Three kinds of classes

Primitive

These are classes which correspond to primitive data types like short. Note that these are different from the classes java.lang.Short and such. These classes are preloaded by Jato and used only by Java Standard Library.

Array

These are classes which correspond to types of references like

int [] ia = new int[10];

There cannot be any subclasses for these classes.

Regular

All other "normal" classes fall into this category.