java - 即使输入了值，文本字段也会出现空指针错误
问题描述
我不断收到一个空异常错误，我似乎无法修复它。我知道当您尝试使用空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。寻找反馈：
确切的错误代码：（这是我将 actionlistner 添加到按钮 bt28 的地方）
"AWT-EventQueue-0" java.lang.NullPointerException at GUI$4.actionPerformed(GUI.java:231) ...等"
Ps 这个类在主类中被调用，这就是为什么不在这里。我已经导入了 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);
}
}
解决方案
问题是您的 cardNum 和 pin 值从未在 txt 和 txt2 的 actionListeners 中分配。如果用户单击继续按钮，您应该提取这些值。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");
}
}
});
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