Validate input of jTextField using setter java

I implement OOP in my Java assignment. But when I started creating user interface and accept the input I have faced a problem. I wanted to validate the user input of jTextField from user interface using my setter method. I want a pop up to appear when user input is invalid instead of just error message. I know it can be done easily if I implement the validation code directly in the user interface. I don’t know which way is better but since I already have all my setter method so I wanted to validate using setter method.

Employee Class

public void setUsername(String username){
    if(username.equals(null)){
        //validation method
    }

User Interface

private void jButton2ActionPerformed(java.awt.event.ActionEvent evt) {                                         
    Admin ad = new Admin();
    String username = jTextField6.getText();
    ad.setUsername(username);
}
  

String username = jTextField6.getText();

if(userName.isEmpty()){
    //Place your error message logic here
}else{
     ad.setUsername(username);
}