From Ruby to Java with interactive Tutor canal.

Main difference with Java

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 several different popular third-party 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).

If you're really want to Ruby has a Kick Start , here is the interactive Tutor session given by Ruby. Learn Ruby within 20mins  http://tryruby.org/levels/1/challenges/0