maybe Character.digit(s.charAt(i), 10) is better?

sorry I was upset on not getting to work another solution and committed this dumb solution.

In Java, variables are passed by reference, but references are passed by value (bend your mind around that for a minute).

Practically speaking, this means you can't change input parameters by assignment, you end up only changing the local reference of the parameter to a different value. In other words:

int content = 100;
int evap_per_day = 10;
int threshold = 20;
Evaporator.evaporator(content, evap_per_day, threshold);
System.out.println(evap_per_day); //Still 10
System.out.println(threshold); //Still 20