object
(8) (Almost) Everything is a Message
(7) Ruby is Way More Dynamic Than You Expect
(6) Objects are Strongly Typed, Not Statically Typed
(5) Don’t Worry About Interfaces
(4) Mix it up with Mix-ins
(3) Embrace Closures
(2) ri is Your Friend, irb is Your Other Friend
(1) Write Less Code
(0) Ruby Makes Programming Fun Again


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