J3D provides a box constructor Box(float xdim, float ydim, float zdim, Appearance ap).

The documentation states:

public Box(float xdim,
           float ydim,
           float zdim,
           Appearance ap)

Constructs a box of a given dimension and appearance. Normals are generated by default, texture coordinates are not.

Parameters:

xdim – X-dimension size.

ydim – Y-dimension size.

zdim – Z-dimension size.

ap – Appearance

This is an error.  xdim, ydim, zdim are actually  one-half the x,y, and z dimension sizes.