How To Use Single Button To Insert And Update In Java While Using MySQL Database

I am developing my first Java application and the database I use is MySQL. I have created two separate buttons; one to insert new data into my database and the other to update the database when changes are made.

Each button is working perfectly, but I now want to combine both functions into just the save button. Thus, whether I am entering new data or modifying existing data, the save button should use an IF ELSE condition to decide whether to use the INSERT or UPDATE command.

My problem is, how do I write this IF ELSE statement? What should be the condition? For example;

IF(what? ){
  String sql ="Insert into Table1 (classID,className,counselorID,startDate,endDate) values (?,?,?,?,?)";
  }ELSE{
  String sql2 = "update Table1 set classID = '"+value1+"',className='"+value2+"',counselorID='"+value3+"',startDate='"+value4+"',endDate='"+value5+"'
where classID = '"+value1+"'";
}