How to convert 15 to 18 digit Id using apex in Salesforce?

Here is the simple way to convert Salesforce 15 digit ID into 18 digit:

String fifteenDigit = '500i0000006oHua'; // Your 15 digit salesforce Id
Id eighteenDigit = fifteenDigit;        
System.debug('15 Digit Id is => ' + fifteenDigit);
System.debug('18 Digit Id is => ' + eighteenDigit);

Output:

15 Digit Id is 500i0000006oHua
18 Digit Id is 500i0000006oHuaAAE

Read More: Which Salesforce API Do I Use in Integration?
                    When To Use Salesforce REST API  
                   When To Use Salesforce SOAP API 
                   When To Use Salesforce Bulk API 
                   When To Use Salesforce Chatter API  

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