Program not Painting [closed]

问题

I intended for this to paint a square on my JPanel, however, it does not show up. What am I doing wrong?

class GUI extends JPanel {
    private static Game game = new Game();
    ...
    public GUI () {
        SwingUtilities.invokeLater(new Runnable() {
            public void run() {
                setAttributes();
                makeMenu();
            }
        });
    }
    ... 
    public void paintComponent(Graphics g) {
        super.paintComponent(g);
        g.setColor(Color.white);
        g.drawRect(20, 20, 100, 100);
    }
}

Edit: the code

import javax.swing.*;
import java.awt.*;
import java.awt.event.*;

class GUI extends JPanel {
    private static Game game = new Game();

    private static JPanel panel = new JPanel();
    private static JFrame frame = new JFrame();
    final private static int FRAME_HEIGHT = 500;
    final private static int FRAME_WIDTH = 500;
    //Board size 25x25px
    final private static int PIXEL_SIZE = 20;
    public GUI () {
      SwingUtilities.invokeLater(new Runnable() {
         public void run() {
            setAttributes();
            makeMenu();
         }
      });
    }
    public static void setAttributes() {
        frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        frame.setTitle("");
        frame.setBackground(Color.black);
        frame.setVisible(true);
    }

    private static void makeMenu() {
        JButton start = new JButton("Start");
        start.addActionListener(new ActionListener() {
            public void actionPerformed(ActionEvent e) {
                game.startGame();
            }
        });
        panel.add(start);
        frame.add(panel);
        frame.pack();
    }
    public void setGameFrame() {
        panel.removeAll();
        frame.setTitle("Snake v0.1");
        frame.setSize(FRAME_WIDTH, FRAME_HEIGHT);
    }
    public void paintComponent(Graphics g) {
      super.paintComponent(g);
      g.setColor(Color.white);
      g.drawRect(20, 20, 100, 100);
   }
    public void paintGraphics() {
        int[][] pixels = Game.getGraphics();
    }
}

回答1:

I looked at your code. Where do you ever add GUI to anything? Answer: you don't, and if you don't, nothing will be painted. Solution: add it to the gui, and read the tutorials as there is much to fix in your code.

Other suggestions:

Get rid of all static variables except for the contants.
Call your invokeLater in your main method, not in a JPanel's constructor
Again, add your painting GUI to your actual gui so that it gets added to something that will display it.
Don't call getGraphics() on any component as that will get you a non-persisting Graphics object.

e.g.,

import javax.swing.*;
import java.awt.*;

class GUI extends JPanel {
   private static final int PREF_W = 200;
   private static final int PREF_H = PREF_W;
   private static final int RECT_X = 20;
   private static final int RECT_Y = RECT_X;
   private static final int RECT_WIDTH = 100;

   public GUI() {
      setBackground(Color.darkGray);
   }

   // use @Override to be sure that your method is a true override
   // Note that paintComponent should be protected, not public
   @Override
   protected void paintComponent(Graphics g) {
      super.paintComponent(g);
      g.setColor(Color.white);

      // avoiding "magic" numbers here.
      g.fillRect(RECT_X, RECT_Y, RECT_WIDTH, RECT_WIDTH);
      g.setColor(Color.red);
      g.drawRect(RECT_X, RECT_Y, RECT_WIDTH, RECT_WIDTH);
   }

   // so that the layout managers know how big I want this JPanel to be.
   // this is a dumb method implementation. Yours will hopefully be smarter
   @Override
   public Dimension getPreferredSize() {
      return new Dimension(PREF_W, PREF_H); 
   }

   private static void createAndShowGui() {
      GUI mainPanel = new GUI();

      JFrame frame = new JFrame("GUI");
      frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
      frame.getContentPane().add(mainPanel);
      frame.pack();
      frame.setLocationByPlatform(true);
      frame.setVisible(true);
   }

   public static void main(String[] args) {
      SwingUtilities.invokeLater(new Runnable() {
         public void run() {
            createAndShowGui();
         }
      });
   }
}

来源：https://stackoverflow.com/questions/20693744/program-not-painting

标签

java

swing

graphics