public Trigger when(Criterion criterion){
  if (criterion != null) {
    criterions.add(criterion);
  }
  return this;
}

public Trigger when ( Criterion criterion ) { if ( criterion != null ) { criterions . add ( criterion ) ; } return this ; }
set a conditional expression that restricts which rows will cause statements to be performed when the trigger is activated . by default , the statements will execute for every row affected by the statement that activates the trigger .
