将Download Manager与回收站视图一起使用

Question

Here is my DataAdapter class:

 package com.example.ashish.fileserver; import android.app.DownloadManager; import android.content.Context; import android.content.Intent; import android.net.Uri; import android.support.v7.widget.CardView; import android.support.v7.widget.RecyclerView; import android.view.LayoutInflater; import android.view.View; import android.view.ViewGroup; import android.widget.Switch; import android.widget.TextView; import android.widget.Toast; import java.util.ArrayList; import retrofit2.Callback; /** * Created by ashish on 23/8/16. */ public class DataAdapter extends RecyclerView.Adapter<DataAdapter.ViewHolder> { private ArrayList<AndroidVersions> android; private static String url; private Context context; public DataAdapter(Context context,ArrayList<AndroidVersions> android) { this.android = android; this.context = context; } @Override public DataAdapter.ViewHolder onCreateViewHolder(ViewGroup viewGroup, int i) { View view = LayoutInflater.from(viewGroup.getContext()).inflate(R.layout.card_row, viewGroup, false); return new ViewHolder(view); } @Override public void onBindViewHolder(DataAdapter.ViewHolder viewHolder, int i) { viewHolder.tv_name.setText(android.get(i).getName()); viewHolder.tv_version.setText(android.get(i).getVer()); //viewHolder.tv_api_level.setText(android.get(i).getApi()); } @Override public int getItemCount() { return android.size(); } public class ViewHolder extends RecyclerView.ViewHolder implements View.OnClickListener{ private TextView tv_name,tv_version,tv_api_level; public CardView card; ArrayList<AndroidVersions> android = new ArrayList<AndroidVersions>(); public ViewHolder(View view) { super(view); this.android = android; tv_name = (TextView)view.findViewById(R.id.tv_name); tv_version = (TextView)view.findViewById(R.id.tv_version); view.setOnClickListener(this); //tv_api_level = (TextView)view.findViewById(R.id.tv_api_level); } @Override public void onClick(View view) { System.out.println(getAdapterPosition()); int position = getAdapterPosition(); if (position == 1){ System.out.println("hey Ashish"); url = "http://vfu.bg/en/e-Learning/Computer-Basics--CTE_I__-_Computer_Basics.ppt"; String servicestring = Context.DOWNLOAD_SERVICE; DownloadManager downloadmanager; downloadmanager = (DownloadManager)getSystemService(servicestring); Uri uri = Uri .parse(url); DownloadManager.Request request = new DownloadManager.Request(uri); long reference = downloadmanager.enqueue(request); Toast.makeText(context.getApplicationContext(), "Your file is now downloading...", Toast.LENGTH_LONG).show(); } } } } 

I want to Download some files at the time when I click on a Cardview but it's showing some error that java.lang.Object android.content.Context.getSystemService(java.lang.String)' on a null object reference . When I click on the card view it says App Stopped Working. How could I solve thsi problem?

Answer 1

If you check DownloadManager, it says:

Note that the application must have the INTERNET permission to use this class.

Are you good with that? Also, this kind operation should be done in a different AsyncTask, so that you can keep your main thread free.

Finally, you will not get any log by using System.out.println . Use Android Log API instead

Answer 2

    String servicestring = Context.DOWNLOAD_SERVICE; 
    downloadmanager  = (DownloadManager)getSystemService(servicestring);

error was in these two lines, I've changed it to:

   String servicestring = Context.DOWNLOAD_SERVICE;
   downloadmanager  = (DownloadManager)context.getSystemService(servicestring);

And in main activity this line was showing error:

    adapter = new DataAdapter(data);

to:

    adapter = new DataAdapter(data, ScrollingActivity_Test.this);