更新JLabel文本錯誤

我在更新JLabel文本時遇到了一些問題。我有一個AddCollection JDialog，它調用它的控制器，然後更新主視圖狀態欄。但是，當我將相同的技術應用到我的AddHolding JDialog和控制器時，我得到一個空指針異常。我現在已經呆了好幾個小時了，但是不知道我哪裏出了問題。這與我的鏈接有關嗎？更新JLabel文本錯誤

下面是我的代碼，涉及到手頭的問題。

public class LMSDriver 
{ 
    public static void main(String[] args) 
    { 
     LMSModel model = new LMSFacade(); 
     AddCollectionPanel colPanel = new AddCollectionPanel(model); 
     colPanel.setVisible(true); 
    } 
} 


public class AddCollectionPanel extends JDialog 
{ 
    private LMSModel model; 
    private AddCollectionController controller; 
    private LibraryView view; 

    private Box mainBox, hBox1, hBox2, hBox3; 
    private JLabel jlCode, jlTitle; 
    private JButton submitBtn; 
    private JTextField codeField; 
    private JTextField titleField; 

    public AddCollectionPanel(LMSModel model) 
    {  
     super(); 
     this.model = model; 
     this.controller = new AddCollectionController(this); 

     setTitle("Add Collection"); 
     setSize(300,160); 
     setLayout(new FlowLayout()); 
     setLocationRelativeTo(null); 

     mainBox = Box.createVerticalBox(); 
     hBox1 = Box.createHorizontalBox(); 
     hBox2 = Box.createHorizontalBox(); 
     hBox3 = Box.createHorizontalBox(); 
     jlCode = new JLabel("Collection Code: "); 
     jlTitle = new JLabel("Collection Title: "); 
     codeField = new JTextField(10); 
     titleField = new JTextField(10);    
     submitBtn = new JButton("Submit"); 

     submitBtn.addActionListener(controller); 

     hBox1.add(jlCode); 
     hBox1.add(Box.createHorizontalStrut(12)); 
     hBox1.add(codeField); 
     hBox2.add(jlTitle); 
     hBox2.add(Box.createHorizontalStrut(10)); 
     hBox2.add(titleField); 
     hBox3.add(Box.createHorizontalStrut(120)); 
     hBox3.add(submitBtn); 

     mainBox.add(Box.createVerticalStrut(10)); 
     mainBox.add(hBox1); 
     mainBox.add(Box.createVerticalStrut(10)); 
     mainBox.add(hBox2); 
     mainBox.add(Box.createVerticalStrut(10)); 
     mainBox.add(hBox3); 

     add(mainBox); 

     setVisible(true); 
     setDefaultCloseOperation(JDialog.DISPOSE_ON_CLOSE); 

     this.view = new LibraryView(model); 
    } 

    public String getCodeField() 
    { 
     String codeText = new String(codeField.getText()); 
     return codeText; 
    } 

    public void clearCodeField() 
    { 
     codeField.setText(""); 
    } 

    public String getTitleField() 
    { 
     String titleText = new String(titleField.getText()); 
     return titleText; 
    } 

    public void clearTitleField() 
    { 
     titleField.setText(""); 
    } 

    public JButton getSubmitBtn() 
    { 
     return submitBtn; 
    } 

    public LMSModel getModel() 
    { 
     return model; 
    } 

    public AddCollectionController getController() 
    { 
     return controller; 
    } 

    public LibraryView getView() 
    { 
     return view; 
    } 
} 


public class AddCollectionController implements ActionListener 
{ 
    private AddCollectionPanel colPanel; 
    private LMSModel model; 
    private LibraryCollection lib; 

    private JLabel colCode; 
    private JLabel totalBooks; 
    private JLabel totalVideos; 

    public AddCollectionController(AddCollectionPanel collectionPanel) 
    { 
     this.colPanel = collectionPanel; 
     model = this.colPanel.getModel(); 
    } 

    @Override 
    public void actionPerformed(ActionEvent event) 
    { 
     if (event.getSource() == colPanel.getSubmitBtn()) 
     {  
      lib = new LibraryCollection(colPanel.getCodeField(), colPanel.getTitleField()); 
      model.addCollection(lib); 
      System.out.println(lib); 
      System.out.println(model.getCollection()); 
      colCode = colPanel.getView().getLibraryStatusbar().getColCode(); 
      colCode.setText("CollectionCode: " + colPanel.getCodeField() + " | "); 
      totalBooks = colPanel.getView().getLibraryStatusbar().getTotalBooks(); 
      totalBooks.setText("Total Books: " + model.countBooks() + " | "); 
      totalVideos = colPanel.getView().getLibraryStatusbar().getTotalVideos(); 
      totalVideos.setText("Total Videos: " + model.countVideos()); 
      colPanel.dispose(); 
     } 
    } 
} 


public class LibraryView extends JFrame 
{ 
    private LMSModel model; 
    private LibraryToolbar toolbar; 
    private LibraryPanel panel; 
    private LibraryStatusbar statusbar; 
    private LibraryMenu menu; 
    private LibraryViewController controller; 
    private AddCollectionPanel addCollectionPanel; 
    private AddBookPanel addBookPanel; 

    public LibraryView(LMSModel model) 
    { 
     this.model = model; 
     this.controller = new LibraryViewController(this); 

     toolbar = new LibraryToolbar(this); 
     panel = new LibraryPanel(this); 
     statusbar = new LibraryStatusbar(this); 
     menu = new LibraryMenu(this); 

     JFrame library = new JFrame("Library"); 
     library.setSize(1024, 720); 
     library.setLayout(new BorderLayout(5,5)); 
     library.setLocationRelativeTo(null); 
     library.add(toolbar, BorderLayout.WEST); 
     library.add(menu, BorderLayout.NORTH); 
     library.add(panel, BorderLayout.CENTER); 
     library.add(statusbar, BorderLayout.SOUTH);  

     library.setVisible(true); 
     library.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE); 
    } 

    public LMSModel getModel() 
    { 
     return model; 
    } 

    public void setModel(LMSModel model) 
    { 
     this.model = model; 
    } 

    public LibraryMenu getLibraryMenu() 
    { 
     return menu; 
    } 

    public LibraryToolbar getLibraryToolbar() 
    { 
     return toolbar; 
    } 

    public LibraryPanel getLibraryPanel() 
    { 
     return panel; 
    } 

    public LibraryStatusbar getLibraryStatusbar() 
    { 
     return statusbar; 
    } 

    public LibraryViewController getController() 
    { 
     return controller; 
    } 

    public void setController(LibraryViewController controller) 
    { 
     this.controller = controller; 
    } 

    public AddCollectionPanel getAddCollectionPanel() 
    { 
     return addCollectionPanel; 
    } 

    public void setAddCollectionPanel(AddCollectionPanel addCollectionPanel) 
    { 
     this.addCollectionPanel = addCollectionPanel; 
    } 

    public AddBookPanel getAddBookPanel() 
    { 
     return addBookPanel; 
    } 

    public void setAddBookPanel(AddBookPanel addBookPanel) 
    { 
     this.addBookPanel = addBookPanel; 
    } 
} 



public class AddBookPanel extends JDialog 
{ 
    private LMSModel model; 
    private AddBookController controller; 
    private LibraryView view; 

    private Box mainBox, hBox1, hBox2, hBox3; 
    private JLabel jlCode, jlTitle; 
    private JButton addBookBtn; 
    private JTextField codeField; 
    private JTextField titleField; 

    public AddBookPanel(LMSModel model) 
    { 
     super(); 
     this.model = model; 
     this.controller = new AddBookController(this); 

     setTitle("Add Book"); 
     setSize(300,160); 
     setLayout(new FlowLayout()); 
     setLocationRelativeTo(null); 

     mainBox = Box.createVerticalBox(); 
     hBox1 = Box.createHorizontalBox(); 
     hBox2 = Box.createHorizontalBox(); 
     hBox3 = Box.createHorizontalBox(); 
     jlCode = new JLabel("Book Code: "); 
     jlTitle = new JLabel("Book Title: "); 
     codeField = new JTextField(10); 
     titleField = new JTextField(10); 

     addBookBtn = new JButton("Add Book"); 

     addBookBtn.addActionListener(controller); 

     hBox1.add(jlCode); 
     hBox1.add(Box.createHorizontalStrut(12)); 
     hBox1.add(codeField); 
     hBox2.add(jlTitle); 
     hBox2.add(Box.createHorizontalStrut(10)); 
     hBox2.add(titleField); 
     hBox3.add(Box.createHorizontalStrut(120)); 
     hBox3.add(addBookBtn); 

     mainBox.add(Box.createVerticalStrut(10)); 
     mainBox.add(hBox1); 
     mainBox.add(Box.createVerticalStrut(10)); 
     mainBox.add(hBox2); 
     mainBox.add(Box.createVerticalStrut(10)); 
     mainBox.add(hBox3); 

     add(mainBox); 

     setVisible(true); 
     setDefaultCloseOperation(JDialog.DISPOSE_ON_CLOSE);  
    } 

    public String getCodeField() 
    { 
     String codeText = new String(codeField.getText()); 
     return codeText; 
    } 

    public void clearCodeField() 
    { 
     codeField.setText(""); 
    } 

    public String getTitleField() 
    { 
     String titleText = new String(titleField.getText()); 
     return titleText; 
    } 

    public void clearTitleField() 
    { 
     titleField.setText(""); 
    } 

    public JButton getAddBookBtn() 
    { 
     return addBookBtn; 
    } 

    public LMSModel getModel() 
    { 
     return model; 
    } 

    public AddBookController getController() 
    { 
     return controller; 
    } 

    public LibraryView getView() 
    { 
     return view; 
    } 
} 



public class AddBookController implements ActionListener { 

    private AddBookPanel bookPanel; 
    private LMSModel model; 
    private int code; 
    private LibraryView view; 

    private JLabel colCode; 
    private JLabel totalBooks; 
    private JLabel totalVideos; 

    public AddBookController(AddBookPanel bookPanel) { 

     this.bookPanel = bookPanel; 
     model = this.bookPanel.getModel(); 
    } 

    @Override 
    public void actionPerformed(ActionEvent event) 
    { 
     if (event.getSource() == bookPanel.getAddBookBtn()) 
     { 
      code = Integer.parseInt(bookPanel.getCodeField().trim()); 
      Holding h = new Book(code, bookPanel.getTitleField()); 
      model.addHolding(h); 
      System.out.println(h); 
      colCode = bookPanel.getView().getLibraryStatusbar().getColCode(); 
      colCode.setText("CollectionCode: " + bookPanel.getCodeField() + " | "); 
      totalBooks = bookPanel.getView().getLibraryStatusbar().getTotalBooks(); 
      totalBooks.setText("Total Books: " + model.countBooks() + " | "); 
      totalVideos = bookPanel.getView().getLibraryStatusbar().getTotalVideos(); 
      totalVideos.setText("Total Videos: " + model.countVideos()); 
      bookPanel.dispose(); 
     } 
    } 
}

來源

2015-11-14 shakarn7

請將您的代碼降低到最小（即[Short但可運行的示例]（http://stackoverflow.com/help/mcve）），我們仍然可以看到該錯誤。併發布堆棧跟蹤。 – Frakcool

*但是，當我將相同的技術應用到我的AddHolding JDialog和控制器時，我得到一個空指針異常。*您忘記在此處放置該代碼（即'AddHolding'代碼）。 – Frakcool

我的答案是否有效？如果沒有，請隨時問我任何問題。如果確實如此，請確保將其標記爲最佳答案！ :) –

這個問題可能出在你所做的所有方法中JLabel.setText。在設置文本之前，您可能需要先調用初始化方法，然後再調用。當您正在更改的對象爲空時，如果在更改JLabel的文本時得到NullPointerException，則表示您的JLabel爲空，則發生異常NullPointerException。

我注意到你正在使用getters來初始化你的對象......不...... Why getters and setters are evil，但這是一個你應該稍後進入的主題。目前，您需要初始化您的JLabel！

看，聲明和初始化變量之間有很大的區別。這裏是simplist這樣我可以把它：

Object imAnObject; //Declaring

現在，因爲你沒做：

Object imAnObject = new Object(); //Initializing

然後，imAnObject仍然是空！所以顯然會有NullPointerException。

如何初始化你的JLabel：

所以，現在，你只有這樣做如下聲明它：

private JLabel jlCode, jlTitle;

但是你從來沒有初始化jlCode或jlTitle。因此，在嘗試更改文本之前，在某處

您需要初始化它。怎麼樣？

jlCode = new JLabel();

來源

2015-11-14 01:07:00

更新JLabel文本錯誤

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