我试图获取包含文件夹内的文件的数组的长度，但它说数组为null。
@Override
public void onItemClick(View view, int position) {
String string1 = adapter.getItem(position);
String path = Environment.getExternalStorageDirectory().getAbsolutePath()+ string1;
File directory = new File(path);
File[] files = directory.listFiles();
Toast.makeText(this, Integer.toString(files.length), Toast.LENGTH_SHORT) //this line throws NPE exception
.show();
}
当我用String path1 = Environment.getExternalStorageDirectory().getAbsolutePath()+ string1;替换它时，它工作正常
String path1 = Environment.getExternalStorageDirectory().getAbsolutePath()+ "/Pictures";
包含getItem方法的适配器：
public class MyRecyclerViewAdapter extends RecyclerView.Adapter<MyRecyclerViewAdapter.ViewHolder> {
private ArrayList<String> mData;
private ArrayList<String> mData2;
private LayoutInflater mInflater;
private ItemClickListener mClickListener;
private Context context;
// data is passed into the constructor
public MyRecyclerViewAdapter(Context context, ArrayList<String> data, ArrayList<String> data2) {
this.mInflater = LayoutInflater.from(context);
this.mData = data;
this.mData2 = data2;
this.context = context;
}
// inflates the cell layout from xml when needed
@Override
public ViewHolder onCreateViewHolder(ViewGroup parent, int viewType) {
View view = mInflater.inflate(R.layout.recyclerview_item, parent, false);
return new ViewHolder(view);
}
// binds the data to the textview in each cell
@Override
public void onBindViewHolder(ViewHolder holder, int position) {
String animal = mData.get(position);
String animal2 = mData2.get(position);
int THUMBSIZE = 100;
Bitmap ThumbImage = ThumbnailUtils.extractThumbnail(BitmapFactory.decodeFile(animal2),
THUMBSIZE, THUMBSIZE);
Bitmap thumb = ThumbnailUtils.createVideoThumbnail(animal2, MediaStore.Video.Thumbnails.MINI_KIND);
holder.myTextView.setText(animal);
if(animal!= null && animal.endsWith(".mp3")){
holder.myImage.setImageResource(R.drawable.song);
}
else if(animal!= null && animal.endsWith(".pdf")){
holder.myImage.setImageResource(R.drawable.pdficon2);
}
else
if(animal!= null && animal.endsWith(".jpeg") && BitmapFactory.decodeFile(animal2)!=null ){
holder.myImage.setImageBitmap(ThumbImage);
}
else
if(animal!= null && animal.endsWith(".mp4")){
holder.myImage.setImageBitmap(thumb);
}
else
if(animal!= null && animal.endsWith(".zip")){
holder.myImage.setImageResource(R.drawable.zip);
}
else
if(animal!= null && animal.endsWith(".aac")){
holder.myImage.setImageResource(R.drawable.song);
}
else
if(animal!= null && animal.endsWith(".txt")){
holder.myImage.setImageResource(R.drawable.text);
}
else if(animal!= null && animal.endsWith(".apk")){
PackageInfo packageInfo = context.getPackageManager()
.getPackageArchiveInfo(animal2, PackageManager.GET_ACTIVITIES);
if(packageInfo != null) {
ApplicationInfo appInfo = packageInfo.applicationInfo;
appInfo.sourceDir = animal2;
appInfo.publicSourceDir = animal2;
Drawable icon = appInfo.loadIcon(context.getPackageManager());
Bitmap bmpIcon = ((BitmapDrawable) icon).getBitmap();
holder.myImage.setImageBitmap(bmpIcon);
}
}
else {
holder.myImage.setImageResource(R.drawable.folder);
}
}
// total number of cells
@Override
public int getItemCount() {
return mData.size();
}
// stores and recycles views as they are scrolled off screen
public class ViewHolder extends RecyclerView.ViewHolder implements View.OnClickListener {
TextView myTextView;
ImageButton myImage;
ViewHolder(View itemView) {
super(itemView);
myImage = (ImageButton) itemView.findViewById(R.id.buttonimage);
myTextView = (TextView) itemView.findViewById(R.id.info_text);
myImage.setOnClickListener(this);
}
@Override
public void onClick(View view) {
if (mClickListener != null) mClickListener.onItemClick(view, getAdapterPosition());
}
}
public String getItem(int id) {
return mData.get(id);
}
// allows clicks events to be caught
public void setClickListener(ItemClickListener itemClickListener) {
this.mClickListener = itemClickListener;
}
// parent activity will implement this method to respond to click events
public interface ItemClickListener {
void onItemClick(View view, int position);
}
}
logcat的：
E/AndroidRuntime: FATAL EXCEPTION: main
Process: com.example.dell_1.Myapp3, PID: 3755
java.lang.NullPointerException: Attempt to get length of null array
at com.example.dell_1.myapp3.InternalStorage.onItemClick(InternalStorage.java:43)
at com.example.dell_1.myapp3.MyRecyclerViewAdapter$ViewHolder.onClick(MyRecyclerViewAdapter.java:120)
at android.view.View.performClick(View.java:5609)
at android.view.View$PerformClick.run(View.java:22263)
at android.os.Handler.handleCallback(Handler.java:751)
at android.os.Handler.dispatchMessage(Handler.java:95)
at android.os.Looper.loop(Looper.java:154)
at android.app.ActivityThread.main(ActivityThread.java:6077)
at java.lang.reflect.Method.invoke(Native Method)
at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:865)
at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:755)
清单文件包含以下权限：
<uses-permission android:name="android.permission.WRITE_EXTERNAL_STORAGE" />
<uses-permission android:name="android.permission.WRITE_MEDIA_STORAGE" />
<uses-permission android:name="android.permission.READ_EXTERNAL_STORAGE" />
<uses-permission android:name="android.permission.INTERNET" />
<uses-permission android:name="android.permission.MEDIA_CONTENT_CONTROL" />
另外，我也在运行时请求权限。
private void buttonClicked(View view) {
if (ContextCompat.checkSelfPermission(this,
Manifest.permission.READ_EXTERNAL_STORAGE) + ContextCompat
.checkSelfPermission(this,
Manifest.permission.INTERNET)
!= PackageManager.PERMISSION_GRANTED) {
Snackbar.make(view, "Permission not Granted, Requesting permission.", Snackbar.LENGTH_LONG).show();
if (ActivityCompat.shouldShowRequestPermissionRationale(this,
Manifest.permission.READ_EXTERNAL_STORAGE)) {
Snackbar.make(view, "We need permission to internal storage for displaying songs", Snackbar.LENGTH_LONG).show();
} else {
Snackbar.make(view, "Allow myapp3 to access this device's internal storage", Snackbar.LENGTH_LONG).show();
ActivityCompat.requestPermissions(this,
new String[]{Manifest.permission.READ_EXTERNAL_STORAGE},
MY_PERMISSIONS_REQUEST_READ_EXTERNAL_STORAGE);
// MY_PERMISSIONS_REQUEST_READ_CONTACTS is an
// app-defined int constant. The callback method gets the
// result of the request.
}
}
}
@Override
public void onRequestPermissionsResult(int requestCode, String[] permissions, int[] grantResults) {
switch (requestCode) {
case 1: {
// If request is cancelled, the result arrays are empty.
if (grantResults.length > 0
&& grantResults[0] == PackageManager.PERMISSION_GRANTED) {
Toast.makeText(Bacon1.this, "WRITE_CONTACTS granted", Toast.LENGTH_SHORT)
.show();
} else {
Toast.makeText(Bacon1.this, "WRITE_CONTACTS Denied", Toast.LENGTH_SHORT)
.show();
}
return;
}
// other 'case' lines to check for other
// permissions this app might request
}
}
您可能需要文件的绝对路径。所以，改为使用：
String path = Environment.getExternalStorageDirectory().toString() + string1;
尝试使用：
String path = Environment.getExternalStorageDirectory().getAbsolutePath() + string1;
在清单中添加此权限以读取文件夹内容，否则将返回null：
<uses-permission android:name="android.permission.READ_EXTERNAL_STORAGE" />
无论如何你的文件夹可能是空的，所以检查value != null是否也是一个好习惯：
@Override
public void onItemClick(View view, int position) {
String string1 = adapter.getItem(position);
String path = Environment.getExternalStorageDirectory().toString()+ string1;
File directory = new File(path);
File[] files = directory.listFiles();
if (files != null){
Toast.makeText(this, files.length, Toast.LENGTH_SHORT) //this line throws NPE exception
.show();
}
}