What do those strange class names in a java heap dump mean?

You’ll find the complete list documented under Class.getName():

If this class object represents a
reference type that is not an array
type then the binary name of the class
is returned, as specified by the Java™
Language Specification, Second
Edition.

If this class object represents a
primitive type or void, then the name
returned is a String equal to the Java
language keyword corresponding to the
primitive type or void.

If this class object represents a
class of arrays, then the internal
form of the name consists of the name
of the element type preceded by one or
more ‘[‘ characters representing the
depth of the array nesting. The
encoding of element type names is as
follows:

Element Type        Encoding
boolean             Z
byte                B
char                C
class or interface  Lclassname;
double              D
float               F
int                 I
long                J
short               S