can anyone give me apex trigger examples for better understanding of the apex trigger 

// Map to keep key (caregory) and value related to each other
        Map<String, String> mailValues = new Map<String, String>();
        
        // Seperate keys and values and put it in the map 
       	List<String> listOfLines = plainText.split('\n');
        for(String line : listOfLines){
            String key   = line.substringBetween('<', '>');
            String value = line.substringBetween('<' + key + '>', '</' + key + '>');
            
            if (key != null) {                                    // this works just fine, null key not added 

            // if (key != null || value != null){           // works partial, null key not added, but empty value added
            //  if (key != null || value != ''){              // doesn't work at all, null key + empty value get added 

                mailValues.put(key, value);                
            }

Hi guys, I'm getting tagged values per mail in this format:

<key>value</key>
<key2>value</key2>

Somethimes there are linebreaks, so I get a null key added to the map. Got that solved with "key != null".

But I'm struggling with excluding no-values like <key></key>.
Probably easy, but I'm absulte beginner … can someone give me a hand here?
 

for(Apttus_Billing__Invoice__c invoice:invoiceObjectList1) // loop over invoices
              {
                  List<InvoiceLineItems> listToStoreLineItems = new List<InvoiceLineItems>();
                  APTS_InvoiceJournalWrapper invoiceJournWrapper= new APTS_InvoiceJournalWrapper();
                   countHeader=0;
                   
                   count=count+1;
                  Decimal Sum=0;
                   
                                    invoiceJournWrapper.id=String.valueof(invoice.id);
                                    invoiceJournWrapper.HeaderKey=String.valueof(count);
                                    invoiceJournWrapper.ZCurrency=(invoice.CurrencyIsoCode!=null?invoice.CurrencyIsoCode:'USD');
                                    //invoiceJournWrapper.AccountingJournal='null';
                                    invoiceJournWrapper.AccountingJournalID=String.valueOf(invoice.Name + '_' +myDate);
                                    invoiceJournWrapper.JournalNumber=String.valueOf(invoice.Name + '_' +myDate);
                                    invoiceJournWrapper.ExternalReferenceID=String.valueOf(invoice.Name);                  
                                    invoiceJournWrapper.AccountingDate=String.valueof(Date.valueof(invoice.Apttus_Billing__InvoiceDate__c));
                  //-------------------
                  list<Apttus_Billing__InvoiceLineItem__c> invlineitems=[Select id,Apttus_Billing__AssetLineItemId__r.Apttus_Config2__ProductId__r.ProductCode,Apttus_Billing__AssetLineItemId__r.Apttus_Config2__OptionId__r.ProductCode,Apttus_Billing__InvoiceId__r.Name,Apttus_Billing__ProductId__r.Name,Apttus_Billing__InvoiceId__r.CurrencyIsoCode,
                                                                      Apttus_Billing__Type__c,Apttus_Billing__InvoiceId__c,Name,Apttus_Billing__Amount__c,Apttus_Billing__SoldToAccountId__r.EQ_ClientRef__c,Apttus_Billing__LocationId__c,
                                                                      Apttus_Billing__ProductId__r.EQ_FeeCode__c,Apttus_Billing__BillToAccountId__r.EQ_ClientRef__c,Apttus_Billing__ProductId__r.ProductCode,Apttus_Billing__InvoiceId__r.EQ_TotalInvoiceAmount__c 
                                                             from Apttus_Billing__InvoiceLineItem__c where Apttus_Billing__InvoiceId__c=:invoice.id and Apttus_Billing__Type__c='Contracted' and Apttus_Billing__Amount__c!=0];
                  //----
                  //
                  //-------------
                   InvoiceLineItems FirstInvItems= new InvoiceLineItems();
                            countHeader=countHeader+1;
                            FirstInvItems.LineKey=string.valueof(countHeader);