Never store record’s Id as a String (Variable type) if you are comparing Ids in your Apex code

If you are writing apex code then make sure that you are treating Ids’ carefully.  You should emphasize on declaring your variables of Id-type instead of String since storing record Id in a variable which type is Id will do the 15-character Id to 18-character Id conversion. Let’s take an example.

Code uses String type variable for holding record Ids

Map<String, Opportunity> mapOpportunities = new Map<String, Opportunity>();
mapOpportunities.put('006w000000tVvtt', Opp);
006w000000tVvtt - 15_Character_Opp_Id 

String oppRecordId = '006w000000tVvttAAG';
mapOpportunities.containsKey( oppRecordId ); // Will return False
006w000000tVvttAAG - 18_Character_Opp_Id
// Same as above
String character_15_id = '006w000000tVvtt';
String character_18_id = '006w000000tVvttAAG';
character_15_id == character_18_id  // return false

Code uses Id type variable for holding record Ids

Map<Id, Opportunity> mapOpportunities = new Map<Id, Opportunity>();
mapOpportunities.put('006w000000tVvtt', Opp);
006w000000tVvtt - 15_Character_Opp_Id 
Id oppRecordId = '006w000000tVvttAAG';
// Performing containsKey operation will give you true
// since declaring the map key as Id has done the required ID's conversion
mapOpportunities.containsKey( oppRecordId ); // Will return True
006w000000tVvttAAG - 18_Character_Opp_Id

// Same as above
Id character_15_id = '006w000000tVvtt';
Id character_18_id = '006w000000tVvttAAG';
character_15_id == character_18_id // return true

Declaring such variables as Id will also help you to deal with invalid Ids. Happy coding!! 🙂