public class crud extends javax.swing.JFrame {
    DefaultTableModel model;
    //declare koneksi disini
    private Connection koneksi;

    /**
     * Creates new form crud
     */
    public crud() {
        initComponents();
        ...bla bla bla
        //instance koneksi di sini
        koneksi =  DriverManager.getConnection("jdbc:mysql://localhost:3306/crud","root","");
    }

private void tampilkan() {
...bla blaa
//jd tinggal panggil koneksi di button dll, gak perlu di instance lg
ResultSet rs = koneksi.createStatement().executeQuery("select * from datasiswa");

....bla blaaa

}