I was then a Java Developer like 8yrs ago but since the conception and famous web 2.0 dream is about to change the web technology, I switch to Ruby. So I’ve been working on Ruby on Rails for quite some time already.

So to Differ from the 2 programming languages,  I posted here the article from Ruby-Lang website.

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 several different popular third-party GUI toolkits. Ruby users can try WxRuby, FXRuby, Ruby-GNOME2, Qt, 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 “mixins” 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).

Reference : https://www.ruby-lang.org/en/documentation/ruby-from-other-languages/to-ruby-from-java/