Attaching email template to Messaging.SingleEmailMessage throws the error

Question

while calling below code of sendEmail throwing an error as:

First error: SendEmail failed. First exception on row 0; first error: INVALID_FIELD_WHEN_USING_TEMPLATE, When a template is specified the plain text body, html body, subject and charset may not be specified : []

if I remove line - mail.setTemplateId(emailTemp.id); below code works fine. Is there anything wrong I am doing?

Messaging.SingleEmailMessage mail = new Messaging.SingleEmailMessage();
List<Messaging.SingleEmailMessage> mails = new List<Messaging.SingleEmailMessage>();
EmailTemplate emailTemp=[Select Id,Subject, HtmlValue, body from EmailTemplate where name='Notify on oppy closed'];
        List<String> sendTo = new List<String>();
        for(opportunity_contact__c opContact : oppyContact) {
           // sendTo.add(opContact.email__c);
            mail.setSubject(emailTemp.Subject);
            mail.setSenderDisplayName('Test Email');
            mail.setHtmlBody(emailTemp.HtmlValue);
            mail.setPlainTextBody(emailTemp.body);
            mail.setTargetObjectId('0031100000YDHru');
            mail.setSaveAsActivity(false);
            mail.setWhatId(opContact.opportunity__c);
            mail.setTemplateId(emailTemp.id); // #throwing error here
            mails.add(mail);
        }
        Messaging.sendEmail(mails);

Custom Email Template :

Dear {!Opportunity_Contact__c.Name__c}, 
TEXT Here.....
TEXT Here.....
TEXT Here.....TEXT Here.....
Opportunity : {!Opportunity.Id}
Regards,
{!User.FirstName} {!User.LastName} 
{!Organization.Name}

Thanks in advance!

Answer 1

Since you are specifying the attribute "mail.setTemplateId(emailTemp.id)", again no need to specify all those attributes. The below link will give you more information

https://www.salesforce.com/us/developer/docs/apexcode/Content/apex_classes_email_outbound_single.htm#apex_Messaging_SingleEmailMessage_methods

Answer 2

If you remove these two lines

 mail.setHtmlBody(emailTemp.HtmlValue);
 mail.setPlainTextBody(emailTemp.body);

your code should work

Answer 3

AFTER this step Messaging.sendEmail(mails);

PUT this: mails.clear();

This works for me.

Answer 4

    for(opportunity_contact__c opContact : oppyContact) {
       // sendTo.add(opContact.email__c);
        mail.setTargetObjectId('0031100000YDHru');
        mail.setWhatId(opContact.opportunity__c);
        mail.setTemplateId(emailTemp.id); // #throwing error here
        mails.add(mail);
    }

You cannot modify subject and content when using template to send an email via APEX.