There seems to be something wrong with your encoding for some phrases on some test cases after trying to deliver the solution.
I even tried just returning str and still got this message for Java solution:

Error: Command failed: javac -verbose -cp /home/codewarrior -d /home/codewarrior -sourcepath /home/codewarrior -cp /usr/local/groovy/lib/junit-4.12.jar:/usr/local/groovy/lib/hamcrest-core-1.3.jar /home/codewarrior/TGG.java /home/codewarrior/TestE.java ./frameworks/java/CwRunListener.java
[parsing started RegularFileObject[/home/codewarrior/TGG.java]]
[parsing completed 36ms]
/home/codewarrior/TestE.java:10: error: unmappable character for encoding ASCII
"world that a child can think; and, possibly, do it practically; you wouldn???t constantly run\n" +

...

[parsing started RegularFileObject[/home/codewarrior/TestE.java]]
[parsing completed 14ms]
[parsing started RegularFileObject[./frameworks/java/CwRunListener.java]]
[parsing completed 3ms]
[total 130ms]
27 errors

I think some of your test cases are making assumptions that return is different from null before asserting.
Maybe you could add assertNotNull to answers before asserting something else. That woul avoid nullPointers on the test case, which makes it harder to figure out what the problem with our code is.

Is there a way to check which inputs you have on your testcases?

I'm getting a Runtime Error Occurred java.lang.NullPointerException at SumOfKTest.BasicTests2(SumOfKTest.java:95) but I don't have access to that line of code.

It's the "Basic Tests bigger numbers" test, but I've tried inputing even Integer.MAX_VALUE without my code throwing exceptions, so I'm not sure what to look for.