Let A be a non-empty set and $f:\emptyset \rightarrow A$, why f must be 1-1? Let A be a non-empty set and $f:\emptyset \rightarrow A$, why f must be 1-1 ?
This is a part of my Sets Numbers and Functions' mid-term test, I'm still confusing about it... 

If a function is not injective, this means there exists $x,y$ in the domain of the function such that $x\neq y$ and $f(x)=f(y)$.
But the domain of $f$ is $\varnothing$...
