Android NullPointerException imageView.setImageBitmap(位图)
我正在尝试从网站下载一张图片。这是我的代码: 主要活动Android NullPointerException imageView.setImageBitmap(位图),android,image,nullpointerexception,download,Android,Image,Nullpointerexception,Download,我正在尝试从网站下载一张图片。这是我的代码: 主要活动 ... public class MainActivity extends Activity { public final static String URL = "com.example.test3.URL"; ImageView imageView = null; @Override protected void onCreate(Bundle savedInstanceState) { super.onCreate(saved
...
public class MainActivity extends Activity {
public final static String URL = "com.example.test3.URL";
ImageView imageView = null;
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
if (savedInstanceState == null) {
getFragmentManager().beginTransaction()
.add(R.id.container, new PlaceholderFragment())
.commit();
}
}
@Override
public boolean onCreateOptionsMenu(Menu menu) {
getMenuInflater().inflate(R.menu.main, menu);
return true;
}
@Override
public boolean onOptionsItemSelected(MenuItem item) {
int id = item.getItemId();
if (id == R.id.action_settings) {
return true;
}
return super.onOptionsItemSelected(item);
}
public static class PlaceholderFragment extends Fragment {
public PlaceholderFragment() {
}
@Override
public View onCreateView(LayoutInflater inflater, ViewGroup container,
Bundle savedInstanceState) {
View rootView = inflater.inflate(R.layout.fragment_main, container, false);
return rootView;
}
}
// execute by pressing button
public void send(View view){
EditText enter = (EditText) findViewById(R.id.enter);
String url = enter.getText().toString();
ImageView imageView = (ImageView) findViewById(R.id.image1);
new ImageDownload(imageView).execute(url);
}
}
图像下载
public class ImageDownload extends AsyncTask<String, Void, Bitmap>{
ImageView imageView = null;
public ImageDownload(ImageView imageView){
this.imageView = imageView;
}
/*@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_image_download);
if (savedInstanceState == null) {
getFragmentManager().beginTransaction()
.add(R.id.container, new PlaceholderFragment()).commit();
}
Intent intent = getIntent();
String urlString = intent.getStringExtra(MainActivity.URL); */
@Override
protected Bitmap doInBackground(String... url) {
if(url.length <= 0){
return null;
}
return downloadBitmap(url[0]);
}
public Bitmap downloadBitmap(String s) {
String urlString = s;
URL url;
InputStream i;
Bitmap bitmap = null;
try {
url = new URL(urlString);
i = url.openStream();
bitmap = BitmapFactory.decodeStream(i);
} catch (MalformedURLException e) {
e.printStackTrace();
} catch (IOException e){
e.printStackTrace();
}
return bitmap;
}
public void onPostExecute(Bitmap bitmap){
Log.d("done", "onPostExecute");
if(bitmap != null){
this.imageView.setImageBitmap(bitmap);
}
}
}
fragment_image_download.xml:
<RelativeLayout xmlns:android="http://schemas.android.com/apk/res/android"
xmlns:tools="http://schemas.android.com/tools"
android:layout_width="match_parent"
android:layout_height="match_parent"
android:paddingBottom="@dimen/activity_vertical_margin"
android:paddingLeft="@dimen/activity_horizontal_margin"
android:paddingRight="@dimen/activity_horizontal_margin"
android:paddingTop="@dimen/activity_vertical_margin"
tools:context="com.example.test3.ImageDownload$PlaceholderFragment" >
<ImageView
android:contentDescription="@string/desc"
android:id="@+id/image1"
android:layout_width="wrap_content"
android:layout_height="wrap_content" >
</ImageView>
</RelativeLayout>
这个NullPointerException来自哪里?有人能帮我吗?错误出现在
ImageDownload
的第90行:
at com.example.test3.ImageDownload.onPostExecute(ImageDownload.java:90)
编辑:
image1
出现在片段的布局中fragment\u main
不在活动的布局中activity\u main
,这就是NullPointerException
的原因
在片段布局中输入
EditText
,或者更改image1
访问逻辑。这里的问题是,您在片段内部使用的视图不正确send
函数是Activity
类的成员,不是Fragment
类,而是ImageView
在Fragment
中声明,因此在findviewbyd
调用之前,应该提供它的根视图(它不等于Activity
的根视图)。大概是这样的:
ImageView imageView = getFragmentManager().findFragmentById(...).getView().findViewById(R.id.image1);
但最好的解决方案是将所有与片段相关的逻辑移到片段类中。调试…开发人员的最佳朋友ImageDownload类的第90行写了什么代码?@user3615363 no,我的意思是将处理占位符片段
的逻辑移动到占位符片段
中,我以前从未处理过占位符片段。你能给我介绍一下吗?@user3615363但是PlaceholderFragment
只是你自己的类,你可以随意添加任何方法。网上有很多关于片段的教程。不管怎样,至少请尝试一下我在答案上发布的解决方案。
ImageView imageView = getFragmentManager().findFragmentById(...).getView().findViewById(R.id.image1);