Java – Use AsyncTask to download many images and publish them to ImageView

I sat down and tried to do some exercises with Android. My focus today is to make a simple application that downloads the data (images from the URL) and displays them in an ImageView control in the layout. I saw some examples online and finished my application. Everything seems fine, but when I press the button, I start its work, but then I can’t display the error: NULL POINTER 9error reading file). Here is my code :

package com.example.htmlcontent;

import java.io.BufferedInputStream;

import java.io.InputStream;
import java.net.URL;
import java.util.ArrayList;

import android.app.Activity;

import android.graphics.Bitmap;
import android.graphics.BitmapFactory;
import android.graphics.drawable.BitmapDrawable;
import android.graphics.drawable.Drawable;

import android.os.AsyncTask;
import android.os.Bundle;
import android.widget.Button;
import android.widget.ImageView;
import android.util.Log;
import android.view.View;
import android.view.View.OnClickListener;

public class MainActivity extends Activity {
        private ImageView mImageView;
        private ImageView mImageView2;
        public Button button;
        public static ArrayList<Drawable> drawable;

public static String[] URLs = {"http://zitterman.com/wp-content/uploads/2013/07/19194927_1371972212.jpg","http://i.imgur.com/CQzlM.jpg"};

/** Called when the activity is first created. */
        @Override
        public void onCreate(Bundle savedInstanceState) {

super.onCreate(savedInstanceState);
            setContentView(R.layout.activity_main);

mImageView = (ImageView) findViewById(R.id.test_image);
            mImageView2 = (ImageView) findViewById(R.id.test_image2);
            button = (Button) findViewById(R.id.download1);

button.setOnClickListener(new OnClickListener() {

@Override
                public void onClick(View v) {

new DownloadImage().execute();
                }
            });
        }

/**
         * Simple functin to set a Drawable to the image View
         * @param drawable
         */
        @SuppressWarnings("deprecation")
        private void setImage()
        {
            if(drawable.get(0) == null)
            {
                System.out.println("DRAWABLE JEST NULL");
            }
            mImageView.setBackgroundDrawable(drawable.get(0));
            mImageView2.setBackgroundDrawable(drawable.get(1));
        }

public class DownloadImage extends AsyncTask<Void, Void, Void> {

/**
             * Called after the image has been downloaded
             * -> this calls a function on the main thread again
             */
            protected void onPostExecute(Drawable image)
            {
                setImage();
            }
            protected void onPreExecute()
            {
                Log.i("333333", "Uruchamiam WATEK SCIAGANIA ASYNCTASKIEM PLIKU Z NETA");
            }

@Override
            protected Void doInBackground(Void... params) {

downloadImage();
                return null;
            }
            /**
             * Actually download the Image from the _url
             * @param _url
             * @return
             */
            @SuppressWarnings("deprecation")
            private void downloadImage()
            {
                Prepare to download image

URL url;        

InputStream in;
                BufferedInputStream buf;

BufferedInputStream buf;
                for(int i = 0; i<URLs.length; i++)
                {
                    try {
                    url = new URL(URLs[i]);
                    in = url.openStream();

 Read the inputstream 
                    buf = new BufferedInputStream(in);

 Convert the BufferedInputStream to a Bitmap
                    Bitmap bMap = BitmapFactory.decodeStream(buf);
                    if (in != null) {
                        in.close();
                    }
                    if (buf != null) {
                        buf.close();
                    }

drawable.add(new BitmapDrawable(bMap));

} catch (Exception e) {
                    Log.e("Error reading file", e.toString());
                }

}

}

}
    }

and My XML File Layout:

<?xml version="1.0" encoding="utf-8"?>
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
    android:orientation="vertical"
    android:layout_width="fill_parent"
    android:layout_height="fill_parent"
    >

<Button
        android:id="@+id/download1"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:text="Button" />

<TextView
    android:layout_width="102dp"
    android:layout_height="wrap_content"
    android:text="hello" />

<ImageView
        android:id="@+id/test_image"
        android:layout_width="match_parent"
        android:layout_height="200dp"
        android:background="@drawable/ic_launcher" />
    <ImageView
        android:id="@+id/test_image2"
        android:layout_width="match_parent"
        android:layout_height="match_parent"
        android:background="@drawable/ic_launcher" />

</LinearLayout>

As you can see in the code ArrayList, I made a drawable list. There are no errors in the code. ONLY NULL POINTER.

I think it’s because you forgot to initialize the drawable. Change to:

public static ArrayList<Drawable> drawable = new ArrayList<Drawable>();

Next, because your AsyncTask is <Void, Void, Void> Your post execution should look like this

    @Override
    protected void onPostExecute(Void aVoid) {
        setImage();
    }

Generic types <A, B, C> correspond to the parameters and return types of different methods. You should read more about this here: https://stackoverflow.com/a/6053673/827110

You also need to get the Internet permission addition in your AndroidManifest.xml (just before <application:) in your AndroidManifest (for completeness).

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