JFrame从Chooser打开文件

Question

因此，我有一个JFrame和一个选项，该选项说“列出文件”作为菜单，如果单击该菜单，则会打开一个窗口，您可以选择一个文件。 但是我遇到的问题是打开文件并将其加载到新窗口中，请问有什么帮助吗？ 我的代码在下面，这是我尝试过的内容，但是我似乎遇到了错误，因此我认为我做错了，在ActionPerformed方法的末尾也出现了语法错误，说“}期望”，但是程序运行，不知道那是什么。

public class MyJFrame extends JFrame {

JMenuBar menubar;
JMenu menu, submenu;
JMenuItem mi;

public MyJFrame(String title) {
    super(title);
    menubar = new JMenuBar();
    setJMenuBar(menubar);
    buildMenu();
}

void buildMenu() {
    menu = new JMenu("File");

    mi = new JMenuItem("New");
    menu.add(mi);
    menubar.add(menu);

    mi = new JMenuItem("List Files");
    menu.add(mi);
    menubar.add(menu);
    menu.addSeparator();
    mi.addActionListener(

            new ActionListener() {
                private Component frame;

                @Override
                public void actionPerformed(ActionEvent enterPress) {
                    JFileChooser chooser = new JFileChooser();
                    File file1;
                    JFileChooser selection = null;
                    chooser.setCurrentDirectory(new java.io.File("."));
                    chooser.setSelectedFile(new File(""));
                    chooser.setFileSelectionMode(JFileChooser.FILES_AND_DIRECTORIES);
                    if (chooser.showOpenDialog(frame) == JFileChooser.OPEN_DIALOG) {
                        file1 = selection.getSelectedFile();
                    } else {

                    }
                }
            };

    mi = new JMenuItem("Save As");
    menu.add(mi);
    menubar.add(menu);
    menu.addSeparator();

    mi = new JMenuItem("Close");
    menu.add(mi);
    menubar.add(menu);
}
}

从选择器中选择文件时的错误日志：

run:
Exception in thread "AWT-EventQueue-0" java.lang.NullPointerException
at MyJFrame$1.actionPerformed(MyJFrame.java:51)
at javax.swing.AbstractButton.fireActionPerformed(AbstractButton.java:2022)
at     javax.swing.AbstractButton$Handler.actionPerformed(AbstractButton.java:2346)
at     javax.swing.DefaultButtonModel.fireActionPerformed(DefaultButtonModel.java:402)
at javax.swing.DefaultButtonModel.setPressed(DefaultButtonModel.java:259)
at javax.swing.AbstractButton.doClick(AbstractButton.java:376)
at javax.swing.plaf.basic.BasicMenuItemUI.doClick(BasicMenuItemUI.java:833)
at javax.swing.plaf.basic.BasicMenuItemUI$Handler.mouseReleased(BasicMenuItemUI.java:877)
at java.awt.Component.processMouseEvent(Component.java:6527)
at javax.swing.JComponent.processMouseEvent(JComponent.java:3321)
at java.awt.Component.processEvent(Component.java:6292)
at java.awt.Container.processEvent(Container.java:2234)
at java.awt.Component.dispatchEventImpl(Component.java:4883)
at java.awt.Container.dispatchEventImpl(Container.java:2292)
at java.awt.Component.dispatchEvent(Component.java:4705)
at java.awt.LightweightDispatcher.retargetMouseEvent(Container.java:4898)
at java.awt.LightweightDispatcher.processMouseEvent(Container.java:4533)
at java.awt.LightweightDispatcher.dispatchEvent(Container.java:4462)
at java.awt.Container.dispatchEventImpl(Container.java:2278)
at java.awt.Window.dispatchEventImpl(Window.java:2739)
at java.awt.Component.dispatchEvent(Component.java:4705)
at java.awt.EventQueue.dispatchEventImpl(EventQueue.java:746)
at java.awt.EventQueue.access$400(EventQueue.java:97)
at java.awt.EventQueue$3.run(EventQueue.java:697)
at java.awt.EventQueue$3.run(EventQueue.java:691)
at java.security.AccessController.doPrivileged(Native Method)
at java.security.ProtectionDomain$1.doIntersectionPrivilege(ProtectionDomain.java:75)
at java.security.ProtectionDomain$1.doIntersectionPrivilege(ProtectionDomain.java:86)
at java.awt.EventQueue$4.run(EventQueue.java:719)
at java.awt.EventQueue$4.run(EventQueue.java:717)
at java.security.AccessController.doPrivileged(Native Method)
at java.security.ProtectionDomain$1.doIntersectionPrivilege(ProtectionDomain.java:75)
at java.awt.EventQueue.dispatchEvent(EventQueue.java:716)
at java.awt.EventDispatchThread.pumpOneEventForFilters(EventDispatchThread.java:201)
at java.awt.EventDispatchThread.pumpEventsForFilter(EventDispatchThread.java:116)
at java.awt.EventDispatchThread.pumpEventsForHierarchy(EventDispatchThread.java:105)
at java.awt.EventDispatchThread.pumpEvents(EventDispatchThread.java:101)
at java.awt.EventDispatchThread.pumpEvents(EventDispatchThread.java:93)
at java.awt.EventDispatchThread.run(EventDispatchThread.java:82)

Answer 1

您有两个 JFileChoosers，一个为null：

JFileChooser chooser = new JFileChooser(); // *** number 1
File file1;
JFileChooser selection = null;             // *** number 2
chooser.setCurrentDirectory(new java.io.File("."));
chooser.setSelectedFile(new File(""));
chooser.setFileSelectionMode(JFileChooser.FILES_AND_DIRECTORIES);
if (chooser.showOpenDialog(frame) == JFileChooser.OPEN_DIALOG) {
    file1 = selection.getSelectedFile();
} else {

}

不要这样做-仅使用一个非空选择器，例如，

JFileChooser chooser = new JFileChooser(); // *** Keep
File file1;
//   JFileChooser selection = null;        // *** get rid of
chooser.setCurrentDirectory(new java.io.File("."));
chooser.setSelectedFile(new File(""));
chooser.setFileSelectionMode(JFileChooser.FILES_AND_DIRECTORIES);
if (chooser.showOpenDialog(frame) == JFileChooser.OPEN_DIALOG) {
    // file1 = selection.getSelectedFile(); // **** NO ****
    file1 = chooser.getSelectedFile(); // **** YES ****

} else {

}

更重要的是，您需要学习有关如何调试NPE的一般概念（NullPointerException）。 您应该仔细检查抛出该行的行，找出哪个变量为null，然后追溯到您的代码以了解原因。 您会一次又一次地遇到这些，请相信我。