I'm trying to populate/set/put/initialize a custom setting's field with a calculated value within my apex code. The field type is number. Is there a way to do this?
For Hierarchy custom settings, you can get an instance, then populate and insert it:
//fetches or creates a Custom Setting at the organization level HierarchySetting__c setting = HierarchySetting__c.getOrgDefaults(); setting.CustomField__c = 'http://salesforce.stackexchange.com/'; upsert setting;
Or with List custom settings: new one up with the fields and insert:
upsert new ListSetting__c( Name = 'sandbox', CustomField__c = 'https://test.salesforce.com/' ); upsert new ListSetting__c( Name = 'production', CustomField__c = 'https://login.salesforce.com/' );
Check out the docs: