I used javac of j2sdk1.4.1 to compile the following codes:
public class comment {
  public static void main(String argv[]) {
    System.out.println("comment");
  }
} //end comment

But the compiler complaint:
comment.java:5: unclosed comment
} //end comment
1 error

The more intesting thing is when I add a blank line after the whole source file, it is ok.

I think you may search a \n after // as the end of the comment, but if the comment locates at the last line of the file, it cause problem.

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