显示java.lang.NullPointerException：尝试调用虚拟方法android.view.View .MainActivity.findViewById（INT）'对空对象引用 [英] java.lang.NullPointerException: Attempt to invoke virtual method 'android.view.View .MainActivity.findViewById(int)' on a null object reference
问题描述
我有一类叫做 MainActivity.java 调用一个的AsyncTask 类。最后一类具有 findViewById（）在执行这个错误：
显示java.lang.NullPointerException：尝试调用虚函数的空对象引用android.view.View＆LT; .MainActivity.findViewById（INT）; mypackage的＆GT
我不明白我怎么可以编辑的ImageView 位于 R.layout.activity_main ，一个后的AsyncTask 完成工作。
MainActivity.java
公共类MainActivity延伸活动{
公共MainActivity（）{}
@覆盖
保护无效的onCreate（包savedInstanceState）{
super.onCreate（savedInstanceState）;
的setContentView（R.layout.activity_main）;
新的连接（）执行（）;
}
}
Connection.java
公共类连接扩展的AsyncTask＆LT;字符串，太虚，字符串＆GT; {
公共字符串的结果;
//我知道，这是不正确的，我该怎么办？
公共MainActivity MainActivity;
@覆盖
保护字符串doInBackground（字符串...为arg0）{
// TODO自动生成方法存根
// ...
返回字符串;
}
保护无效onPostExecute（字符串结果）{
super.onPostExecute（结果）;
// ...
//当生成错误
ImageView的形象=（ImageView的）MainActivity.findViewById（R.id.image）;
// ...
}
}
该错误是
公共MainActivity MainActivity;
从未初始化，从而指向空。 为了让您的code正常工作的最低步骤是 在MainActivity
新的连接（本）.execute（）;
在连接
公共类连接扩展的AsyncTask＆LT;字符串，太虚，字符串＆GT; {
公共MainActivity MainActivity;
公共连接（MainActivity活动）{
MainActivity =活动;
}
但创造的onCreate任务，并通过一个活动是不是反正最好的主意。 此外，字段名应该总是以一个小写字母。
最好的办法是通过一个ImageView的到AsyncTask的。 不要启动任务，直到活动开始，也不要忘了取消时，活动停止的任务。
公开最后一类MainActivity延伸活动{
公共MainActivity（）{}
专用连接的连接;
私人ImageView的ImageView的;
@覆盖
保护无效的onCreate（包savedInstanceState）{
super.onCreate（savedInstanceState）;
的setContentView（R.layout.activity_main）;
ImageView的=（ImageView的）findViewById（R.id.image）;
}
@覆盖
保护无效的OnStart（）{
super.onStart（）;
如果（连接== NULL || connection.getStatus（）！= AsyncTask.Status.RUNNING）{
连接=新的连接（ImageView的）;
connection.execute（）;
}
}
@覆盖
保护无效的onStop（）{
super.onStop（）;
如果（连接= NULL和放大器;！＆安培; connection.getStatus（）== AsyncTask.Status.RUNNING）{
connection.cancel（真正的）;
}
}
}
在Connection.java，存储一个ImageView的一个WeakReference的，防止泄漏。
公开最后一类连接扩展的AsyncTask＆LT;字符串，太虚，字符串＆GT; {
私人最终的WeakReference＆LT; ImageView的＆GT; imageViewRef;
公共连接（ImageView的视图）{
imageViewRef =新的WeakReference＆LT; ImageView的＆GT;（视图）;
}
@覆盖
保护字符串doInBackground（字符串...为arg0）{
// TODO自动生成方法存根
// ...
返回字符串;
}
@覆盖
保护无效onPostExecute（字符串结果）{
super.onPostExecute（结果）;
// ...
最后的ImageView ImageView的= imageViewRef.get（）;
//如果活动还活着，ImageView的将不能为空
如果（ImageView的！= NULL）{
//设置图像或任何你需要
image.setImageResource（666）;
}
}
I have a class called MainActivity.java that call an AsyncTask class. The last class have a findViewById() that in execution return this error:
java.lang.NullPointerException: Attempt to invoke virtual method 'android.view.View <mypackage>.MainActivity.findViewById(int)' on a null object reference
I don't understand how can I edit an ImageView positioned in R.layout.activity_main after that an AsyncTask finish to work.
MainActivity.java
public class MainActivity extends Activity {
public MainActivity() {}
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
new Connection().execute();
}
}
Connection.java
public class Connection extends AsyncTask<String, Void, String> {
public String result;
//I know, this isn't correct, how can i do?
public MainActivity MainActivity;
@Override
protected String doInBackground(String... arg0) {
// TODO Auto-generated method stub
//...
return "a string";
}
protected void onPostExecute(String result) {
super.onPostExecute(result);
//...
// Where the error is generated
ImageView image = (ImageView) MainActivity.findViewById(R.id.image);
//...
}
}
The error is that
public MainActivity MainActivity;
is never initialized, thus pointing to null. To make your code work the minimum step is in MainActivity
new Connection(this).execute();
In Connection
public class Connection extends AsyncTask<String, Void, String> {
public MainActivity MainActivity;
public Connection(MainActivity activity) {
MainActivity = activity;
}
But creating a task in onCreate and passing an Activity is not the best idea anyway. Also, field names should always start with a lowercase letter.
The best way is passing an ImageView to the AsyncTask. Don't start a task until the Activity is started and also, don't forget to cancel the task when the Activity is stopped.
public final class MainActivity extends Activity {
public MainActivity() {}
private Connection connection;
private ImageView imageView;
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
imageView = (ImageView) findViewById(R.id.image);
}
@Override
protected void onStart() {
super.onStart();
if (connection == null || connection.getStatus() != AsyncTask.Status.RUNNING) {
connection = new Connection(imageView);
connection.execute();
}
}
@Override
protected void onStop() {
super.onStop();
if (connection != null && connection.getStatus() == AsyncTask.Status.RUNNING) {
connection.cancel(true);
}
}
}
In Connection.java, store an ImageView as a WeakReference to avoid leaks.
public final class Connection extends AsyncTask<String, Void, String> {
private final WeakReference<ImageView> imageViewRef;
public Connection(ImageView view) {
imageViewRef = new WeakReference<ImageView>(view);
}
@Override
protected String doInBackground(String... arg0) {
// TODO Auto-generated method stub
//...
return "a string";
}
@Override
protected void onPostExecute(String result) {
super.onPostExecute(result);
//...
final ImageView imageView = imageViewRef.get();
// if the Activity is still alive, the ImageView will not be null
if (imageView != null) {
// set an image or whatever you need
image.setImageResource(666);
}
}
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