java - 遍历文本文件中的行，显示 Jlabel 中的行

Question

我有一个程序可以将 txt 文档的行设置为 Jlabel。单击 JButton 时，它应该显示 txt 文件中的下一行，

问题是程序显示第一行并正常运行，但从不显示另一行。它允许我继续点击按钮但永远不会切换线路......

这是我的代码。

import java.awt.EventQueue;
import java.awt.GridLayout;
import java.awt.event.*;
import java.io.File;
import java.io.FileNotFoundException;
import java.util.*;

import javax.swing.*;

public class Driver {

        List<String> lines;

        static String line = "";

        static Scanner scanner = new Scanner(System.in);

        String s = "Welcome Students!";
        String b = "Start!";
        private JFrame f;
        private JPanel p;

        JFrame frame = new JFrame();

        JButton b1 = new JButton(b);

        JLabel jl = new JLabel(s);

        int i;

        private int clicked;

        public Driver() {
                gui();
        }

        public void gui() {
                lines = readLinesFromFile();
                i = 0;
                f = new JFrame("Flash Card Program");
                p = new JPanel();
                f.setLayout(new GridLayout(2, 1));
                f.add(jl);
                f.add(p);
                p.setLayout(new GridLayout(2, 1));
                p.add(b1);

                jl.setHorizontalAlignment(JLabel.CENTER);

                // pack the frame for better cross platform support
                f.pack();
                // Make it visible
                f.setVisible(true);
                f.setSize(500, 400); // default size is 0,0
                f.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);

                b1.addActionListener(new ActionListener() {
                    public void actionPerformed(ActionEvent e) {
                        jl.setText(lines.get(i));
                        i++;
                        if ( i > lines.size() ) {
                            i = 0;
                        }
                        if (b1.getText().equals("Click For Answer")) {
                            b1.setText("Next Question");
                        } else {
                            b1.setText("Click For Answer");
                        }
                    }
                });


            b1.addActionListener(new ActionListener() {
                    public void actionPerformed(ActionEvent e) {

                            if (clicked++ == 10) {

                                    Object[] options = { "No, thanks", "Yes, please" };

                                    int response = JOptionPane.showOptionDialog(frame,
                                                    "Would you like more math questions? ",
                                                    "Math Questions", JOptionPane.YES_NO_CANCEL_OPTION,
                                                    JOptionPane.QUESTION_MESSAGE, null, options,
                                                    options[1]);

                                    if (response == 1)
                                            clicked = 1; // reset
                                    else
                                            System.exit(0);
                            }
                    }
            }); 
    }


        public static List<String> readLinesFromFile() {
            List<String> lines = new ArrayList<String>();
            try {
                scanner = new Scanner(new File("upload.txt"));
                if (scanner.hasNext()){
                    lines.add(scanner.nextLine());
                }
            } catch (FileNotFoundException e) {
                e.printStackTrace();
            }
            return lines;
        }

        private static void readFile(File file) throws FileNotFoundException{
            Scanner scanner = new Scanner(file);
            while (scanner.hasNextLine()) {
            line = scanner.nextLine();
            }
            scanner.close();
        }


        public static void main(String[] args) {
            EventQueue.invokeLater(new Runnable() {
                @Override
                public void run() {
                    try {
                        new Driver();
                        readFile(new File("upload.txt"));
                    } catch (FileNotFoundException e) {
                        e.printStackTrace();
                    }
                }
            });
        }
}

Answer 1

好的，穷人的调试器来拯救:

public static List<String> readLinesFromFile() {
  List<String> lines = new ArrayList<String>();
  try {
     // !! scanner = new Scanner(new File("upload.txt"));
     scanner = new Scanner(new File(UPLOAD_TXT));
     if (scanner.hasNext()) {
        lines.add(scanner.nextLine());
     }
  } catch (FileNotFoundException e) {
     e.printStackTrace();
  }

  System.out.println(lines); // ***** I added this
  return lines;
}

而且它只读一行。然后我回头看看原因:您使用的是 if block 而不是 while 循环。

改变这个:

     if (scanner.hasNext()) {
        lines.add(scanner.nextLine());
     }

为此:

     // also it should be hasNextLine() not hasNext()
     while (scanner.hasNextLine()) {
        lines.add(scanner.nextLine());
     }

所以教训又是这样的:仅仅因为你的代码可以编译并不意味着它没有错误。使用简单的调试技术，如 println 语句(同样，“穷人的调试器”)通常是查找逻辑错误所需的全部。强大的 Eclipse 调试器可以帮助您找到更多。

---

顺便说一句，你会想要改变这个:

        if (i > lines.size()) {
           i = 0;
        }

为此:

        if (i >= lines.size()) {
           i = 0;
        }

避免数组越界异常。

您的静态 readFile(...) 方法和静态行变量也没有用。