问打开文件会产生一个java.lang.NullPointerException

EN

我有一个JFrame，在我有JButton的框架上，我想要的是当这个文件被点击时，用户可以使用java JFileChooser加载一个文件。

我宣布FileChooser是这样的。

JFileChooser fc;

下面是按钮的动作监听器中的代码。

JButton btnLoad = new JButton("Load .txt");
    btnLoad.addActionListener(new ActionListener() {
        public void actionPerformed(ActionEvent e) {

            int returnVal = fc.showOpenDialog(OpenFile.this);

            if (returnVal == JFileChooser.APPROVE_OPTION) {
                File file = fc.getSelectedFile();
                //This is where a real application would open the file.
                System.out.println("Opening: " + file.getName() + ".");
            } else {
                System.out.println("Open command cancelled by user.");
            }


        }
    });

它给我带来的错误是

线程“AWT 0”中的异常: maple.Netflix$2.actionPerformed(Netflix.java:73) at javax.swing.AbstractButton.fireActionPerformed(Unknown Source) at javax.swing.AbstractButton$Handler.actionPerformed(Unknown Source) at javax.swing.DefaultButtonModel.fireActionPerformed(Unknown Source) at javax.swing.DefaultButtonModel.setPressed(Unknown Source) at javax.swing.plaf.basic.BasicButtonListener.mouseReleased(Unknown Source) at java.awt.AWTEventMulticaster。未知源(未知源)在javax.swing.JComponent.processMouseEvent(Unknown源(java.awt.Component.processMouseEvent(未知源))，java.awt.Component.processEvent(未知源)，java.awt.Container.processEvent(未知源)，java.awt.Component.dispatchEventImpl(未知源)，java.awt.Container.dispatchEventImpl(未知源)，java.awt.Component.dispatchEvent(未知源)，java.awt.Component.processEvent(未知源)，java.awt.Container.processEvent(未知源)，java.awt.Component.dispatchEventImpl(未知源)，java.awt.Container.dispatchEventImpl(未知源)，java.awt.Component.dispatchEvent(未知源)，java.awt.LightweightDispatcher (未知源)。java.awt.LightweightDispatcher.processMouseEvent(Unknown源(未知源)在java.awt.LightweightDispatcher.dispatchEvent(Unknown源(RetargetMouseEvent)，java.awt.Container.dispatchEventImpl(未知源)，java.awt.Window.dispatchEventImpl(未知源)，java.awt.Component.dispatchEvent(未知源)，java.awt.EventQueue.dispatchEventImpl(未知源)，java.awt.EventQueue.access$200(未知源)，java.awt.EventQueue$3 (未知源)。运行(未知源)在java.awt.EventQueue$3.运行(未知源)在java.security.AccessController.doPrivileged(Native方法)在java.security.ProtectionDomain$1.doIntersectionPrivilege(Unknown源)在java.awt.EventQueue$4.运行(未知源)在java.awt.EventQueue$4.运行(未知源)在java.security.AccessController.doPrivileged(Native方法)在java.security.ProtectionDomain$1)。(未知源)在java.awt.EventDispatchThread.pumpOneEventForFilters(Unknown源)在java.awt.EventDispatchThread.pumpEventsForFilter(Unknown源)在java.awt.EventDispatchThread.pumpEventsForHierarchy(Unknown源)在java.awt.EventDispatchThread.pumpEvents(Unknown源)在java.awt.EventDispatchThread.pumpEvents(Unknown源)在java.awt.EventDispatchThread.run(未知源)

这是第73线。

int returnVal = fc.showOpenDialog(Netflix.this);