如何從我的內部類引用「顯示」變量，以在我的JLabel中使用外部類

Question

我想將文本輸入到第四個JPanel中，並且每次按Enter鍵以更新帶有文本的第一個JPanel 。我爲返回鍵添加了一個keylistener，它在內部類AddSong中運行一個函數，該函數接受一個文本的數組列表，並返回一個字符串。我想在第一個quadrent中的JPanel上顯示該字符串，但它不起作用。現在我相信這是刪除第一個JPanel，而不是更新。我很困惑，因爲這是我第一次遇到關鍵事件，因爲我是編程新手。

下面的代碼：

public class MusicPlayer extends JFrame{ 

    private static final int WIDTH = 400; 
    private static final int HEIGHT = 400; 
    private JLabel songList,songPlaying,c; 
    private JTextField enterSong; 
    private AddSong input; 

    public MusicPlayer(){ 

    //Text box enterSong, input passed to actionListener 
    input = new AddSong(); 
    enterSong = new JTextField(10); 
    enterSong.addActionListener(input); 

    //Numbers the quadrents 
    songList = new JLabel(input.printSongs(input.songList),SwingConstants.CENTER); 
    songPlaying = new JLabel("2",SwingConstants.CENTER); 
    c = new JLabel("c",SwingConstants.CENTER); 

    //Makes and sets size of pane 
    Container pane = getContentPane(); 
    pane.setLayout(new GridLayout(2,2)); 

    //Add JLabels to panes in the container 
    pane.add(songList); 
    pane.add(songPlaying); 
    pane.add(c); 
    pane.add(enterSong); 




    //KeyListener - return 
    addKeyListener(new KeyListener() { 
     @Override 
     public void keyPressed(KeyEvent ke){} 
     @Override 
     public void keyReleased(KeyEvent ke){} 
     @Override 
     public void keyTyped(KeyEvent ke){ 
      if(ke.getKeyCode()==KeyEvent.VK_ENTER){ 
       input.printSongs(input.songList); 
      } 
     } 
    }); 
    add(songList); 
    pack(); 

    setTitle("Andrew's Music Player"); 
    setSize(WIDTH,HEIGHT); 
    setVisible(true); 
    setDefaultCloseOperation(EXIT_ON_CLOSE); 

    } 

    public class AddSong implements ActionListener{ 

    //List of Songs 
    ArrayList<String> songList = new ArrayList<String>(); 
    String display; 

    public AddSong(){ 
     display = ""; 
    } 
    public void actionPerformed(ActionEvent e){ 

     String song; 
     //get the text 
     song = enterSong.getText(); 

     //add current text string to list 
     songList.add(song); 

     //print songlist 
     System.out.print(printSongs(songList)); 

    } 

    public String printSongs(ArrayList<String> songList){ 
     display = ""; 
     for(int i = 0; i < songList.size(); i++){ 
      display += songList.get(i) + "\n"; 
     } 
     return display; 
    } 
    } 

    public static void main(String args[]){ 
    MusicPlayer boi = new MusicPlayer(); 
    } 

}

Answer 1

歡迎來到SO。有一個動作監聽器，你可以放下監聽器。
請注意評論：

public class MusicPlayer extends JFrame{ 

    private static final int WIDTH = 400; 
    private static final int HEIGHT = 400; 
    private JLabel songsList,songPlaying,c; 
    private JTextField enterSong; 
    private AddSong input; 

    public MusicPlayer(){ 

     //Text box enterSong, input passed to actionListener 
     input = new AddSong(); 
     enterSong = new JTextField(10); 
     enterSong.addActionListener(input); 

     //Numbers the quadrents 
     songsList = new JLabel("No songs in list",SwingConstants.CENTER); 
     songPlaying = new JLabel("2",SwingConstants.CENTER); 
     c = new JLabel("c",SwingConstants.CENTER); 

     //Makes and sets size of pane 
     Container pane = getContentPane(); 
     pane.setLayout(new GridLayout(2,2)); 

     //Add JLabels to panes in the container 
     pane.add(songsList); 
     pane.add(songPlaying); 
     pane.add(c); 
     pane.add(enterSong); 

     //adding a key listenr to the Jframe is not need 
     //the enterSong.addActionListener(input); does the work 

     add(songsList); 
     pack(); 

     setTitle("Andrew's Music Player"); 
     setSize(WIDTH,HEIGHT); 
     setVisible(true); 
     setDefaultCloseOperation(EXIT_ON_CLOSE); 

    } 

    public class AddSong implements ActionListener{ 

     //List of Songs 
     ArrayList<String> songList = new ArrayList<>(); 

     public AddSong(){ } 

     @Override 
     public void actionPerformed(ActionEvent e){ 

      String song; 
      //get the text 
      song = enterSong.getText(); 

      //add current text string to list 
      songList.add(song); 

      String songListAsString = printSongs(songList); 

      //update display with new song 
      songsList.setText(songListAsString); 

      //clear text field 
      enterSong.setText(""); 
     } 

     public String printSongs(ArrayList<String> songList){ 

      //using string builder is more efficient 
      //to have multiple lines in a JLabel you have to use 
      //html tags 
      //a JList would be more appropriiate 
      StringBuilder sb = new StringBuilder("<html>"); 

      for(int i = 0; i < songList.size(); i++){ 
       sb.append("<p>") 
        .append(songList.get(i)) 
        .append("</p>"); 
      } 

      sb.append("</html>"); 
      return sb.toString(); 
     } 
    } 

    public static void main(String args[]){ 
     MusicPlayer boi = new MusicPlayer(); 
    } 
} 

Answer 2

要訪問你需要創建它的一個實例，它持有該變量的具體值類的變量。另一方面，addSong.display會嘗試訪問不帶cocrete實例的變量。這隻適用於靜態變量。

您已經在幾行後面創建了addSong的實例 input = new addSong（）;

從這個實例中，您可以訪問display變量。接下來的問題是，如果它已經具有正確的值，或者您需要首先執行某些代碼才能正確初始化該值。

請僅使用以Java中的大寫字母開頭的類名稱，並嘗試將名詞用作類名稱。