我一直在编写这个程序，出于某种原因，当我尝试使用JButton的Action Listener时，我得到了一个Null指针异常。很抱歉，我的大多数代码都在main方法中，我对面向对象编程的面向对象部分感到很沮丧。问题始于main方法中while（true）语句之后的第一个actionlistener。
public class SignInFrame extends JFrame {
/**
*
*/
private static final long serialVersionUID = 1L;
private JPanel contentPane;
private static JTextField input;
private static JButton btnNext;
private static JButton btnYes;
private static JButton btnNo;
private static JLabel output;
private static String idstring;
private static int yesno = 0;
private static int id = 0;
/**
* Launch the application.
*/
public static int getIndex(int[] array, int x){
int index = -1;
for(int i = 0; i < array.length; i++){
if(array[i] == x)
index = i;
}
return index;
}
public static void main(String[] args) throws Exception {
EventQueue.invokeLater(new Runnable() {
public void run() {
try {
SignInFrame frame = new SignInFrame();
frame.setVisible(true);
} catch (Exception e) {
e.printStackTrace();
}
}
});
int i;
DateFormat dateFormat = new SimpleDateFormat("MM_dd_yyyy");
Date date = new Date();
File file = new File("Attendance/" + (dateFormat.format(date) + "_Attendance.txt"));
File numfile = new File("DataBase/Student Number DataBase.txt");
File namfile = new File("DataBase/Student Name DataBase.txt");
file.getParentFile().mkdirs();
PrintWriter info = new PrintWriter(file);
PrintWriter numdata = new PrintWriter(new FileWriter(numfile,true));
PrintWriter namdata = new PrintWriter(new FileWriter(namfile,true));
@SuppressWarnings("resource")
Scanner sc = new Scanner(namfile);
List<String> lines = new ArrayList<String>();
while (sc.hasNextLine()) {
lines.add(sc.nextLine());
}
String[] namedatabase = lines.toArray(new String[150]);
@SuppressWarnings("resource")
Scanner scc = new Scanner(numfile);
List<Integer> numbers = new ArrayList<Integer>();
while (scc.hasNextInt()) {
numbers.add(scc.nextInt());
}
int[] numberdatabase = new int[150];
for(int p=0, len = numbers.size(); p < len; p++){
numberdatabase[p] = numbers.get(p);
}
while(true){
btnNext.addActionListener(new ActionListener() {
public void actionPerformed(ActionEvent e) {
idstring = input.getText();
id = Integer.parseInt(input.getText());
}
});
if(idstring == "000") break;
while(idstring.length() != 9){
output.setText("Error(not 9-digits), retype it");
btnNext.addActionListener(new ActionListener() {
public void actionPerformed(ActionEvent e) {
idstring = input.getText();
id = Integer.parseInt(input.getText());
}
});
}
while(getIndex(numberdatabase, id) == -1){
output.setText("Is your Employee Number " + id + "?");
btnNext.setVisible(false);
btnYes.setVisible(true);
btnNo.setVisible(true);
btnYes.addActionListener(new ActionListener() {
public void actionPerformed(ActionEvent e) {
yesno = 1;
}
});
btnNo.addActionListener(new ActionListener() {
public void actionPerformed(ActionEvent e) {
yesno = 2; }
});
if(yesno == 1){
for(i = 0; i < numberdatabase.length; i++){
if(numberdatabase[i] == 0) break;
}
numberdatabase[i] = id;
numdata.println(id);
output.setText("Write your FIRST AND LAST name: ");
btnNext.setVisible(true);
btnYes.setVisible(false);
btnNo.setVisible(false);
namedatabase[i] = input.getText();
namdata.println(namedatabase[i]);
}
if(yesno == 2){
output.setText("Retype your Employee ID: ");
btnNext.setVisible(true);
btnYes.setVisible(false);
btnNo.setVisible(false);
btnNext.addActionListener(new ActionListener() {
public void actionPerformed(ActionEvent e) {
idstring = input.getText();
id = Integer.parseInt(input.getText());
}
});
while(idstring.length() != 9){
output.setText("Error(not 9-digits), retype it: ");
btnNext.addActionListener(new ActionListener() {
public void actionPerformed(ActionEvent e) {
idstring = input.getText();
id = Integer.parseInt(input.getText());
}
});
}
}
}
DateFormat timeFormat = new SimpleDateFormat("hh:mm a");
Date time = new Date();
System.out.println("");
System.out.println("You logged in at " + timeFormat.format(time) + " as " + namedatabase[getIndex(numberdatabase, id)] + " : " + id);
info.print("Employee ID: " + id);
info.print(" Name: " + namedatabase[getIndex(numberdatabase, id)]);
info.println(" " + timeFormat.format(time));
}
info.close();
numdata.close();
namdata.close();
}
/**
* Create the frame.
*/
public SignInFrame() {
setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
setBounds(100, 100, 450, 300);
contentPane = new JPanel();
contentPane.setBorder(new EmptyBorder(5, 5, 5, 5));
setContentPane(contentPane);
contentPane.setLayout(null);
JLabel attendance = new JLabel("Attendance");
attendance.setBounds(5, 5, 424, 25);
attendance.setFont(new Font("Tahoma", Font.BOLD, 20));
attendance.setHorizontalAlignment(SwingConstants.CENTER);
contentPane.add(attendance);
output = new JLabel("Write your 9-digit Employee ID");
output.setHorizontalAlignment(SwingConstants.CENTER);
output.setBounds(84, 73, 271, 14);
contentPane.add(output);
input = new JTextField();
input.setBounds(176, 98, 86, 20);
contentPane.add(input);
input.setColumns(10);
btnNext = new JButton("Next");
btnNext.setBounds(173, 129, 89, 23);
contentPane.add(btnNext);
btnYes = new JButton("Yes");
btnYes.setVisible(false);
btnYes.setBounds(123, 129, 89, 23);
contentPane.add(btnYes);
btnNo = new JButton("No");
btnNo.setVisible(false);
btnNo.setBounds(225, 129, 89, 23);
contentPane.add(btnNo);
}
}
exception trace
java.lang.NullPointerException
at SignInFrame.main(SignInFrame.java:94)
at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
at sun.reflect.NativeMethodAccessorImpl.invoke(Unknown Source)
at sun.reflect.DelegatingMethodAccessorImpl.invoke(Unknown Source)
at java.lang.reflect.Method.invoke(Unknown Source)
at edu.rice.cs.drjava.model.compiler.JavacCompiler.runCommand(JavacCompiler.java:272)
答案 0 :(得分：1)
异常发生在
行btnNext.addActionListener(new ActionListener() {
除了btnNext之外，此行可能为空？没有。所以btnNext为空。
事实上，btnNext在执行此代码行时尚未初始化，因为它在SignInFrame的构造函数中初始化，在内部调用
EventQueue.invokeLater(new Runnable() {
public void run() {
try {
SignInFrame frame = new SignInFrame();
frame.setVisible(true);
} catch (Exception e) {
e.printStackTrace();
}
}
});
正如其名称所示，您传递给invokeLater()的内容将在稍后调用。
坦率地说，你的代码是一团糟。应始终从事件派发线程访问每个Swing组件。因而不是直接在主方法下。这就是在invokeLater调用中初始化帧的原因。
构造函数不应初始化静态字段。按钮应该是框架的私有实例字段。主要方法不应该处理它们。并且有一个无限循环可以将听众添加到按钮中没有任何意义。
阅读Swing教程。
答案 1 :(得分：0)
JButton尚未初始化。您在空白的JButton上调用btnNext.addActionListener。因此，空指针就是这样。
例如：
JButton btnNewButton_1 = new JButton("New");
btnNewButton_1.setBackground(Color.LIGHT_GRAY);
btnNewButton_1.setBounds(236, 228, 89, 23);
frame.getContentPane().add(btnNewButton_1);
btnNewButton_1.addActionListener(new ActionListener() {
public void actionPerformed(ActionEvent e) {
//actions here
}
});