Capturing a drop-down value and then display the result in a scripted field

[Sanket Mane]

We have 4 custom fields (Type- Drop-down) where users can select a numeric value. 

We want to do calculation based upon the values selected within the dropdown fields and display the results in a scripted field.

Code:

import com.atlassian.jira.component.ComponentAccessor
import com.atlassian.jira.issue.search.SearchProvider
import com.atlassian.jira.jql.parser.JqlQueryParser
import com.atlassian.jira.web.bean.PagerFilter

def customFieldManager = ComponentAccessor.getCustomFieldManager()
def customField1 = customFieldManager.getCustomFieldObject(10000L)
def customField2 = customFieldManager.getCustomFieldObject(10001L)
def customField3 = customFieldManager.getCustomFieldObject(10002L)
def customField4 = customFieldManager.getCustomFieldObject(10003L)

double calculated

if(issue.getCustomFieldValue(customField1) && issue.getCustomFieldValue(customField2) && issue.getCustomFieldValue(customField3) && issue.getCustomFieldValue(customField4))

{
def cf1= issue.getCustomFieldValue(customField1) as double
def cf2= issue.getCustomFieldValue(customField2) as double
def cf3= issue.getCustomFieldValue(customField3) as double
def cf4= issue.getCustomFieldValue(customField4) as double

return ((cf1+ cf2+ cf3) / cf4)
} 

else {
return null
}

Error:
org.codehaus.groovy.runtime.typehandling.GroovyCastException: Cannot cast object '[8]' with class 'java.util.ArrayList' to class 'java.lang.Double'