//swimming competition class, that you made without UI, assume there's a Swimmer class exist already
public class Competition {
    public final List<Swimmer> swimmers = new ArrayList<Swimmer>();

    //All the swimming compeition based stuff here
    .....
    public static void main(String[] args) {
        Competition comp = new Competition();
        comp.swimmers.add(new Swimmer());
    }
}

//now the Main UI form which starts when your app starts
public class MainFrame extends javax.swing.JFrame{
    ....
}

//another third JFrame class or a JDialog class, that has a JButton, when you press it
// you want to create a new Swimmer object and store it in some list
public class AddSwimmerDialog extends javax.swing.JDialog {
    .....
    private void jButton1ActionPerformed(java.awt.event.ActionEvent evt) {
        //you create the object
        Swimmer s = new Swimmer();
        //now you need to store somewhere
    }
}