如何从其他类更新JLabel？

Question

I need to use a thread to change the position of my JLabel (movingDisplay) every second when I click on my button (btnDisplay) and for the thread to stop when I click on my other button (btnDStop). I have a class called MoveDisplay that implements Runnable and does this action when I click on btnDisplay. After MoveDisplay has randomized x and y for my JLabel, it's supposed to send x and y back to my main class where it updates the position of the JLabel. I have a method in my mainclass to update the JLabel position however I get a NullPointerException when trying to do so. In fact it doesn't work changing any component at all from MoveDisplay class.

public class Main {
    public static void main(String[] args) {
        GUIFrame test = new GUIFrame();
    }
}

MoveDisplay class:

public class MoveDisplay implements Runnable {
 private GUIFrame gui;
 private boolean moving = true;
 private Thread thread;

 public void run() {
  gui = new GUIFrame();
  if (moving) {
   Random rand = new Random();
   while (moving) {
    int x = rand.nextInt(150) + 1;
    int y = rand.nextInt(150) + 1;
    gui.moveDisplay(x, y, 100, 100);
    try {
     thread.sleep(1000);
    } catch (InterruptedException e) {
     break;
    }
   }
  }
 }

 public void start() {
  thread = new Thread(new MoveDisplay());
  thread.start();
 }

 public void stop() {
  thread.interrupt();
  System.out.println("Stopped");
 }
}

GUIFrame class:

public class GUIFrame {

 private JFrame frame; // The Main window
 private JLabel movingDisplay;
 private boolean playing = true;
 private boolean moving = true;
 private MoveDisplay moveDisplay = new MoveDisplay();

 /**
  * Starts the application
  */
 public void Start() {
  frame = new JFrame();
  frame.setBounds(0, 0, 494, 437);
  frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
  frame.setLayout(null);
  frame.setTitle("Multiple Thread Demonstrator");
  InitializeGUI(); // Fill in components
  frame.setVisible(true);
  frame.setResizable(false); // Prevent user from change size
  frame.setLocationRelativeTo(null); // Start middle screen
 }

 public void InitializeGUI() {

  // The moving display outer panel
  JPanel pnlDisplay = new JPanel();
  Border b2 = BorderFactory.createTitledBorder("Display Thread");
  pnlDisplay.setBorder(b2);
  pnlDisplay.setBounds(12, 118, 222, 269);
  pnlDisplay.setLayout(null);

  // Add buttons and drawing panel to this panel
  btnDisplay = new JButton("Start Display");
  btnDisplay.setBounds(10, 226, 121, 23);
  pnlDisplay.add(btnDisplay);

  btnDStop = new JButton("Stop");
  btnDStop.setBounds(135, 226, 75, 23);
  pnlDisplay.add(btnDStop);

  pnlMove = new JPanel();
  pnlMove.setBounds(10, 19, 200, 200);
  Border b21 = BorderFactory.createLineBorder(Color.black);
  pnlMove.setBorder(b21);
  pnlDisplay.add(pnlMove);
  // Then add this to main window
  frame.add(pnlDisplay);

  movingDisplay = new JLabel("DisplayThread");
  pnlMove.add(movingDisplay);
  btnDStop.setEnabled(false);

  btnDisplay.addActionListener(new ActionListener() {
   public void actionPerformed(ActionEvent arg0) {
    moving = true;
    btnDisplay.setEnabled(false);
    btnDStop.setEnabled(true);
    startMoveDisplay();
   }
  });

  btnDStop.addActionListener(new ActionListener() {
   public void actionPerformed(ActionEvent arg0) {
    moving = false;
    btnDisplay.setEnabled(true);
    btnDStop.setEnabled(false);
    startMoveDisplay();
   }
  });
 }

 public void startMoveDisplay() {
  if(moving) {
   moveDisplay.start();
  }
  else {
   moveDisplay.stop();
  }
 }


 public void moveDisplay(int x, int y, int a, int b) {
  movingDisplay.setBounds(10,10,150,150);
 }

}

Answer 1

You have to get a reference to the existing GUIFrame instance instead of creating a new one, try this:

MoveDisplay

public class MoveDisplay implements Runnable {
    private GUIFrame gui;
    Random rand = new Random();
    volatile boolean moving;

    public MoveDispaly(GUIFrame gui){
        this.gui = gui;
    }
    public void run() { 
       while (moving) {
          int x = rand.nextInt(150) + 1;
          int y = rand.nextInt(150) + 1;
          try {
              SwingUtilities.invokeAndWait(new Runnable(){
                   public void run(){
                       gui.moveDisplay(x, y, 100, 100);
                   }
              });
             thread.sleep(1000);
           } catch (InterruptedException e) {
             break;
           }
       }
   }

   public void start() {
      moving = true;
      thread = new Thread(new MoveDisplay());
      thread.start();
   }

   public void stop() {
      moving = false;
   }
}

GUIFrame

public class GUIFrame{
   MoveDisplay moveDisplay = new MoveDisplay(this);
   ...
}

Also, you have to run the part of the code that changes the GUI on the EDT thread.

Is not a good idea to stop a thread by interrupting it, you can stop it by setting the moving variable to false .

Answer 2

I've checked the code and in the MoveDisplay.run() you were creating a new frame, but in it's constructor the panel wasn't initialized, which triggered the NPE. Because of this I've refactored the GUIFrame constructor to initialeze all components (invoked the Start() method) and removed the new frame's initialization from the run method. Here are the modified classed

public class MoveDisplay {
private GUIFrame gui;
private volatile boolean moving;

public MoveDisplay(GUIFrame gui) {
    this.gui = gui;
}

public void start() {
    moving = true;
    Random rand = new Random();
    while (moving) {
        int x = rand.nextInt(150) + 1;
        int y = rand.nextInt(150) + 1;
        gui.moveDisplay(x, y, 100, 100);
        try {
            Thread.sleep(1000);
        } catch (InterruptedException e) {
            e.printStackTrace();
        }
    }
}

public void stop() {
    moving = false;
}

}

And here is the new frame class

public class GUIFrame {

private JFrame frame; // The Main window
private JLabel movingDisplay;
private boolean playing = true;
private boolean moving = true;
private MoveDisplay moveDisplay;

public GUIFrame() {
    Start();
}

/**
 * Starts the application
 */
private void Start() {
    frame = new JFrame();
    frame.setBounds(0, 0, 494, 437);
    frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
    frame.setLayout(null);
    frame.setTitle("Multiple Thread Demonstrator");
    InitializeGUI(); // Fill in components
    frame.setVisible(true);
    frame.setResizable(false); // Prevent user from change size
    frame.setLocationRelativeTo(null); // Start middle screen

    moveDisplay = new MoveDisplay(this);
}

public void InitializeGUI() {

    // The moving display outer panel
    JPanel pnlDisplay = new JPanel();
    Border b2 = BorderFactory.createTitledBorder("Display Thread");
    pnlDisplay.setBorder(b2);
    pnlDisplay.setBounds(12, 118, 222, 269);
    pnlDisplay.setLayout(null);

    // Add buttons and drawing panel to this panel
    JButton btnDisplay = new JButton("Start Display");
    btnDisplay.setBounds(10, 226, 121, 23);
    pnlDisplay.add(btnDisplay);

    JButton btnDStop = new JButton("Stop");
    btnDStop.setBounds(135, 226, 75, 23);
    pnlDisplay.add(btnDStop);

    JPanel pnlMove = new JPanel();
    pnlMove.setBounds(10, 19, 200, 200);
    Border b21 = BorderFactory.createLineBorder(Color.black);
    pnlMove.setBorder(b21);
    pnlDisplay.add(pnlMove);
    // Then add this to main window
    frame.add(pnlDisplay);

    movingDisplay = new JLabel("DisplayThread");
    pnlMove.add(movingDisplay);
    btnDStop.setEnabled(false);

    btnDisplay.addActionListener(new ActionListener() {
        public void actionPerformed(ActionEvent arg0) {
            moving = true;
            btnDisplay.setEnabled(false);
            btnDStop.setEnabled(true);
            startMoveDisplay();
        }
    });

    btnDStop.addActionListener(new ActionListener() {
        public void actionPerformed(ActionEvent arg0) {
            moving = false;
            btnDisplay.setEnabled(true);
            btnDStop.setEnabled(false);
            startMoveDisplay();
        }
    });
}

public void startMoveDisplay() {
    if(moving) {
        new Thread(new Runnable() {
            @Override
            public void run() {
                moveDisplay.start();
            }
        }).start();
    } else {
        moveDisplay.stop();
    }
}

public void moveDisplay(int x, int y, int a, int b) {
    movingDisplay.setBounds(x, y, a, b);
}

}

Answer 3

Maybe you can also use the Singleton Pattern to get the original instance of GUIFrame :

public static class instanceClassA { 
    private static instanceClassA = null; 

    public static instanceClassA(){ 
        if(instance == null){
            instance = instanceClassA();
        }
        return instance;
    }
}