code: public float getNormalZ(int vertex) {
          if (!hasNormals()) {
              throw new IllegalStateException("mesh has no normals");
          }
          
          checkVertexIndexBounds(vertex);
          
          return m_normals.getFloat((vertex * 3 + 2) * SIZEOF_FLOAT);
      }
docs: Returns the z-coordinate of a vertex normal.
  
  @param vertex the vertex index
  @return the z coordinate