How to send email in TriggerClass in Salesforce

TriggerClass2 .java

Messaging.SingleEmailMessage mail = new Messaging.SingleEmailMessage();    
string body = 'Hi ';  
String[] toAddresses = new String[] {'sksworld10@gmail.com'};   
mail.setToAddresses(toAddresses);  
mail.setSubject('Test Subject');    
mail.setSaveAsActivity(false);    
for(OrgWideEmailAddress owa : [select id, Address, DisplayName from OrgWideEmailAddress])   
{  
 if(owa.Address.contains('testuser'))   
  mail.setOrgWideEmailAddressId(owa.id);   
}  
mail.setHtmlBody(body);    
Messaging.sendEmail(new Messaging.SingleEmailMessage[] { mail });