  In this paper, we give a new proof of a celebrated theorem of J\"orgens which
states that every classical convex solution of \[ \det\nabla^2 u (x)=1\quad
{in} \mathbb{R}^2 \] has to be a second order polynomial. Our arguments do not
use complex analysis, and can be applied to establish such Liouville type
theorems for solutions of a class of degenerate Monge-Amp\`ere equations. We
prove that every convex generalized (or Alexandrov) solution of \[ \det
\nabla^2 u(x_1,x_2)=|x_1|^{\alpha} \quad {in} \mathbb{R}^2, \] where
$\alpha>-1$, has to be \[ u(x_1,x_2)=
\frac{a}{(\alpha+2)(\alpha+1)}|x_1|^{2+\alpha}+\frac{a b^2}{2}x_1^2 +bx_1x_2+
\frac{1}{2a} x_2^2+\ell(x_1,x_2) \] for some constants $a>0$, $b$ and a linear
function $\ell(x_1,x_2)$.
  This work is motivated by the Weyl problem with nonnegative Gauss curvature.
