错误：java.lang.NullPointerException：尝试获取空数组的长度

Question

我的错误：

for (int i = 0; i <listfiles.length; i ++) {

我不明白，因为如果我已经拥有所有权限。

public static ArrayList<String> getFilePaths(String directory){
    ArrayList<String> pathArray = new ArrayList<>();
    File file = new File(directory);
    File[] listfiles = file.listFiles();
    for(int i = 0; i < listfiles.length; i++){
        if(listfiles[i].isFile()){
            pathArray.add(listfiles[i].getAbsolutePath());
        }
    }
    return pathArray;
}

权限

检查是否已获得许可。

错误：java.lang.NullPointerException：尝试获取空数组的长度

Answer 1

您需要先请求权限，然后才能像下面这样动态地进行任何操作

if (ContextCompat.checkSelfPermission(thisActivity,Manifest.permission.WRITE_EXTERNAL_STORAGE)!= PackageManager.PERMISSION_GRANTED) {
    // Permission is not granted
     if (ActivityCompat.shouldShowRequestPermissionRationale(thisActivity,
            Manifest.permission.WRITE_EXTERNAL_STORAGE)) {
        // Show an explanation to the user *asynchronously* -- don't block
        // this thread waiting for the user's response! After the user
        // sees the explanation, try again to request the permission.
    } else {
        // No explanation needed; request the permission
        ActivityCompat.requestPermissions(thisActivity,
                new String[]{Manifest.permission.WRITE_EXTERNAL_STORAGE},
                MY_PERMISSIONS_WRITE_EXTERNAL_STORAGE);
    }
}

Answer 2

从file.Java关于listFiles()方法：

 * @return  An array of abstract pathnames denoting the files and
 *          directories in the directory denoted by this abstract pathname.
 *          The array will be empty if the directory is empty.  Returns
 *          {@code null} if this abstract pathname does not denote a
 *          directory, or if an I/O error occurs.
 *
 * @throws  SecurityException
 *          If a security manager exists and its {@link
 *          SecurityManager#checkRead(String)} method denies read access to
 *          the directory

由于您没有得到SecurityException并且列表为null，因此
唯一的其他选择是：

1. 此抽象路径名不表示目录
2. 发生I / O错误

Answer 3

您应该查看javadoc中的listFiles（）方法。

https://developer.android.com/reference/java/io/File.html#listFiles()

如果文件不是目录，则返回

null。为了防止空指针，您可以首先使用isDirectory（）检查文件对象，如果为false，则不要执行循环。