java - How to get User Input to Populate Jira Fields -

i new java , using jira misc custom fields add-on , require logic assistance solve math functions between 2 drop down fields.

field 1 "user cost" field contains 4 string selections user price posted @ end of string.

1. sam costs .21
2. mitch costs .419
3. lance costs 2.66
4. xmen costs 13.338

field 2 "usage" field contains 2 string selections:

1. 24 hours (unless maintenance)
2. 12 hours (7a-7p)

the argument should invoked new field called "total user cost." field automatically display correct price user , usage amount.

the equation blueprint follows:

cost*31(calendar days)*usage(12 || 24) 

i want form update based on user input selection of these 2 fields , other variables in equation.

this have far:

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thank in advance feedback!

if understand correctly, first need initialize issue something. (it looks red in images variable doesn't exist)

then, can

double costsam = 0.21; string usersam = issue.get("customfield_10620"); 

then, if needing convert or otherwise math on usersam, need this

double samtotal = costsam * double.parsedouble(usersam); 

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some flaws code

1. you have define types values string or double.
2. if have string x = "hello" on 1 line, x = 0.4 on next, won't work because of incompatible types
3. if did have compatible types on consecutive lines, first line pointless unless using value first, second line overwrites value of first one