ciao!

grazie per l'ottima sopiegazione.

ho optato per un flowlayout.
per quello che devo fare basta e avanza:
codice:
public class MainWindow extends JFrame { 
 
    private JPanel mainPanel; 
    private JTextField txtSource; 
    private JTextField txtDestination; 
    private JButton btnStart; 
    private int generalW = 300; 
    private int generalH = 20; 
 
    public MainWindow() { 
        mainPanel = new JPanel(new BorderLayout()); 
        txtSource = new JTextField(); 
        txtSource.setPreferredSize(new Dimension(generalW, generalH)); 
        txtDestination = new JTextField(); 
        txtDestination.setPreferredSize(new Dimension(generalW, generalH)); 
        btnStart = new JButton("Start"); 
        btnStart.setPreferredSize(new Dimension(generalW, generalH)); 
 
        mainPanel.add(txtSource, BorderLayout.NORTH); 
        mainPanel.add(txtDestination, BorderLayout.CENTER); 
        mainPanel.add(btnStart, BorderLayout.SOUTH); 
 
        setLayout(new FlowLayout()); 
        getContentPane().add(mainPanel); 
 
        setPreferredSize(new Dimension(500, 100)); 
        setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE); 
        setResizable(false); 
        pack(); 
        setLocationRelativeTo(null); 
    } 
}