java - 将文件夹中的文件列出到 JOptionPane

Question

最近我一直在开发一个程序来列出文件夹中的文件，以便它们显示在 JOptionPane 上。 .

我试图获得 File[]调用File.listFiles在文件夹对象上，但结果是 NullPointerException在线for (int i = 0; i < listOfFiles.length; i++) { .

我的代码是这样的:

if (OS.contains("Mac")){
        button.addActionListener(new ActionListener() {
            @Override
            public void actionPerformed(ActionEvent e) {
                File folder = new File("/Users/"+ user +"/Desktop");

                File[] listOfFiles = folder.listFiles();
                String herd = "";
                for (int i = 0; i < listOfFiles.length; i++) {
                      StringBuffer sb = new StringBuffer(2000);
                      String s = listOfFiles[i].getName();
                      herd = sb.append(s).append(", ").toString();
                }

                msgbox(herd);

            }
        });
    }

消息框是:

public void msgbox(String s){
    JOptionPane.showMessageDialog(null, s);
}

NullPointerException堆栈跟踪如下:

Exception in thread "AWT-EventQueue-0" java.lang.NullPointerException
    at goatSoftware.CreateJFrame$1.actionPerformed(CreateJFrame.java:93)
    at javax.swing.AbstractButton.fireActionPerformed(AbstractButton.java:2018)
    at javax.swing.AbstractButton$Handler.actionPerformed(AbstractButton.java:2341)
    at javax.swing.DefaultButtonModel.fireActionPerformed(DefaultButtonModel.java:402)
    at javax.swing.DefaultButtonModel.setPressed(DefaultButtonModel.java:259)
    at javax.swing.plaf.basic.BasicButtonListener.mouseReleased(BasicButtonListener.java:252)
    at java.awt.Component.processMouseEvent(Component.java:6516)
    at javax.swing.JComponent.processMouseEvent(JComponent.java:3320)
    at java.awt.Component.processEvent(Component.java:6281)
    at java.awt.Container.processEvent(Container.java:2229)
    at java.awt.Component.dispatchEventImpl(Component.java:4872)
    at java.awt.Container.dispatchEventImpl(Container.java:2287)
    at java.awt.Component.dispatchEvent(Component.java:4698)
    at java.awt.LightweightDispatcher.retargetMouseEvent(Container.java:4832)
    at java.awt.LightweightDispatcher.processMouseEvent(Container.java:4492)
    at java.awt.LightweightDispatcher.dispatchEvent(Container.java:4422)
    at java.awt.Container.dispatchEventImpl(Container.java:2273)
    at java.awt.Window.dispatchEventImpl(Window.java:2719)
    at java.awt.Component.dispatchEvent(Component.java:4698)
    at java.awt.EventQueue.dispatchEventImpl(EventQueue.java:735)
    at java.awt.EventQueue.access$200(EventQueue.java:103)
    at java.awt.EventQueue$3.run(EventQueue.java:694)
    at java.awt.EventQueue$3.run(EventQueue.java:692)
    at java.security.AccessController.doPrivileged(Native Method)
    at java.security.ProtectionDomain$1.doIntersectionPrivilege(ProtectionDomain.java:76)
    at java.security.ProtectionDomain$1.doIntersectionPrivilege(ProtectionDomain.java:87)
    at java.awt.EventQueue$4.run(EventQueue.java:708)
    at java.awt.EventQueue$4.run(EventQueue.java:706)
    at java.security.AccessController.doPrivileged(Native Method)
    at java.security.ProtectionDomain$1.doIntersectionPrivilege(ProtectionDomain.java:76)
    at java.awt.EventQueue.dispatchEvent(EventQueue.java:705)
    at java.awt.EventDispatchThread.pumpOneEventForFilters(EventDispatchThread.java:242)
    at java.awt.EventDispatchThread.pumpEventsForFilter(EventDispatchThread.java:161)
    at java.awt.EventDispatchThread.pumpEventsForHierarchy(EventDispatchThread.java:150)
    at java.awt.EventDispatchThread.pumpEvents(EventDispatchThread.java:146)
    at java.awt.EventDispatchThread.pumpEvents(EventDispatchThread.java:138)
    at java.awt.EventDispatchThread.run(EventDispatchThread.java:91)

Answer 1

试试这个:

if(OS.contains("Mac"))

{
    button.addActionListener(new ActionListener() {
        @Override
        public void actionPerformed(ActionEvent e) {
            String path = "/Users/" + user + "/Desktop";
            File folder = new File(path);

            File[] listOfFiles = folder.listFiles();
            if (listOfFiles != null && listOfFiles.length > 0) {
                StringBuffer sb = new StringBuffer(2000);
                for (int i = 0; i < listOfFiles.length; i++) {
                    String s = listOfFiles[i].getName();
                    sb.append(s).append(", ");
                }
                msgbox(sb.toString());

            } else {
                msgbox("No files in the folder " + path);
            }
        }
    }); 
}

初始代码中的错误:

1. 您在 for 循环中声明了 StringBuffer。在每个循环中都会构造一个新对象。
2. 不需要在每个循环中执行 sb.toString()。通常只做一次。
3. File.listFiles() 可能会为无效路径返回 null。所以 listOfFiles 的值应该被评估为 null。

需要改进的地方:

1. “无效路径”和“空目录”的不同消息。现在一切都一样，但不应该。

2. 尾随逗号!对于包含一些文件的有效目录，输出将是:“a, b, c,”。最后一个逗号是不必要的。您必须以某种方式避免或删除它，这是一个单独的(也是一个非常受欢迎的)问题。祝你好运。 )