Question

我在名为tree的类中创建了这个方法：

public void printTree(Graphics g, int x, int y) {
MediaTracker mt = new MediaTracker(this);
tree = getImage(getCodeBase(), "tree.png");
mt.addImage(tree, 0);
g.drawImage(tree, x, y, this);

我尝试在paint方法的main中使用它：

Tree tree = new Tree();
    tree.printTree(g, 60, 50);

我没有错误，但是当我运行应用程序时，我得到了这些错误：

    Exception in thread "AWT-EventQueue-1" java.lang.NullPointerException
at java.applet.Applet.getCodeBase(Unknown Source)
at Tree.printTree(Tree.java:12)
at Main.paint(Main.java:146)
at Main.update(Main.java:187)
at sun.awt.RepaintArea.updateComponent(Unknown Source)
at sun.awt.RepaintArea.paint(Unknown Source)
at sun.awt.windows.WComponentPeer.handleEvent(Unknown Source)
at java.awt.Component.dispatchEventImpl(Unknown Source)
at java.awt.Container.dispatchEventImpl(Unknown Source)
at java.awt.Component.dispatchEvent(Unknown Source)
at java.awt.EventQueue.dispatchEventImpl(Unknown Source)
at java.awt.EventQueue.access$000(Unknown Source)
at java.awt.EventQueue$1.run(Unknown Source)
at java.awt.EventQueue$1.run(Unknown Source)
at java.security.AccessController.doPrivileged(Native Method)
at java.security.AccessControlContext$1.doIntersectionPrivilege(Unknown Source)
at java.security.AccessControlContext$1.doIntersectionPrivilege(Unknown Source)
at java.awt.EventQueue$2.run(Unknown Source)
at java.awt.EventQueue$2.run(Unknown Source)
at java.security.AccessController.doPrivileged(Native Method)
at java.security.AccessControlContext$1.doIntersectionPrivilege(Unknown Source)
at java.awt.EventQueue.dispatchEvent(Unknown Source)
at java.awt.EventDispatchThread.pumpOneEventForFilters(Unknown Source)
at java.awt.EventDispatchThread.pumpEventsForFilter(Unknown Source)
at java.awt.EventDispatchThread.pumpEventsForHierarchy(Unknown Source)
at java.awt.EventDispatchThread.pumpEvents(Unknown Source)
at java.awt.EventDispatchThread.pumpEvents(Unknown Source)
at java.awt.EventDispatchThread.run(Unknown Source)

如果有人可以帮助我，我会很高兴。

Answer 1

您尝试使用的对象为null。使用调试器并找出哪一个，并找出如何确保它不为null，或修复使其为null的错误。

Answer 2

Applet在AppletContext中运行是否由appletviewer提供或浏览器页面。如果你跑了来自主方法的applet或加载将applet放入应用程序中不是AppletContext和getCodeBase 将返回null。在这种情况下你可以使用备用/其他图像加载方法

Reference

使用另一种方式import a picture into an applet。

Answer 3

“我没有错误，但是当我运行应用程序时，我得到了这些错误：”

您可能意味着没有编译错误。 NullPointerException是一个相当常见但严重的错误。您必须在使用它们之前初始化对象。

无法在另一个类中创建print方法并在主类中调用它

3 个答案: