@AndreaLaforgia, in practice you should never "expect" too much about code; anything not declared final is modifiable.

IMO best practice is make code intent clearer by getting into habit of using final modifier wherever appropriate - eg Unless you are going to change p0 then it should be declared final p0.

+1 for using the += operation to cast to int and auto floor it! We can't increase the population by 0.25 of a person.