Show that P={X \in P(Z+)|X finite} is countably infinite.

I have no idea where to start.

Let $\displaystyle P= \{X \in \wp(Z^+)|X finite \}$. Prove that $\displaystyle P$ is denumerable (countably infinite), where $\displaystyle \wp(Z^+)$ is the power set of $\displaystyle Z^+$

I tried to find $\displaystyle f:Z^+ \longrightarrow P$ that is one-to-one and onto. But that attemt was failed. I also tried to find something that might be equivalent to $\displaystyle Z^+$ and $\displaystyle P$ and show that they are equivalent, but failed as well.

Please guide me. Thanks.