NullPointerException異常

Question

在下面的代碼我使用雙緩衝，以避免圖像的閃爍在我的

import java.awt.Graphics; 
import java.awt.Image; 
import java.awt.event.KeyEvent; 
import java.awt.event.KeyListener; 
import java.awt.image.BufferedImage; 
import java.io.IOException; 
import javax.imageio.ImageIO; 
import javax.swing.JFrame; 
import javax.swing.JPanel; 


public class TestProgram extends JFrame implements KeyListener { 
    private Image doubleBuffer; 
    private Graphics myGraphics; 
    private BufferedImage TestImage; 
    private int cordX = 100; 
    private int cordY = 100; 

    public TestProgram() { 
     setTitle("Testing...."); 
     setSize(500,500); 
     imageLoader(); 
     setVisible(true); 
    } 

    public static void main(String[] args) { 
     new TestProgram(); 
    } 

    public void imageLoader() { 
     try { 
      String testPath = "test.png"; 
      TestImage = ImageIO.read(getClass().getResourceAsStream(testPath)); 

     } catch (IOException ex) { 
      ex.printStackTrace(); 
     } 

     addKeyListener(this); 

     doubleBuffer = createImage(getWidth(), getHeight()); 
     myGraphics = doubleBuffer.getGraphics(); 
     drawImages(); 
    } 

    @Override 
    public void update(Graphics g) { 
     drawImages(); 
     g.drawImage(doubleBuffer, 0, 0, this);   
    } 

    public void drawImages() { 
     myGraphics.drawImage(TestImage, cordX, cordY, this); 
    } 



    public void keyPressed(KeyEvent ke) { 
     switch (ke.getKeyCode()) { 
      case KeyEvent.VK_RIGHT: { 
       cordX+=5; 
      } 
      break; 
      case KeyEvent.VK_LEFT: { 
       cordX-=5; 
      } 
      break; 
      case KeyEvent.VK_DOWN: { 
       cordY+=5; 
      } 
      break; 
      case KeyEvent.VK_UP: { 
       cordY-=3; 
      } 
      break; 
     } 
     repaint(); 
    } 

    public void keyTyped(KeyEvent ke) {} 

    public void keyReleased(KeyEvent ke) {} 
} 

問題的this question建議是，即時得到這一個NullPointerException line

myGraphics = doubleBuffer.getGraphics(); 

是我的方法正確嗎？ 請幫忙。 感謝

Answer 1

1. 不用油漆到JFrame直接，放在那裏JPanel或JComponent

2. Swing GUI的應Initial Thread

3. 什麼TestImage和路徑Image首發???，

4. KeyListener未指定爲Swing JComponents，使用KeyBindings代替

5. 後的座標改變你要調用repaint()

6. 把那乾脆

。

import java.awt.*; 
import java.awt.event.*; 
import java.awt.image.BufferedImage; 
import java.io.IOException; 
import java.net.*; 
import java.util.HashMap; 
import java.util.Map; 
import javax.imageio.ImageIO; 
import javax.swing.*; 

public class MoveIcon extends JPanel { 

    private static final long serialVersionUID = 1L; 
    private static final String IMAGE_PATH = "http://duke.kenai.com/misc/Bullfight.jpg"; 
    private static final String IMAGE_PATH_PLAYER = "http://duke.kenai.com/iconSized/duke4.gif"; 
    public static final int STEP = 3; 
    private static final int TIMER_DELAY = STEP * 8; 
    private BufferedImage bkgrndImage = null; 
    private BufferedImage playerImage = null; 
    private Map<Direction, Boolean> directionMap = new HashMap<Direction, Boolean>(); 
    private int playerX = 0; 
    private int playerY = 0; 

    enum Direction { 

     UP(KeyEvent.VK_UP, 0, -1), DOWN(KeyEvent.VK_DOWN, 0, 1), 
     LEFT(KeyEvent.VK_LEFT, -1, 0), RIGHT(KeyEvent.VK_RIGHT, 1, 0); 
     private int keyCode; 
     private int xDirection; 
     private int yDirection; 

     private Direction(int keyCode, int xDirection, int yDirection) { 
      this.keyCode = keyCode; 
      this.xDirection = xDirection; 
      this.yDirection = yDirection; 
     } 

     public int getKeyCode() { 
      return keyCode; 
     } 

     public int getXDirection() { 
      return xDirection; 
     } 

     public int getYDirection() { 
      return yDirection; 
     } 
    } 

    public MoveIcon() { 
     try { 
      URL bkgrdImageURL = new URL(IMAGE_PATH); 
      URL playerImageURL = new URL(IMAGE_PATH_PLAYER); 
      bkgrndImage = ImageIO.read(bkgrdImageURL); 
      playerImage = ImageIO.read(playerImageURL); 
      setPreferredSize(new Dimension(bkgrndImage.getWidth(), bkgrndImage.getHeight())); 
     } catch (MalformedURLException e) { 
      e.printStackTrace(); 
     } catch (IOException e) { 
      e.printStackTrace(); 
     } 
     for (Direction direction : Direction.values()) { 
      directionMap.put(direction, false); 
     } 
     setKeyBindings(); 
     Timer timer = new Timer(TIMER_DELAY, new TimerListener()); 
     timer.start(); 
    } 

    private void setKeyBindings() { 
     InputMap inMap = getInputMap(JComponent.WHEN_IN_FOCUSED_WINDOW); 
     ActionMap actMap = getActionMap(); 
     for (final Direction direction : Direction.values()) { 
      KeyStroke pressed = KeyStroke.getKeyStroke(direction.getKeyCode(), 0, false); 
      KeyStroke released = KeyStroke.getKeyStroke(direction.getKeyCode(), 0, true); 
      inMap.put(pressed, direction.toString() + "pressed"); 
      inMap.put(released, direction.toString() + "released"); 
      actMap.put(direction.toString() + "pressed", new AbstractAction() { 

       private static final long serialVersionUID = 1L; 

       @Override 
       public void actionPerformed(ActionEvent e) { 
        directionMap.put(direction, true); 
       } 
      }); 
      actMap.put(direction.toString() + "released", new AbstractAction() { 

       private static final long serialVersionUID = 1L; 

       @Override 
       public void actionPerformed(ActionEvent e) { 
        directionMap.put(direction, false); 
       } 
      }); 
     } 

    } 

    @Override 
    protected void paintComponent(Graphics g) { 
     super.paintComponent(g); 
     if (bkgrndImage != null) { 
      g.drawImage(bkgrndImage, 0, 0, null); 
     } 
     if (playerImage != null) { 
      g.drawImage(playerImage, playerX, playerY, null); 
     } 
    } 

    private class TimerListener implements ActionListener { 

     @Override 
     public void actionPerformed(ActionEvent e) { 
      boolean moved = false; 
      for (Direction direction : Direction.values()) { 
       if (directionMap.get(direction)) { 
        playerX += STEP * direction.getXDirection(); 
        playerY += STEP * direction.getYDirection(); 
        moved = true; 
       } 
      } 
      if (moved) { 
       int x = playerX - 2 * STEP; 
       int y = playerY - 2 * STEP; 
       int w = playerImage.getWidth() + 4 * STEP; 
       int h = playerImage.getHeight() + 4 * STEP; 
       MoveIcon.this.repaint(x, y, w, h); // !! repaint just the player 
      } 
     } 
    } 

    private static void createAndShowUI() { 
     JFrame frame = new JFrame("MoveIcon"); 
     frame.getContentPane().add(new MoveIcon()); 
     frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE); 
     frame.pack(); 
     frame.setLocationRelativeTo(null); 
     frame.setVisible(true); 
    } 

    public static void main(String[] args) { 
     java.awt.EventQueue.invokeLater(new Runnable() { 

      @Override 
      public void run() { 
       createAndShowUI(); 
      } 
     }); 
    } 
}