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Sorry, you need to use the javac compiler to get good compiler error messages. The error messages I get compiling your code have nothing to do with the length attribute.

What you posted is an execution time error, not a compiler error. There is no stack trace for compiler errors.

Can you post the full text of the compiler error message? The error messages I get...

Please post the full text of the compiler's error message.

Have you coded the class and compiled it? What happened?
Have you tried the code in post#1?

For that to work, concat() needs to return a TestClass object.