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Java is mature. It’s tested. And it’s fast (contrary to what the anti-Java crowd may
still claim). It’s also quite verbose. Going from Java to Ruby, expect your code size
to shrink down considerably. You can also expect it to take less time to knock together
quick prototypes.

Similarities

As with Java, in Ruby,…

  • Memory is managed for you via a garbage collector.
  • Objects are strongly typed.
  • There are public, private, and protected methods.
  • There are embedded doc tools (Ruby’s is called RDoc). The docs generated by rdoc look
    very similar to those generated by javadoc.

Differences

Unlike Java, in Ruby,…

  • You don’t need to compile your code. You just run it directly.
  • There are different GUI toolkits. Ruby users can try
    WxRuby,
    FXRuby,
    Ruby-GNOME2,
    or the bundled-in Ruby Tk for example.
  • You use the end keyword after defining things like classes, instead of having
    to put braces around blocks of code.
  • You have require instead of import.
  • All member variables are private. From the outside, you access everything via methods.
  • Parentheses in method calls are usually optional and often omitted.
  • Everything is an object, including numbers like 2 and 3.14159.
  • There’s no static type checking.
  • Variable names are just labels. They don’t have a type associated
    with them.
  • There are no type declarations. You just assign to new variable names as-needed
    and they just “spring up” (i.e. a = [1,2,3] rather than int[] a = {1,2,3};).
  • There’s no casting. Just call the methods. Your unit tests should tell you
    before you even run the code if you’re going to see an exception.
  • It’s foo = Foo.new( "hi") instead of Foo foo = new Foo( "hi" ).
  • The constructor is always named “initialize” instead of the name of the class.
  • You have “mixin’s” instead of interfaces.
  • YAML tends to be favored over XML.
  • It’s nil instead of null.
  • == and equals() are handled differently in Ruby. Use == when you want to test equivalence in Ruby (equals() is Java). Use equal?() when you want to know if two objects are the same (== in Java).