将项目添加到不同类的ArrayList中

Question

For an assignment I have to make a Java application and one of the functions I'm having a problem with is adding to an ArrayList . The ArrayList is in a different class then the method and I get a nullpointer (doesn't happen when I make it static).

This is the code where I'm trying to add:

if (e.getSource() == voegToe) {
   if (naam.equals("") || straat.equals("") || huisnummer.equals("")
                || postcode.equals("") || plaats.equals("")
                || telefoonnummer.equals("") || email.equals("")) {

            allesoke = false;
            foutmelding += "\n";
  }

  if (allesoke == true) {
      Klant newKlant = new Klant (naam, postcode, huisnummer);
      System.out.println("HOI");
      klantenBindingRef.alleKlanten.add(newKlant);
      System.out.println(klantenBindingRef.alleKlanten);
  } else {
      JOptionPane.showMessageDialog(null, foutmelding, "Foutmelding",
            JOptionPane.INFORMATION_MESSAGE, null);
  }

And this is part of the code of the class with the ArrayList :

public class Klantenbinding {
private String naam;
protected List<Klant> alleKlanten = new ArrayList<Klant> ();

// klanten
public void voegKlantToe(Klant nweKlant) {

    if (!heeftKlant(nweKlant.getNaam())) {
        alleKlanten.add(nweKlant);
    }
}
    public boolean heeftKlant(String naam) {
    boolean b = false;
    for (Klant k : alleKlanten) {
        if (k.getNaam() == naam) {
            b = true;
        }
    }
    return b;
}

The nullpointer comes from the line:

klantenBindingRef.alleKlanten.add(newKlant);

public class KlantToevoegen extends JFrame implements ActionListener {
/**
 * 
 */
JFrame frame;
private static final long serialVersionUID = 1L;
private JButton voegToe, terug;
private JPanel contentPane;
private JTextField nmTf, strTf, hnrTf, pcTf, plTf, telnrTf, emailTf;
private JLabel lblAanhef, lblNaam, lblStraat, lblNieuweKlantToevoegen,
        lblHuisnr, lblPostcode, lblPlaats, lblTelnr, lblEmail;
private JComboBox aanhefBox;
private Klantenbinding klantenBindingRef;
private AutoToevoegenFrame autoToevoegenFrameRef;
private JSeparator separator;
private JSeparator separator_1;

/**
 * Launch the application.
 */

Answer 1

Did you try importing the class with the ArrayList in it? (can't comment yet)

Answer 2

我敢打赌，您的变量klantenBindingRef永远不会初始化。

Answer 3

Try comparing your strings in this way to avoid NPE

if ("".equals(naam) || "".equals(straat) || ...) {
    allesoke = false;
    foutmelding += "\n";
}

then verify that klantenBindingRef is instantiated before access to his variables

//klantenBindingRef is null
klantenBindingRef = new Klantenbinding();
klantenBindingRef.alleKlanten
...

Answer 4

initialize your varibale klantenBindingRef before use klantenBindingRef = new Klantenbinding();

if( klantenBindingRef == null ) 
    klantenBindingRef = new Klantenbinding();
klantenBindingRef.alleKlanten.add(newKlant);