我一直收到一个null异常错误，我似乎无法解决它。我知道当你尝试使用null值但我将值传递给文本字段时，通常会引发空指针错误，所以我很困惑。寻求反馈：
确切的错误代码:(这是我将actionlistner添加到按钮bt28的地方）
＆＃34; AWT-EventQueue的-0＆＃34;显示java.lang.NullPointerException 在GUI $ 4.actionPerformed（GUI.java:231）......等＆＃34;
P.S。这个类在主类中调用，这就是为什么不在这里。我已经进口了awt，swing和util。
public class GUI{
CardLayout cl=new CardLayout();
private String cardNum, pin;
JLabel header1,header2,header3,header4;
JButton bt01= new JButton("");
//bt01.setBounds(100,150,100,40);
JButton bt02= new JButton("");
JButton bt03= new JButton("");
JButton bt04= new JButton("");
JButton bt05= new JButton("");
JButton bt06= new JButton("");
JButton bt07= new JButton("");
JButton bt08= new JButton("Continue");
JButton bt21= new JButton("");
JButton bt22= new JButton("");
JButton bt23= new JButton("");
JButton bt24= new JButton("");
JButton bt25= new JButton("");
JButton bt26= new JButton("");
JButton bt27= new JButton("");
JButton bt28= new JButton("Continue");
JButton bt31= new JButton("");
JButton bt32= new JButton("");
JButton bt33= new JButton("");
JButton bt34= new JButton("");
JButton bt35= new JButton("Balance Query");
JButton bt36= new JButton("Withdraw");
JButton bt37= new JButton("Deposit");
JButton bt38= new JButton("");
JButton bt41= new JButton("A");
JButton bt42= new JButton("B");
JButton bt43= new JButton("C");
JButton bt44= new JButton("D");
JButton bt45= new JButton("E");
JButton bt46= new JButton("");
JButton bt47= new JButton("Business Accounts");
JButton bt48= new JButton("Cancel");
JPanel panelCont=new JPanel();
JPanel GUI1= new JPanel(new BorderLayout());
JPanel GUI2= new JPanel(new BorderLayout());
JPanel GUI3= new JPanel(new BorderLayout());
JPanel GUI4= new JPanel(new BorderLayout());
public GUI(){
JFrame frame1 = new JFrame ("JLCB Automated Banking Machine v 1.0 ");
frame1.setPreferredSize (new Dimension(500, 200));
header1= new JLabel("JLCB Automated Banking Machine");
JPanel nPanel1= new JPanel();
nPanel1.add(header1);
GUI1.add(nPanel1,BorderLayout.NORTH);
JLabel cardnumLabel =new JLabel("Enter Card Number");
JTextField txt= new JTextField(15);
txt.addActionListener(new ActionListener(){
public void actionPerformed(ActionEvent cl){
cardNum=txt.getText();
txt.setText(null);
}
});
JPanel cPanel1= new JPanel();
cPanel1.add(cardnumLabel);
cPanel1.add(txt);
GUI1.add(cPanel1, BorderLayout.CENTER);
JPanel wPanel1=new JPanel(new GridLayout(4,0));
wPanel1.add(bt01);
wPanel1.add(bt02);
wPanel1.add(bt03);
wPanel1.add(bt04);
JPanel ePanel1=new JPanel(new GridLayout(4,0));
ePanel1.add(bt05);
ePanel1.add(bt06);
ePanel1.add(bt07);
ePanel1.add(bt08);
GUI1.add(wPanel1,BorderLayout.WEST);
GUI1.add(ePanel1,BorderLayout.EAST);
//................................
// .
// GUI2 .
//................................
JFrame frame2 = new JFrame ("JLCB Automated Banking Machine v 1.0 ");
frame2.setPreferredSize (new Dimension(500, 200));
header2= new JLabel("JLCB Automated Banking Machine");
JPanel nPanel2= new JPanel();
nPanel2.add(header1);
GUI2.add(nPanel2,BorderLayout.NORTH);
JLabel pinLabel =new JLabel("Enter your pin");
JTextField txt2= new JTextField(15);
txt2.addActionListener(new ActionListener(){
public void actionPerformed(ActionEvent cl){
pin=txt2.getText();
txt2.setText(null);
}
});
JPanel cPanel2= new JPanel();
cPanel2.add(pinLabel);
cPanel2.add(txt2);
GUI2.add(cPanel2, BorderLayout.CENTER);
JPanel wPanel2=new JPanel(new GridLayout(4,0));
wPanel2.add(bt21);
wPanel2.add(bt22);
wPanel2.add(bt23);
wPanel2.add(bt24);
JPanel ePanel2=new JPanel(new GridLayout(4,0));
ePanel2.add(bt25);
ePanel2.add(bt26);
ePanel2.add(bt27);
ePanel2.add(bt28);
GUI2.add(wPanel2,BorderLayout.WEST);
GUI2.add(ePanel2,BorderLayout.EAST);
//................................
// .
// GUI3 .
//................................
JFrame frame3 = new JFrame ("JLCB Automated Banking Machine v 1.0 ");
frame3.setPreferredSize (new Dimension(500, 200));
header3= new JLabel("JLCB Automated Banking Machine");
JPanel nPanel3= new JPanel();
nPanel3.add(header1);
GUI3.add(nPanel3,BorderLayout.NORTH);
JLabel txt3 =new JLabel("Please select from the buttons on the right");
JPanel cPanel3= new JPanel();
cPanel3.add(txt3);
GUI3.add(cPanel3, BorderLayout.CENTER);
JPanel wPanel3=new JPanel(new GridLayout(4,0));
wPanel3.add(bt31);
wPanel3.add(bt32);
wPanel3.add(bt33);
wPanel3.add(bt34);
JPanel ePanel3=new JPanel(new GridLayout(4,0));
ePanel3.add(bt35);
ePanel3.add(bt36);
ePanel3.add(bt37);
ePanel3.add(bt38);
GUI3.add(wPanel3,BorderLayout.WEST);
GUI3.add(ePanel3,BorderLayout.EAST);
//..................................
// .
// GUI4 .
//..................................
JFrame frame4 = new JFrame ("JLCB Automated Banking Machine v 1.0 ");
frame4.setPreferredSize (new Dimension(500, 200));
header4= new JLabel("JLCB Automated Banking Machine");
JPanel nPanel4= new JPanel();
nPanel4.add(header4);
GUI4.add(nPanel4,BorderLayout.NORTH);
JPanel cPanel4= new JPanel();
GUI4.add(cPanel4, BorderLayout.CENTER);
JPanel wPanel4=new JPanel(new GridLayout(4,0));
wPanel4.add(bt41);
wPanel4.add(bt42);
wPanel4.add(bt43);
wPanel4.add(bt44);
JPanel ePanel4=new JPanel(new GridLayout(4,0));
ePanel4.add(bt45);
ePanel4.add(bt46);
ePanel4.add(bt47);
ePanel4.add(bt48);
GUI4.add(wPanel4,BorderLayout.WEST);
GUI4.add(ePanel4,BorderLayout.EAST);
//---------------------------------------------------
panelCont.setLayout(cl);
panelCont.add(GUI1, "1");
panelCont.add(GUI2, "2");
panelCont.add(GUI3, "3");
cl.show(panelCont,"1");
String a ="100001 1111 15984789.74";
bt08.addActionListener(new ActionListener(){
@Override
public void actionPerformed(ActionEvent e){
cl.show(panelCont,"2");
}
});
bt28.addActionListener(new ActionListener(){
@Override
public void actionPerformed(ActionEvent r){
if (cardNum.equals(a.substring(0, a.indexOf(" "))) && pin.equals(a.substring(a.indexOf(" ")+1,a.lastIndexOf(" ")))){
cl.show(panelCont,"3");
String acc="a";
}
else{
JOptionPane.showMessageDialog(null,"Invalid accoun number or pin");
cl.show(panelCont,"3");
}
}
});
bt35.addActionListener(new ActionListener(){
@Override
public void actionPerformed(ActionEvent e){
cl.show(panelCont,"3");
}
});
bt36.addActionListener(new ActionListener(){
@Override
public void actionPerformed(ActionEvent e){
cl.show(panelCont,"3");
}
});
bt37.addActionListener(new ActionListener(){
@Override
public void actionPerformed(ActionEvent e){
cl.show(panelCont,"3");
}
});
frame1.add(panelCont);
//(JFrame.DO_NOTHING_ON_CLOSE)
frame1.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
frame1.pack();
frame1.setVisible(true);
}
}
答案 0 :(得分：0)
问题是你的cardNum和pin的值永远不会在txt和txt2的actionListeners中分配。您应该在用户单击“继续”按钮的情况下提取值。请参阅下面的代码。
bt28.addActionListener(new ActionListener() {
@Override
public void actionPerformed(ActionEvent r) {
cardNum = txt.getText();
pin = txt2.getText();
if (cardNum.equals(a.substring(0, a.indexOf(" "))) && pin.equals(a.substring(a.indexOf(" ") + 1, a.lastIndexOf(" ")))) {
cl.show(panelCont, "3");
String acc = "a";
} else {
JOptionPane.showMessageDialog(null, "Invalid accoun number or pin");
cl.show(panelCont, "3");
}
}
});