当我从欢迎的JFrame中单击一个按钮以加载到我的主框架中时，我一直收到以下错误:
Exception in thread "AWT-EventQueue-0" java.lang.NullPointerException
at SRC_Main.actionPerformed(SRC_Main.java:291)
at javax.swing.AbstractButton.fireActionPerformed(Unknown Source)
at javax.swing.AbstractButton$Handler.actionPerformed(Unknown Source)
at javax.swing.DefaultButtonModel.fireActionPerformed(Unknown Source)
at javax.swing.DefaultButtonModel.setPressed(Unknown Source)
在我的主要
frame上，我有一个next和previous按钮，并且我使用对象数组填充一个数组来存储Cars的记录。但是如果我自己运行Main Jframe，则可以运行
next和prev按钮，因此没有无法归档的文本文件，依此类推。为什么不“初始化”？
这是主要的代码
import java.io.*;
import java.util.*;
import java.awt.*;
import java.awt.event.*;
import javax.mail.Message;
import javax.mail.MessagingException;
import javax.mail.PasswordAuthentication;
import javax.mail.Session;
import javax.mail.Transport;
import javax.mail.internet.InternetAddress;
import javax.mail.internet.MimeMessage;
import javax.swing.*;
import java.awt.color.*;
import java.awt.font.*;
import javax.swing.ImageIcon;
import javax.swing.border.Border;
import sun.audio.*; //import the sun.audio package
import java.io.*;
import javax.sound.sampled.AudioInputStream;
import javax.sound.sampled.AudioSystem;
import javax.sound.sampled.Clip;
//end imports
public class SRC_Main extends JFrame implements ActionListener
{
private JPanel pnl = new JPanel();
private JPanel pnlQuote=new JPanel();
private JButton btnNext=new JButton(new ImageIcon("C:/Users/GordonS/Desktop/SRC_MOTORS/src/Next-48.png"));
private JButton btnPrevious=new JButton(new ImageIcon("C:/Users/GordonS/Desktop/SRC_MOTORS/src/Previous-48.png"));
private JButton btnSound=new JButton(new ImageIcon("C:\\Users\\GordonS\\Desktop\\SRC_MOTORS\\src\\Sound-32.png"));
private JButton btnQuoteDoc=new JButton(new ImageIcon("C:/Users/GordonS/Desktop/SRC_MOTORS/src/Document-32.png"));
private JButton btnCalculate=new JButton(new ImageIcon("C:/Users/GordonS/Desktop/SRC_MOTORS/src/Calculator-32.png"));
private JButton btnPrintQuote=new JButton(new ImageIcon("C:/Users/GordonS/Desktop/SRC_MOTORS/src/Print-32.png"));
//Labels for Display
private JLabel CarImage=new JLabel();
private JLabel lblTitle=new JLabel("SRC_MOTORS");
private JLabel lblSlogan=new JLabel("We search and strike for the best prices,so you don't have to!");
private JLabel lblLikes = new JLabel();
private JLabel lblDLikes = new JLabel();
private JLabel quote=new JLabel("Interested in owning? Let us create a Quote for you");
//***add Finance labels here
//TextFields and Areas
private JTextArea txtCarDetails = new JTextArea();
//for MenuBar
private JMenuBar menuB=new JMenuBar();
private JMenu File = new JMenu("File");
private JMenu Edit = new JMenu("Edit");
private JMenu Home = new JMenu("Home");
private JMenu Exit = new JMenu("Exit");
private JMenu Quote = new JMenu("Develope Quote");
private JMenu Print = new JMenu("Print");
//====================================================
private String [] carPathImg=
{
"C:/Users/GordonS/Desktop/SRC_MOTORS/src/RS3.jpg","C:/Users/GordonS/Desktop/SRC_MOTORS/src/FocusSTedit.jpg",
"C:/Users/GordonS/Desktop/SRC_MOTORS/src/GR7.jpg","C:/Users/GordonS/Desktop/SRC_MOTORS/src/A45M.jpg",
"C:/Users/GordonS/Desktop/SRC_MOTORS/src/SCIRR.jpg","C:/Users/GordonS/Desktop/SRC_MOTORS/src/M313.jpg"
};
//========ScannerOBJ========
private Scanner x;
//==========================================================================================================
private CarsRec[] arrMasterRec;
private static int i=0;//counter for record numbers
private static int counter=0;
private String RecordContent="";
//components for finance panel
private JLabel lblInstr=new JLabel(" Car Price:");
private JLabel lblDepo=new JLabel("Initial Deposit:");
private JLabel lblInt=new JLabel("Interest rate:");
private JLabel lblyears=new JLabel("Payment Period:");
private JLabel lblTotalcalc=new JLabel("Calculate All Info");
private JTextField txtPrice=new JTextField(10);
private JLabel lblTotalIntEar=new JLabel();
private JLabel lblTotalCarAmt=new JLabel();
private JLabel lblMonthly=new JLabel();
private JLabel lblPaymentP=new JLabel();
private JLabel lblPrice=new JLabel();
private JTextField txtInitial=new JTextField("0", 10);
private JComboBox cmbInterest =new JComboBox(new String[]{"5%","6%","7%","8%","9%"});
private JComboBox cmbPeriod=new JComboBox(new String[]{"12","24","36","48","60"});
private JCheckBox chkTotPayDetail=new JCheckBox();
private JButton btnCalc=new JButton("Process");
private JButton btnView=new JButton("View Full Quote");
private JButton btnPrint=new JButton("Print");
private JButton btnEmail=new JButton("Email");
private JButton btnCancel=new JButton("Cancel Quote");
private String Emailcont;
private String EmailOwner;
//==========================================================================================================
//Constructor
public SRC_Main()
{
super("SRC_MotorsMain");
setSize(1015,741);
setVisible(true);
setLayout(null);
setDefaultCloseOperation(EXIT_ON_CLOSE);
setLocationRelativeTo(null);
setResizable(false);
pnl.setLayout(null);
pnlQuote.setLayout(new GridLayout(4,5,20,20));
pnl.setSize(1015,741);
pnl.setBackground(Color.DARK_GRAY);
//Look and Feel of Components
lblTitle.setForeground(Color.RED);
lblTitle.setFont(new Font("Blade Runner Movie Font",0,38));
quote.setFont(new Font("Tahoma",1,13));
quote.setForeground(Color.RED);
lblSlogan.setFont(new Font("Berlin Sans FB Demi",2,14));
lblSlogan.setForeground(Color.RED);
lblLikes.setForeground(Color.WHITE);
lblDLikes.setForeground(Color.WHITE);
Border border2 = BorderFactory.createLineBorder(Color.RED,5);
pnl.setBorder(BorderFactory.createCompoundBorder(border2,
BorderFactory.createEmptyBorder(5, 5, 5, 5)));
//Editing TXTFIELDS****************************************
txtCarDetails.setBorder(javax.swing.BorderFactory.createTitledBorder(javax.swing.BorderFactory.createLineBorder(new java.awt.Color(255, 0, 0)), "Vehicle_Information", javax.swing.border.TitledBorder.DEFAULT_JUSTIFICATION, javax.swing.border.TitledBorder.DEFAULT_POSITION, new java.awt.Font("Copperplate Gothic Light", 3, 14), new java.awt.Color(255, 255, 255)));
txtCarDetails.setRows(20);
txtCarDetails.setColumns(25);
txtCarDetails.setEditable(false);
txtCarDetails.setFont(new Font("Berlin Sans FB Demi",2,13));
txtCarDetails.setBackground(Color.darkGray);
txtCarDetails.setForeground(Color.WHITE);
txtCarDetails.setText("Use the directional"+"\n"+" buttons to navigate through the records");
CarImage.setIcon(new ImageIcon("C:\\Users\\GordonS\\Desktop\\SRC_MOTORS\\src\\car_front_intro"));
//==========================================================
CarImage.setSize(360,420);
//Part that adds MenuBars and items
File.add(Home);
File.add(Exit);
Edit.add(Quote);
Edit.add(Print);
menuB.add(File);
menuB.add(Edit);
setJMenuBar(menuB);
//:::::::::::::::::::::::::::::::::::::
//******Layouts of Components*********************************************************************
pnl.add(lblTitle);
lblTitle.setBounds(320, 0, 290, 70);
pnl.add(lblSlogan);
lblSlogan.setBounds(270, 60, 390, 15);
pnl.add(CarImage);
CarImage.setIcon(new ImageIcon("C:\\Users\\GordonS\\Desktop\\SRC_MOTORS\\src\\car_front_intro.png"));
CarImage.setBounds(30, 80, 360, 410);
pnl.add(txtCarDetails);
txtCarDetails.setBounds(520, 90, 420, 185);
pnl.add(quote);
quote.setBounds(510, 360, 360, 30);
pnl.add(btnQuoteDoc);
btnQuoteDoc.setBounds(870, 350, 30, 40);
pnl.add(btnSound);
btnSound.setBounds(715, 280, 40, 40);
btnPrevious.setEnabled(false);
pnl.add(btnNext);
btnNext.setBounds(870, 280, 70, 40);
pnl.add(btnPrevious);
btnPrevious.setBounds(520, 280, 70, 40);
btnPrevious.setEnabled(false);
//ADD EXTRA PANEL FOR FINANCE
pnlQuote.setBackground(Color.GRAY);
chkTotPayDetail.setBackground(Color.GRAY);
pnlQuote.setBorder(javax.swing.BorderFactory.createTitledBorder(javax.swing.BorderFactory.createLineBorder(new java.awt.Color(255, 0, 0)), "SRC_FINANCE", javax.swing.border.TitledBorder.DEFAULT_JUSTIFICATION, javax.swing.border.TitledBorder.DEFAULT_POSITION, new java.awt.Font("Copperplate Gothic Light", 3, 14), new java.awt.Color(255, 255, 255)));
pnlQuote.add(lblInstr);
pnlQuote.add(lblDepo);
pnlQuote.add(lblInt);
pnlQuote.add(lblyears);
pnlQuote.add(lblTotalcalc);
pnlQuote.add(txtPrice);
txtPrice.setEditable(false);
pnlQuote.add(txtInitial);
pnlQuote.add(cmbInterest);
pnlQuote.add(cmbPeriod);
pnlQuote.add(chkTotPayDetail);
// pnlQuote.add(new JLabel("Details: "));
pnlQuote.add(lblPrice);
pnlQuote.add(lblTotalIntEar);
pnlQuote.add(lblTotalCarAmt);
pnlQuote.add(lblMonthly);
pnlQuote.add(lblPaymentP);
pnlQuote.add(btnCalc);
pnlQuote.add(btnView);
pnlQuote.add(btnPrint);
pnlQuote.add(btnEmail);
pnlQuote.add(btnCancel);
pnl.add(pnlQuote);
pnlQuote.setBounds(490,400, 450, 270);
pnlQuote.setVisible(false);
//ActionListeners For Buttons
btnNext.addActionListener(this);
btnPrevious.addActionListener(this);
btnQuoteDoc.addActionListener(this);
btnView.addActionListener(this);
btnCalc.addActionListener(this);
btnPrint.addActionListener(this);
btnEmail.addActionListener(this);
btnCancel.addActionListener(this);
i=0;
add(pnl);
validate();
JOptionPane.showMessageDialog(null, "Welcome to SRC_MOTORS"+"\n"+"Our mission is to find the best Performance Modded Cars at the lowest of prices!"+"\n"+"Visit our ABOUT page for more information");
//JOptionPane.showMessageDialog(null, "");
}
//********************************************END_OF_GUI*****************************************************
//---------------------------------------------------------------------------------------------------------------------------------
//==(1)Open Textfile=====================================
public void OpenFile()
{
try
{
x=new Scanner(new File("C:/Users/GordonS/Desktop/SRC_MOTORS/src/CarRecords.txt"));
}
catch(Exception e)
{
JOptionPane.showMessageDialog(null, "Cannot Find CarRecords! "+e);
}
}
//==(2)ReadTextFile======================================
public CarsRec[] ReadFile()
{
int count=0;
//Array of Objects of The Car Records class-set to Max 100
CarsRec [] arrRec=new CarsRec[100];
while(x.hasNext())
{
String line=x.nextLine();
String arr[]=line.split("#");
arrRec[count]=new CarsRec(arr[0],arr[1],arr[2],arr[3],arr[4],arr[5],arr[6],arr[7],arr[8],arr[9],arr[10],arr[11]);
count ++;
}//close while
x.close();//close Scanner use in txt
return arrRec;
}
//==(3)Preparing ArrObj for use in this class-GETMETHODS
public void setCarRecs(CarsRec[] arr)
{
arrMasterRec=arr;
}
//************************************************End TextFiles Part**********************************************************
//============================================================================================================================
//BUTTONS ACTION AND PROCESSES
public void actionPerformed(ActionEvent e)
{
//==========NEXT==========
if(e.getSource()==btnNext)
{
btnPrevious.setEnabled(true);
btnSound.setEnabled(true);
if(i<carPathImg.length)
{
RecordContent="CarID: "+arrMasterRec[i].getCarID()+"\n"+"Car Name: "+arrMasterRec[i].getName()+"\n"+"Engine Consumption: "+arrMasterRec[i].getLiter()
+"\n"+"Transmission Type: "+arrMasterRec[i].getTrans()+"\n"+"Year of model: "+arrMasterRec[i].getYear()+"\n"+"Mileage: "+arrMasterRec[i].getMileage()
+"\n"+"Modification Company: "+arrMasterRec[i].getModComp()+"\n"+"Software Stage Modded: "+arrMasterRec[i].getModStage()+"\n"+
"Power and Performance output: "+arrMasterRec[i].getPerformance()+"\n"+"Price: "+"R"+arrMasterRec[i].getPrice()+"\n"+"Owner Name: "+arrMasterRec[i].getCusName();
txtCarDetails.setText(RecordContent);
CarImage.setIcon(new ImageIcon(carPathImg[i]));
txtPrice.setText("R "+arrMasterRec[i].getPrice()+" "+i);
Emailcont=RecordContent;
EmailOwner=arrMasterRec[i].getCusEmail();
i++;
}
else
{
JOptionPane.showMessageDialog(null,"Reached end of records!");
btnNext.setEnabled(false);
}
}
//==========================================================================================================
//========PREVIOUS=========
if(e.getSource()==btnPrevious)
{
btnNext.setEnabled(true);
if(i>0)
{
i--;
RecordContent="CarID: "+arrMasterRec[i].getCarID()+"\n"+"Car Name: "+arrMasterRec[i].getName()+"\n"+"Engine Consumption: "+arrMasterRec[i].getLiter()
+"\n"+"Transmission Type: "+arrMasterRec[i].getTrans()+"\n"+"Year of model: "+arrMasterRec[i].getYear()+"\n"+"Mileage: "+arrMasterRec[i].getMileage()
+"\n"+"Modification Company: "+arrMasterRec[i].getModComp()+"\n"+"Software Stage Modded: "+arrMasterRec[i].getModStage()+"\n"+
"Power and Performance output: "+arrMasterRec[i].getPerformance()+"\n"+"Price: "+"R"+arrMasterRec[i].getPrice()+"\n"+"Owner Name: "+arrMasterRec[i].getCusName();
txtCarDetails.setText(RecordContent);
CarImage.setIcon(new ImageIcon(carPathImg[i]));
txtPrice.setText("R "+arrMasterRec[i].getPrice()+" "+i);
}
else
{
JOptionPane.showMessageDialog(null,"You are currently viewing the First Record!");
btnPrevious.setEnabled(false);
btnSound.setEnabled(false);
}
}
//===========================================================================================================
if(e.getSource()==btnQuoteDoc)
{
pnlQuote.setVisible(true);
try{
// open the sound file as a Java input stream
String gongFile = "C:\\Users\\GordonS\\Desktop\\SRC_MOTORS\\src\\VWSCIRROCO.aiff";
InputStream in = new FileInputStream(gongFile);
// create an audiostream from the inputstream
AudioStream audioStream = new AudioStream(in);
// play the audio clip with the audioplayer class
AudioPlayer.player.start(audioStream);
}
catch(Exception ex)
{
JOptionPane.showMessageDialog(null, ex);
}
}
if(e.getSource()==btnCalc)
{
double deposit=0;
deposit=Double.parseDouble(txtInitial.getText());
double intRate=0;
int period=0;
double Total=0;
double AccumulatedAmount=0;
double monthlyIn=0;
double Carprice=Double.parseDouble(arrMasterRec[i].getPrice());
//Setting intRate value
if(cmbInterest.getSelectedIndex()==0)
{
intRate=(5/100);
}
else if(cmbInterest.getSelectedIndex()==1)
{
intRate=(8/100);
}
else if(cmbInterest.getSelectedIndex()==2)
{
intRate=(10/100);
}
else if(cmbInterest.getSelectedIndex()==3)
{
intRate=(12/100);
}
else if(cmbInterest.getSelectedIndex()==4)
{
intRate=(15/100);
}
//END
//Setting period value
if(cmbPeriod.getSelectedIndex()==0)
{
period=1;
}
else if(cmbPeriod.getSelectedIndex()==1)
{
period=2;
}
else if(cmbPeriod.getSelectedIndex()==2)
{
period=3;
}
else if(cmbPeriod.getSelectedIndex()==3)
{
period=4;
}
else if(cmbPeriod.getSelectedIndex()==4)
{
period=5;
}
//=======================================================
AccumulatedAmount= (Carprice-deposit)*(1+(intRate*(period)));
Total=AccumulatedAmount+Carprice;
monthlyIn=Total/(period*12);
lblPrice.setText("R "+Carprice);
lblTotalCarAmt.setText("R "+Total);
lblTotalIntEar.setText("R "+AccumulatedAmount);
lblMonthly.setText("R "+Math.round(monthlyIn));
lblPaymentP.setText(period+" months");
QuoteLayout="\n"+"Initial Price: "+Carprice+"\n"+"Total Amount: "+Total+"\n"+"Interest Earned: "+AccumulatedAmount+
"\n"+"Monthly Installments: "+monthlyIn+"\n"+"Period: "+period+" Months";
}
//==================MENU_OPTIONS====================
if(e.getSource()==Exit)
{
System.exit(0);
}
//==================================================
if(e.getSource()==btnEmail)
{
//******************************************
String name=JOptionPane.showInputDialog("Enter your Name: ");
final String username="src.motortp@gmail.com";
final String password="SRC_MOTORS123";
String cont="Mr/Mrs "+name+"\n"+"is interested in purchasing the following advertised Vehicle :";
System.out.println(cont);
Properties props= new Properties();
props.put("mail.smtp.auth", "true");
props.put("mail.smtp.starttls.enable", "true");
props.put("mail.smtp.host", "smtp.gmail.com");
props.put("mail.smtp.port", "587");
Session session = Session.getInstance(props, new javax.mail.Authenticator()
{
protected PasswordAuthentication getPasswordAuthentication()
{
return new PasswordAuthentication(username, password);
}
});
try
{
Message message = new MimeMessage(session);
message.setFrom(new InternetAddress("src.motortp@gmail.com"));
message.setRecipients(Message.RecipientType.TO,InternetAddress.parse("agordon03@gmail.com"));
message.setSubject("SRC_MOTORS on behalf of "+name);
message.setContent(cont+"\n"+Emailcont,"text/html");
Transport.send(message);
JOptionPane.showMessageDialog(null, "Message Sent!");
System.out.println("This email is successful");
}
catch(MessagingException err)
{
throw new RuntimeException(err);
}
//******************************************
}
}
//================================================================================================================
private String QuoteLayout="";
//================================================================================================================
public static void main(String[] args)
{
//new SRC_Main();
SRC_Main read =new SRC_Main();
read.OpenFile();
CarsRec[] arrMRecords=read.ReadFile();
read.setCarRecs(arrMRecords);
}
}
WelcomeSrc的代码
import java.awt.*;
import java.awt.event.*;
import javafx.scene.control.ProgressBar;
import javax.swing.*;
import java.io.File;
import java.io.IOException;
import java.awt.color.*;
import javax.swing.Timer;
public class WelcomeSRC extends JFrame implements ActionListener
{
private JPanel pnl = new JPanel();
private JPanel pnl2 = new JPanel();
private JLabel bckPic=new JLabel();
private JProgressBar progressBar=new JProgressBar();
private JButton btnLoad=new JButton();
private JButton btnProceed=new JButton("Proceed to records...");
private Timer timer;
final static int interval=100;
int i;
public WelcomeSRC()
{
super("SRC_MOTORS Home");
setSize(1367,677);
setLocationRelativeTo(null);
setVisible(true);
setDefaultCloseOperation(EXIT_ON_CLOSE);
setLayout(new BorderLayout());
btnLoad.setFont(new Font("Berlin Sans FB Demi",2,26));
btnLoad.setForeground(Color.RED);
progressBar.setBackground(Color.white);
progressBar.setForeground(Color.red);
progressBar.setStringPainted(true);
progressBar.setValue(0);
btnLoad.setIcon(new ImageIcon("C:/Users/GordonS/Desktop/SRC_MOTORS/src/loading.gif"));
btnProceed.setIcon(new ImageIcon("C:/Users/GordonS/Desktop/SRC_MOTORS/src/Login_in-32.png"));
// btnLoad.setIcon(new ImageIcon("C:/Users/GordonS/Desktop/SRC_MOTORS/src/25.gif"));
pnl2.setOpaque(false);
pnl2.setLayout(null);
pnl2.setBounds(430, 410, 420, 170);
pnl2.add(progressBar);
progressBar.setBounds(60, 10, 289, 20);
pnl2.add(btnLoad);
btnLoad.setBounds(180, 50, 40, 40);
// pnl2.add(lblProceed);
// lblProceed.setBounds(110, 130, 220, 40);
progressBar.setVisible(false);
pnl.setLayout(new BorderLayout());
pnl.setSize(1367,677);
bckPic.setIcon(new ImageIcon("C:/Users/GordonS/Desktop/SRC_MOTORS/src/back.png"));
pnl.add(bckPic,BorderLayout.CENTER);
getContentPane().add(pnl2);
add(pnl);
validate();
i=0;
btnProceed.addActionListener(this);
btnLoad.addActionListener(this);
//==================================================
timer = new Timer(interval, new ActionListener()
{
public void actionPerformed(ActionEvent ev)
{
if(i==100)
{
Toolkit.getDefaultToolkit().beep();
timer.stop();
// progressBar.setValue(0);
JOptionPane.showMessageDialog(null, "Contents Successfully Loaded!");
btnLoad.hide();
pnl2.add(btnProceed);
btnProceed.setBounds(180, 50, 40, 40);
}
i=i+1;
progressBar.setValue(i);
}
});
//==================================================
}
//==================================================================
public void actionPerformed(ActionEvent e)
{
if(e.getSource()==btnLoad)
{
progressBar.setVisible(true);
timer.restart();
}
if(e.getSource()==btnProceed)
{
SRC_Main load = new SRC_Main();
load.setVisible(true);
this.dispose();
}
}
public static void main(String[] args)
{
WelcomeSRC obj = new WelcomeSRC();
}
}
CarsRec类
import java.io.*;
import java.util.*;
public class CarsRec
{
private String carID="";
private String Name="";
private String Liter="";
private String Transm="";
private String Year="";
private String Mileage="";
private String ModComp="";
private String ModStage="";
private String Perform="";
private String Price="";
private String CusName="";
private String CusEmail="";
public CarsRec(){}
public CarsRec(String cID ,String N ,String L ,String TR ,String Y ,String M,String MDC ,String MDS ,String PF ,String PR,String CSN ,String CSE)
{
carID=cID;
Name=N;
Liter=L;
Transm=TR;
Year=Y;
Mileage=M;
ModComp=MDC;
ModStage=MDS;
Perform=PF;
Price=PR;
CusName=CSN;
CusEmail=CSE;
}
//Accessor methods
public String getCarID()
{ return carID;}
public String getName()
{ return Name;}
public String getLiter()
{ return Liter;}
public String getTrans()
{ return Transm;}
public String getYear()
{ return Year;}
public String getMileage()
{ return Mileage;}
public String getModComp()
{ return ModComp;}
public String getModStage()
{ return ModStage;}
public String getPerformance()
{ return Perform;}
public String getPrice()
{ return Price;}
public String getCusName()
{ return CusName;}
public String getCusEmail()
{ return CusEmail;}
}
最佳答案
见行号289-> if(i<carPathImg.length)
carPathImg包含6个字符串(图像路径)，因此长度为6。
我认为您的输入文件"CarRecords.txt"不包含6行，每行12个单词，中间用＃分隔。检查您的输入文件。carPathImg和arrMasterRec的长度必须相同。
您需要在WelcomeSRC.java中添加以下代码作为if(e.getSource()==btnProceed)
SRC_Main load =new SRC_Main();
load.OpenFile();
CarsRec[] arrMRecords=load.ReadFile();
load.setCarRecs(arrMRecords);
load.setVisible(true);
this.dispose();
此错误是因为您传递的变量之一为null，但代码尝试使用它却并非如此。
我运行代码，它现在运行良好，没有错误，并按用户想要的次数按
next和previous按钮。没有错误。
关于java - JFrame按钮单击NullPointerException，我们在Stack Overflow上找到一个类似的问题： https://stackoverflow.com/questions/32727498/