为什么在目录上调用File.listFiles时可以返回null? [英] Why can File.listFiles return null when called on a directory?

问题描述

我正在创建一个Android应用，我想在目录中列出文件.我通过打电话来

I'm creating an Android app, and I want to list the files in a directory. I do this by calling

File[] files = path.listFiles(new CustomFileFilter());

path是File对象，它是通过调用创建的

path is a File object, which is created by calling

File path = Environment.getExternalStoragePublicDirectory(Environment.DIRECTORY_DOWNLOADS);

然后我尝试通过调用获取files数组的长度

When I then try to get the length of the files array by calling

int length = files.length;

此行给我一个NullPointerException，因为files为空.

This line gives me a NullPointerException, because files is null.

我通过调用检查了试图列出文件的path是否存在

I have checked if the path which I try to list the files in exists by calling

System.out.println("Path exists: " + path.exists());

当我运行该应用程序时，它会打印

And when I run the app, it prints

Path exists: true

在Android Studio控制台中，因此该目录存在.

in the Android Studio console, so the directory exists.

我还打印了路径名

/storage/emulated/0/Download

所以path是目录，而不是文件.

So the path is a directory, and not a file.

我不知道为什么要得到NullPointerException，因为path是目录.

I have no idea about why I'm getting a NullPointerException, because the path is a directory.

编辑:CustomFileFilter类如下:

public class CustomFileFilter implements FileFilter {

    // Determine if the file should be accepted
    @Override
    public boolean accept(File file) {
        // If the file isn't a directory
        if(file.isDirectory()) {
            // Accept it
            return true;
        } else if(file.getName().endsWith("txt")) {
            // Accept it
            return true;
        }
        // Don't accept it
        return false;
    }
}

推荐答案

将此添加到AndroidManifest.xml:

Add this to AndroidManifest.xml:

<uses-permission android:name="android.permission.READ_EXTERNAL_STORAGE" />

此权限允许您读取文件；如果您不使用此权限，则listFiles()和list()都将抛出NullPointerException.

This permission allow you to read files; if you don't use this permission then listFiles() and list() will both throw NullPointerException.

这是显示/storage/emulated/0/Download中所有文件的示例:

And here is an example of showing all files in /storage/emulated/0/Download:

   String dir = "/storage/emulated/0/Download/";
        File f = new File(dir);
        String[] files = f.list();
        for(int i=0; i<files.length; i++){
            Log.d("tag", files[i]);
        }

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