使用JFrame和BufferedReader获取文件内容

Question

the code bellow it is a part of my code.

The problem in "Generate Keys.." button doesn't give any thing on the text area!.

I think the buffered reader cannot get the file path.. i don't know why?!

   public class Finding_Candidate_Keys {   

  static String L[];
  static int size;
  static JFrame frame;
  static JTextArea textArea;
  static String     TextAreaContent = "";
  static File selectedFile;
  static String filePath;
  static JFileChooser fileChooser = new JFileChooser();
  static File file;


  static void displayJFrame()
  {
     frame = new JFrame("Advanced DB Project- Key Definition");
     frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
     textArea = new JTextArea("");
     textArea.setSize(250,250);    
     textArea.setEditable(false);
     textArea.setVisible(true);
     JScrollPane scroll = new JScrollPane(textArea);
     scroll.setVerticalScrollBarPolicy(JScrollPane.VERTICAL_SCROLLBAR_ALWAYS);
     scroll.setPreferredSize(new Dimension(350, 350));
     frame.add(scroll);
     frame.setVisible(true);

     JButton button = new JButton("Select File");

     button.addActionListener(new ActionListener() {
          public void actionPerformed(ActionEvent ae) {

            textArea.setText("");
            FileNameExtensionFilter filter = new FileNameExtensionFilter("Text Files (*.txt)", "txt");
            fileChooser.setFileFilter(filter);


                           int returnValue = fileChooser.showOpenDialog(frame);


                if (returnValue == JFileChooser.APPROVE_OPTION)  
                {
                    if (fileChooser.getSelectedFile() != null) {
                    selectedFile = fileChooser.getSelectedFile();
                    filePath = selectedFile.getAbsolutePath();
                    file = new File (filePath);

                    textArea.append("Opening: " + selectedFile.getName() +  "\n");              
                    }
                } 
        else {
            textArea.setText("");
            textArea.append("Open command cancelled by user." +  "\n");
             }

          }
     });

      frame.setVisible(true);

              //********************
     JButton showDialogButton = new JButton("Generate Keys..");

     showDialogButton.addActionListener(
           new ActionListener()
           {
              public void actionPerformed(ActionEvent e)
              {
                 textArea.setText("");
                 textArea.append(TextAreaContent);
              }
           });

           frame.add(showDialogButton);
           frame.setVisible(true);

     //*********************

     frame.getContentPane().setLayout(new FlowLayout());
     frame.getContentPane().add(button);
     frame.setPreferredSize(new Dimension(500, 500));
     frame.setSize(400,460);
     frame.setLocationRelativeTo(null);
     frame.pack();
     frame.setVisible(true);   
  }

  public static void main(String[] args) throws IOException{    
     SwingUtilities.invokeLater(
           new Runnable()
           {
              public void run()
              {
                 displayJFrame();
              }
           });

     BufferedReader br = new BufferedReader(new FileReader(filePath));
     .
     .
     .

Note that : it is work properly if i use the name of the file directly like:

BufferedReader br = new BufferedReader(new FileReader("input.txt"));

insted of

BufferedReader br = new BufferedReader(new FileReader(filePath));

can any one told me where is the mistake ?

also this appear during the run:

Exception in thread "main" java.lang.NullPointerException
at java.io.FileInputStream.<init>(FileInputStream.java:124)
at java.io.FileInputStream.<init>(FileInputStream.java:87)
at java.io.FileReader.<init>(FileReader.java:58)
at Finding_Candidate_Keys.main(Finding_Candidate_Keys.java:110)

Answer 1

You never initialize your filePath member variable. Because of this it will be initialized to null , which is the default value for member objects. Since it will be set to null , the constructor of FileReader will throw a NullPointerException . You could solve this by initializing your variable:

String filePath = "input.txt";

EDIT:

As noted in the comments, you seem to initialize it in a conditional statement in your displayJFrame method. This method is invoked by the runnable passed to SwingUtilities.invokeLater . As the name implies, this method will not run the Runnable passed to it directly, but will execute it on another thread later on. Thus you value will not be initialized, since your method call will most likely run after you use the value in your main method. You will have to wait until the method is completed, a possible solution is to use the function SwingUtilities.invokeAndWait instead of SwingUtilities.invokeLater . This method will wait for the call to finish.

Answer 2

You are getting no text because you have set it to show no text . Here is how, in your action listener for Generate Keys.. button:

textArea.setText("");
textArea.append(TextAreaContent);

And your TextAreaContent in initialized with empty string, in the initialization section of your class:

...
static String TextAreaContent = "";
...

Set TextAreaContent to have something that you want to see. For example, you could do something like this:

if (returnValue == JFileChooser.APPROVE_OPTION) {
    if (fileChooser.getSelectedFile() != null) {
        selectedFile = fileChooser.getSelectedFile();
        filePath = selectedFile.getAbsolutePath();
        file = new File(filePath);

        textArea.append("Opening: " + selectedFile.getName() + "\n");
        TextAreaContent = "You have selected some file: " + selectedFile.getName();
    }
} else {
     textArea.setText("");
     textArea.append("Open command cancelled by user." + "\n");
     TextAreaContent = "You chose no file";
}

As for the error, due to invokeLater method, the displayJFrame method is executed asynchronously on another thread, and the next line is executed in the main thread. During the execution of the last line, the filePath is still may not be initialised, causing the error. To remove the error, you could do this check:

if (filePath != null) {
    BufferedReader br = new BufferedReader(new FileReader(filePath));
}