public class MainFrame extends javax.swing.JFrame{

}

//create this class to store data
public class Dataset{
    private static Dataset instance = new Dataset();
    private Competition comp = new Competition();

    //private the constructor to make it a singleton
    private Dataset(){

    }

    public static getInstance(){
        return instance;
    }
    public Competition getCompetition(){
                return comp;
        }

}

//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
        Dataset.getInstance().getCompetition().swimmers.add(s);
        }
}