how to view PDF from a list view which is fetch from Firebase database?

问题

I have fetched a PDF list view from Firebase Database

This is the code that i used to fetch

public class ViewFiles extends AppCompatActivity {

ListView myViewFiles;
DatabaseReference databaseReference;
List<uploadFiles> uploadDOCS;

@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_view_files);

    myViewFiles = (ListView) findViewById(R.id.myViewFiles);
    uploadDOCS = new ArrayList<uploadFiles>();

    viewAllFiles();

    myViewFiles.setOnItemClickListener(new AdapterView.OnItemClickListener() {
        @Override
        public void onItemClick(AdapterView<?> parent, View view, int position, long id) {
            uploadFiles uploadFiles = uploadDOCS.get(position);

            Intent intent = new Intent();
            intent.setType(Intent.ACTION_VIEW);
            intent.setData(Uri.parse(uploadFiles.getUrl()));
            intent = new Intent(ViewFiles.this, ViewPdfFiles.class);
            startActivity(intent);
        }
    });
}

private void viewAllFiles() {

    databaseReference = FirebaseDatabase.getInstance().getReference("HNDIT").child("1st Year 2nd Sem").child("ENGLISH");
    databaseReference.addValueEventListener(new ValueEventListener() {
        @Override
        public void onDataChange(@NonNull DataSnapshot dataSnapshot) {

            for (DataSnapshot postSnapshot: dataSnapshot.getChildren()){

                uploadFiles uploadFiles = postSnapshot.getValue(com.example.lms.fileupload.uploadFiles.class);
                uploadDOCS.add(uploadFiles);
            }

            String[] uploads = new String[uploadDOCS.size()];

            for (int i=0; i < uploads.length; i++){
                uploads[i] = uploadDOCS.get(i).getName();

            }
            ArrayAdapter<String> adapter = new ArrayAdapter<String>(getApplicationContext(),android.R.layout.simple_list_item_1,uploads){
                @NonNull
                @Override
                public View getView(int position, @Nullable View convertView, @NonNull ViewGroup parent) {
                    View view = super.getView(position, convertView, parent);
                    TextView myText = (TextView) view.findViewById(android.R.id.text1);
                    myText.setTextColor(Color.BLACK);

                    return view;
                }
            };
            myViewFiles.setAdapter(adapter);
        }

        @Override
        public void onCancelled(@NonNull DatabaseError databaseError) {

        }
    });

}

}

This is how it displays Click here to View

And i have create a intent for view these file in PDF view in android but it won't display keep blank this is the error it shows

E/PDFView: load pdf error java.lang.NullPointerException: Attempt to invoke virtual method 'int java.io.InputStream.read(byte[])' on a null object reference at com.github.barteksc.pdfviewer.util.Util.toByteArray(Util.java:36) at com.github.barteksc.pdfviewer.source.InputStreamSource.createDocument(InputStreamSource.java:37) at com.github.barteksc.pdfviewer.DecodingAsyncTask.doInBackground(DecodingAsyncTask.java:53) at com.github.barteksc.pdfviewer.DecodingAsyncTask.doInBackground(DecodingAsyncTask.java:25) at android.os.AsyncTask$2.call(AsyncTask.java:333) at java.util.concurrent.FutureTask.run(FutureTask.java:266) at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1167) at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:641) at java.lang.Thread.run(Thread.java:764)

this is the code of that

public class ViewPdfFiles extends AppCompatActivity {

private PDFView pdfView;
private String url;


@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_view_pdf_files);

    pdfView=(PDFView) findViewById(R.id.pdfView);


    Bundle bundle = getIntent().getExtras();
    if (bundle != null) {
        url = getIntent().getStringExtra("url");
    }
    new RetrievePDFStream().execute(url);
}


class RetrievePDFStream extends AsyncTask<String,Void, InputStream> {
    @Override
    protected InputStream doInBackground(String... strings) {
        InputStream inputStream = null;

        try{

            URL url=new URL(strings[0]);
            HttpURLConnection urlConnection=(HttpURLConnection) url.openConnection();
            if(urlConnection.getResponseCode()==200){
                inputStream=new BufferedInputStream(urlConnection.getInputStream());

            }
        }catch (IOException e){
            return null;
        }
        return inputStream;

    }

    @Override
    protected void onPostExecute(InputStream inputStream) {
        pdfView.fromStream(inputStream).load();
    }
}

}

This view parts xml code The Xml file

how can i fix this error and view files in android...

This is how the files in my firebase database the database view

回答1:

I want to start my answer with what you did wrong here :

        Intent intent = new Intent();
        intent.setType(Intent.ACTION_VIEW);
        intent.setData(Uri.parse(uploadFiles.getUrl()));
        intent = new Intent(ViewFiles.this, ViewPdfFiles.class);
        startActivity(intent);  

You did all the important stuff in your intent object like setting it's type and data and then finally you ditched that instance for a new Intent instance. So now your intent object does know where to go i.e. ViewPdfFiles.class but does not have data or type set to it. Changing first line of code to
Intent intent = new Intent(ViewFiles.this, ViewPdfFiles.class);
and removing 4th line should work.
Edited answer :

Intent intent = new Intent(ViewFiles.this, ViewPdfFiles.class);;
//send data using putExtra(String, String) method if uploadFiles.getUrl() is string
intent.putExtra("url", uploadFiles.getUrl());
startActivity(intent);

the null object

来源：https://stackoverflow.com/questions/61588920/how-to-view-pdf-from-a-list-view-which-is-fetch-from-firebase-database