Running JavaTest. Whoa, very big crash. Turns out it's from LinkLoopException's test run, where neither its superclass' test nor its sibling's test failed, becuase they had null constructors.

So,
LinkLoopException needs a LinkLoopException() constructor

Turns out in Java you only get the null constructor for free if you don't define any other constructors. I didn't know that before today. For more on this, see "The Java Programming Language" page 35, or JLS 8.6.7 (page 180).