Question

有人可以帮我吗？我已经完成了这个

线程“AWT-EventQueue-0”中的异常java.lang.NullPointerException 在Frame.Painter.paint（Main.java:399）at sun.awt.RepaintArea.paintComponent（RepaintArea.java:264）at sun.awt.RepaintArea.paint（RepaintArea.java:240）at sun.awt.windows.WComponentPeer.handleEvent（WComponentPeer.java:358）at at java.awt.Component.dispatchEventImpl（Component.java:4965）at java.awt.Component.dispatchEvent（Component.java:4711）at java.awt.EventQueue.dispatchEventImpl（EventQueue.java:758）at at java.awt.EventQueue.access $ 500（EventQueue.java:97）at java.awt.EventQueue $ 3.run（EventQueue.java:709）at java.awt.EventQueue $ 3.run（EventQueue.java:703）at java.security.AccessController.doPrivileged（Native Method）at java.security.ProtectionDomain $ JavaSecurityAccessImpl.doIntersectionPrivilege（ProtectionDomain.java:76）在 java.security.ProtectionDomain $ JavaSecurityAccessImpl.doIntersectionPrivilege（ProtectionDomain.java:86）在java.awt.EventQueue $ 4.run（EventQueue.java:731）at java.awt.EventQueue $ 4.run（EventQueue.java:729）at java.security.AccessController.doPrivileged（Native Method）at java.security.ProtectionDomain $ JavaSecurityAccessImpl.doIntersectionPrivilege（ProtectionDomain.java:76）在java.awt.EventQueue.dispatchEvent（EventQueue.java:728）at java.awt.EventDispatchThread.pumpOneEventForFilters（EventDispatchThread.java:201）在 java.awt.EventDispatchThread.pumpEventsForFilter（EventDispatchThread.java:116）在 java.awt.EventDispatchThread.pumpEventsForHierarchy（EventDispatchThread.java:105）在 java.awt.EventDispatchThread.pumpEvents（EventDispatchThread.java:101）在 java.awt.EventDispatchThread.pumpEvents（EventDispatchThread.java:93）在java.awt.EventDispatchThread.run（EventDispatchThread.java:82）

并且第399行中的代码是：

class Painter extends Canvas{
    Image image;
    private URL url;
    public void setImage(String file) {
        url = null;
        try {
            url = new File(file).toURI().toURL();
        } catch (MalformedURLException ex) {
            System.out.println(ex.toString());
        }
        image = getToolkit().getImage(url);
        repaint();
    }
    public void paint(Graphics g) {
        double d = image.getHeight(this) / this.getHeight();
        double w = image.getWidth(this) / d;
        double x = this.getWidth() / 2 - w / 2;
        g.drawImage(image, (int) x, 0, (int) (w), this.getHeight(), this);
    }
}

我不确切知道那里发生了什么。我的程序工作正常，但每次运行都会对此错误感到不安。请告诉我该怎么做。

Answer 1

我假设你有以下代码：

class Painter extends Canvas{
    Image image;
    private URL url;
    public void setImage(String file) {
        image = Toolkit.getDefaultToolkit().getImage(file);
        repaint();
    }
    public void paint(Graphics g) {
      if(image!=null) {
        double d = image.getHeight(this) / this.getHeight();
        double w = image.getWidth(this) / d;
        double x = this.getWidth() / 2 - w / 2;
        g.drawImage(image, (int) x, 0, (int) (w), this.getHeight(), this);
      }
    }
}

然后你只需要调用setImage（“c：/ yourfilepath / yourfilename”）

扩展canvas的类时paint方法出错

1 个答案: