The current description is fine

Gooood

One shouldn't use 2 different naming conventions. Either lower_underscore or lowerCamelCase. :)

Yep, this should be mentioned in description.

I agree - lots of kyu 8 kata's were harder than this one.

Thank you for advices. I wanted to try and make any kata just to see how it works. I will read all your advices, see more examples and I will try again in 6 months (I've just started java course so maybe by then I'll be able to do it right).

Why return type is Object? Well, there's only one reason: You're making us returning error strings while correct output is a double.

That's a very bad design. Please learn how to catch exceptions properly during testing and throw exceptions for input validation instead. Don't teach others bad practices ;-)

Also, the input validation: it's not just besides the point of the task, it's another bad design: in an actually good design you'd instead have different methods for each shape type, along with the appropriate arguments. Again, learn to make good designs before writing a kata: everyone will be learning from you. Please be a responsible role model ;-)

why the rounding? -> use proper method to check for floats: assertEquals(msg,exp,actual,precision)

Needs Random Tests.

circlePipe

What is a pipe? There are nothing related to pipe or wall in the given wiki page.

Actual and expected were flipped in Java. Fixed.

Fixed.

There's a tiny issue with java tests. The wrong-result-message is reversed. For exemple, if I lose "o" in "Hello", I get a message:
expected:<Hell[] Riley!> but was:<Hell[o] Riley!>
It should be:
expected:<Hell[o] Riley!> but was:<Hell[] Riley!>
isn't it? :)