I thought you provided just enough "code" to enable a reply.
It definitely wasn't "working" code, though, so it was pseudocode at best:
private String generateHTML(Event event)
StringBuilder sb = new StringBuilder();
b.append("<HTML><table>"); <-- WRONG NAME FOR THE VARIABLE
You are allowed to post "code for contrived problems like homework assignments".
So to ensure that your post will be considered on-topic, the best thing to do in a case like this would be to write the smallest piece of working code which demonstrates the problem/question you are asking about.
In your question, it wouldn't take much:
Replace the following three comments with a line of code:
//bunch of properties and accessors
//properties and accessors
//append HTML and ExplosionEvent specific data
Ensure that the code you post will actually build correctly and contains no bugs.