使用Java Swing Graphics G在JPanel中绘制

Question

This is my first java project and I am trying to draw a simple rectangle on my JPanel inside my JFrame. Been trying to solve this issue with the help of the same topics on stackoverflow but still no success. The exception I get when I run the program is java.lang.NullPointerException. From my understanding I can not draw on the JPanel itself? which is created in mainWindow.

Main:

public class Main {
public static void main(String[] args) {
    EventQueue.invokeLater(new Runnable() {
        public void run() {
            try {
                GameBoard game = new GameBoard();
                mainWindow view = new mainWindow(game);
                mainModel model = new mainModel();
                mainController cont = new mainController(model, view, game);

                cont.controllerInit();

            } catch (Exception e) {
                e.printStackTrace();
            }
        }
    });
}
}

View:

public class mainWindow{
public JFrame frame;
public JPanel panel;

    GameBoard game = new GameBoard();
    frame = new JFrame();
    frame.getContentPane().setBackground(SystemColor.control);
    frame.setBounds(100, 100, 728, 435);
    frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
    frame.add(game);
    frame.getContentPane().setLayout(null);

    panel = new JPanel();                           
    FlowLayout flowLayout = (FlowLayout) panel.getLayout();
    panel.setBounds(166, 44, 550, 349); 
    frame.getContentPane().add(panel);

    frame.setVisible(true);
}

Game:

public class GameBoard extends JPanel{

@Override
public void paintComponent(Graphics g) 
{
    super.paintComponent(g);
        g.setColor(Color.BLUE);
        g.drawRect(200, 200, 200, 200);
}

}

Answer 1

Never, ever call paintComponent directly, no external source has any reason to do so. Also, what do you thing would happen if you passed it null ?

You should start by having a look at Performing Custom Painting and Painting in AWT and Swing to get a better understand of how paint in Swing works.

The Swing API basically uses a delegate model, where the system delegates responsibility of the paint of each component to the component. This is achieved by the system calling the components paint method, which in-turn calls (among a few others) paintComponent .

Swing also uses a passive rendering approaching, meaning that painting occurs at the discretion of the paint system. You component is notified of the need when its paint method is called. This may occur at any time.

In order for a component to be painted, it must first be added to container which is realised on the screen (has a native peer), in most cases, this means that the component hierarchy needs to resolve to some kind of window based class, like JFrame .

So, the answer to your question is:

• Read the above documentation (and get a better understanding of how the API works)
• Add your GameBoard to a container which can be resolved to a window based class
• Never call paint or paintComponent directly

Reflection....

private mainWindow view;
private mainModel model;
public GameBoard(mainModel m, mainWindow v)
{
    view = v;
    model = m;
}

To me, this makes no sense. There is no reasonable reason why GameBoard needs a reference to mainWindow . GameBoard is, in of itself, a "view". If anything, the only thing you "should" be passing to GameBoard (assuming you're trying to use a MVC) is a controller