Workshop 201, Tutorial 3, Step 4.3, Apex

InvoiceUtilities.cls

public class InvoiceUtilities {

    // class method to renumber Line Items for a given Invoice number
    // returns a string that indicates success or failure
    public static String renumberLineItems(String invoiceName) {

    // create a copy of the target Invoice object and it's Line Items
    Invoice__c invoice = [Select i.Name, (Select Name From Line_Items__r ORDER BY Name) 
                            From Invoice__c i 
                           Where i.Name = :invoiceName LIMIT 1];

    // loop through each Line Item, renumbering as you go
    Integer i = 1;
    for (Line_Item__c item : invoice.Line_Items__r) {
      item.Name = String.valueOf(i);
      System.debug(item.Name);
      i++;
    }

    // update the Line Items in one transaction, rollback if any problems
    // and return error messages to the calling environment
    try {
      database.update(invoice.Line_Items__r);
    }
    catch (DmlException e) {
      return e.getMessage();
    }

    // on success, return a message to the calling program
    return 'Line items renumbered successfully.'; 
  }
}

My code executed pretty ok. I noticed after the execution however that my "Invoice Number" field format is without the dash so that's INV0004 for me.

thanks rafal00785 I have also noted your approach.

Hi Faced same issue..Here is my solution.
My code executed.I just copied the Invoice number from Record and used in code. The dash length is same as developer console.

String s = InvoiceUtilities.renumberLineItems('INV–0011');

Srilaxmi15 is correct, copy and paste the invoice number from the application; for some reason it's a different dash. In the workbook, they use U+2013, En Dash, instead of what's in the record, a simple U+002D, Hyphen Minus sign.