Java：JFrame 图形未绘制矩形

Question

Hello fellow programmers,

I've ran into a little issue in my code that I can't seem to crack. It has to do with the Jframe; Graphics area of Java. The code that I'll post below, is over a drawing method. Which purpose is to draw the "rooms" that are in a ArrayList roomList which is located in another class hence lvl. before. This off-course doesn't happen, hence the post on here.

public class LevelGUI implements Observer {

    private Level lv;
    private Display d;
    
    public LevelGUI(Level level, String name) {

        this.lv = level;
        
        JFrame frame = new JFrame(name);
        frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        d = new Display(lv, 500, 500);
        frame.getContentPane().add(d);
        frame.pack();
        frame.setLocation(0, 0);
        frame.setVisible(true);
    }
    private class Display extends JPanel {

        public Display(Level fp, int x, int y) {
            addKeyListener(new Listener());
            setBackground(Color.GRAY);
            setPreferredSize(new Dimension(x + 20, y + 20));
            setFocusable(true);
        }
        public void paintComponent(Graphics g) {
            super.paintComponent(g);
            draw(g);
        }
        private void draw(Graphics g) {
            Level lvl = new Level();
            for(int i = 0; i < lvl.roomList.size(); i++) {
                Room room = lvl.roomList.get(i);
                g.setColor(room.floorColor);
                g.drawRect(room.posX, room.posY, room.roomWidth, room.roomHeight);
            }   
        }
    }
}

To get some background info on the program . roomList is the ArrayList , and it is filled with various different sized and colored rooms. The rooms themselves are objects .

Here comes first Level class :

public class Level extends Observable {

    private boolean Switch = true;

    public ArrayList<Room> roomList = new ArrayList<Room>(); 

    (...)
}

Here is the Class Room() that is used to create the rooms.

public class Room {
    Color floorColor;
    int roomWidth;
    int roomHeight;
    int posX;
    int posY;

    public Room(int dx, int dy, Color color) {
        this.floorColor = color;
        this.roomHeight = dy;
        this.roomWidth = dx;
        this.posY = 0;
        this.posX = 0;
    }

    (...)
}

I've managed to locate where the problem is thought to occur, and it's the code in the for-loop . I tried switching the roomList.size() for an integer to test if it was the loop., But it wasn't. It is possible to draw a figure outside of the for-loop .

and again, the problem isn't an error message, the program simply doesn't draw the rooms that I've instructed it to draw in the method draw() .

The display output looks like this:

Thanks beforehand!

Answer 1

Be aware that the paintComponent() method is invoked by Swing whenever the framework thinks the component needs to be rendered on screen. This usually is when the window is getting visible - initially or because some other window no longer hides the component. Such events are out of your control.

So your application should create a state and be ready to draw it anytime. Therefore you do not create state (like a level) inside the paint() or paintComponent() method. Put that elsewhere - if need be into the constructor.

Looking at you code: As you are creating a new level inside paintComponent()/draw(), is it correct to assume that this level has no rooms associated? In that case the method is right to return without having painted anything.

If your application thinks the screen should be updated call repaint(), knowing that the paint() method will be called by the framework soon.