public class actionfunctionDemo {
    public Opportunity thisOpportunity{get;set;}
    public boolean reloadPage{get;set;}
    
    public actionfunctionDemo(ApexPages.StandardController controller){
        reloadPage = true;
        thisOpportunity = (Opportunity) controller.getRecord();
    }
    public void updateOpp(){
        thisOpportunity.Name = 'UpdateName';
        try{
            update thisOpportunity;
        }
        catch(Exception ex){
            ApexPages.Message myMsg = new ApexPages.Message(ApexPages.Severity.FATAL, ex.getMessage());
            ApexPages.addMessage(myMsg);    
        }
    }
    
}