我坐着试着用Android做一些练习。我今天的观点是制作简单的应用程序，它将下载数据（来自URL的图像）并在布局中的ImageView控件中显示它们。我在网上看到了一些例子，并完成了我的应用程序。一切似乎都没问题，但当我按下按钮时，我开始工作，但后来显示错误：NULL POINTER 9error读取文件）。这是我的代码：
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());
}
}
}
}
}
和我的XML文件布局：
<?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>
我已经在代码ArrayList中看到了Drawable列表。代码中没有错误。只有那个NULL POINTERs。
答案 0 :(得分：4)
我认为这是因为你忘了初始化drawable。改为：
public static ArrayList<Drawable> drawable = new ArrayList<Drawable>();
接下来，因为你的AsyncTask是<Void, Void, Void>。你的帖子执行应该如下：
@Override
protected void onPostExecute(Void aVoid) {
setImage();
}
泛型类型<A,B,C>对应于不同方法的参数和返回类型。你应该在这里阅读更多相关信息：https://stackoverflow.com/a/6053673/827110
（为了完整起见），您还需要在AndroidManifest.xml添加（<application..之前）获得互联网权限：
<uses-permission android:name="android.permission.INTERNET" />
答案 1 :(得分：1)
将您的异步任务更改为
public class DownloadImage extends AsyncTask<Void, Void, ArrayList<Drawable>> {
/**
* Called after the image has been downloaded
* -> this calls a function on the main thread again
*/
protected void onPostExecute( ArrayList<Drawable> drawable)
{
setImage(drawable);
}
protected void onPreExecute()
{
Log.i("333333", "Uruchamiam WATEK SCIAGANIA ASYNCTASKIEM PLIKU Z NETA");
}
@Override
protected ArrayList<Drawable> doInBackground(Void... params) {
downloadImage();
return drawable;
}
private void setImage(ArrayList<Drawable> drawable)
{
if(drawable.get(0) == null)
{
System.out.println("DRAWABLE JEST NULL");
}
mImageView.setBackgroundDrawable(drawable.get(0));
mImageView2.setBackgroundDrawable(drawable.get(1));
}
答案 2 :(得分：0)
好的，正如你所说，我已经纠正了。是的，我忘了在开始时初始化这个arraylist，但你比我快;）所以
public class MainActivity extends Activity {
private ImageView mImageView;
private ImageView mImageView2;
public Button button;
public static ArrayList<Drawable> drawable = new ArrayList<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( ArrayList<Drawable> drawable)
{
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, ArrayList<Drawable>> {
/**
* Called after the image has been downloaded
* -> this calls a function on the main thread again
*/
protected void onPostExecute( ArrayList<Drawable> drawable)
{
setImage(drawable);
}
protected void onPreExecute()
{
Log.i("333333", "Uruchamiam WATEK SCIAGANIA ASYNCTASKIEM PLIKU Z NETA");
}
@Override
protected ArrayList<Drawable> 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());
}
}
}
}
}
现在它在尝试下载图像时崩溃了。我不明白为什么我应该把ArrayList作为第三个参数....？为什么我声明静态数组存储我的图像？ OnPostExecute它应该只调用将完成其余工作的函数。