Android : use input numbers for calculation : Android

I'm new to programming and I'm developing a simple android app. In the app there is a input editText to enter a number and I need to use that value for further calculation.

int usedUnits = Integer.parseInt(units.getText().toString());

I used above code for convert string to integer but when I'm writing formula I cannot use that value. as a suggestion it says make it "final" but when it make "final" there is no compilation error but there is some runtime error occurred. How can I solve this?? any suggestions??