Question

嘿，当我尝试在paintComponent中使用Graphics对象时，我得到一个NullPointerException，我无法找出原因。它仍然会绘制我想要的东西，但在此之后，它会抛出异常。继承人的代码

public class RacePanel extends JPanel implements MouseListener, ActionListener
{

private static final int PANEL_WIDTH = 640;
private static final int PANEL_HEIGHT = 400;
private Timer time;
boolean firstTime;
private Rabbit hare;

public RacePanel()
{
    setSize(PANEL_WIDTH, PANEL_HEIGHT);
    setVisible(true);
    firstTime = true;
    addKeyListener(new Key());
    addMouseListener(this);
    hare = new Rabbit();
    time = new Timer(40, this);
    time.start();
}

public void paintComponent(Graphics g)
{
    super.paintComponent(g);
    setBackground(Color.WHITE);

    g.setColor(Color.RED);
    g.drawLine(0, this.getHeight(), this.getWidth()/4, this.getHeight()/2);
    g.drawLine(this.getWidth()/4, this.getHeight()/2, 2*this.getWidth()/4, this.getHeight());
    g.drawLine(2*this.getWidth()/4, this.getHeight(), 3*this.getWidth()/4, this.getHeight()/2);
    g.drawLine(3*this.getWidth()/4, this.getHeight()/2, this.getWidth(), this.getHeight());
    g.setColor(Color.PINK);
    g.fillOval(hare.getPosition(), this.getHeight()-10, 10, 10);


}
@Override
public void actionPerformed(ActionEvent arg0)
{
    Graphics g = getGraphics();
    paintComponent(g);
    hare.move();

}
public static void main(String[] args) 
{
    RacePanel panel = new RacePanel();
    panel.setVisible(true);
    JFrame frame = new JFrame();
    frame.getContentPane().add(panel);
    frame.setMinimumSize(panel.getSize());
    frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
    frame.addKeyListener(panel.new Key());
    frame.pack();
    frame.setVisible(true);
}
}

一旦命中g = getGraphics（），g为null，并且对super.paintComponent（g）的调用抛出异常。我从之前的一个项目中复制了很多动画代码，一切都运行良好，所以我很困惑为什么这不起作用。

Answer 1

此...

@Override
public void actionPerformed(ActionEvent arg0)
{
    Graphics g = getGraphics();
    paintComponent(g);
    hare.move();

}

不是Swing中自定义绘画的工作原理。从来没有任何理由直接调用paintComponent（或甚至paint），它们由绘画子系统调用。

首先查看Performing Custom Painting和Painting in AWT and Swing，了解有关绘画系统如何运作的详细信息

如果您想要更新UI，您应该调用repaint，这将在未来一段时间内安排绘画通行证

您可能还想查看How to Use Key Bindings，这有助于解决与KeyListener

JPanel中的getGraphics（）中的NullPointer

1 个答案: