Java 从SharedReferences检索数据时引发NullPointerException
首先，很抱歉我的英语不好 我有一个活动（比如MainActivity.java）和一个类（比如AsynkTaskClass.java） AsynkTaskClass.java包含一个AsynkTask。我在MainActivity.java中将此任务称为AsynkTask 在调用此AsynkTask之前，我正在使用SharedReferences保存一些数据 从AsynkTask的onPostExecute函数中，我调用了一个在MainActivity.java中定义的普通函数（比如displaySharedData（））。在这里，我试图检索使用SharedReferences存储的数据。然后它显示一个空点异常。但是我可以从displaySharedData（）函数之外的任何地方访问共享数据 这是日志输出：Java 从SharedReferences检索数据时引发NullPointerException,java,android,Java,Android,首先，很抱歉我的英语不好 我有一个活动（比如MainActivity.java）和一个类（比如AsynkTaskClass.java） AsynkTaskClass.java包含一个AsynkTask。我在MainActivity.java中将此任务称为AsynkTask 在调用此AsynkTask之前，我正在使用SharedReferences保存一些数据 从AsynkTask的onPostExecute函数中，我调用了一个在MainActivity.java中定义的普通函数（比如displa
W/dalvikvm(3896): threadid=1: thread exiting with uncaught exception
E/AndroidRuntime(3896): FATAL EXCEPTION: main
E/AndroidRuntime(3896): java.lang.NullPointerExceptionE/AndroidRuntime(3896): at com.samApp.project.samApp.ui.MainActivity.displaySharedData(MainActivity.java:52)
E/AndroidRuntime(3896): at com.samApp.project.samApp.s3.AsynkTaskClass$UploadImage.onPostExecute(AsynkTaskClass.java:264)
E/AndroidRuntime(3896): at com.samApp.project.samApp.s3.AsynkTaskClass$UploadImage.onPostExecute(AsynkTaskClass.java:1)
E/AndroidRuntime(3896): at android.os.AsyncTask.finish(AsyncTask.java:417)
E/AndroidRuntime(3896): at android.os.AsyncTask.access$300(AsyncTask.java:127)
E/AndroidRuntime(3896): at android.os.AsyncTask$InternalHandler.handleMessage(AsyncTask.java:429)
E/AndroidRuntime(3896): at android.os.Handler.dispatchMessage(Handler.java:99)
E/AndroidRuntime(3896): at android.os.Looper.loop(Looper.java:130)
E/AndroidRuntime(3896): at android.app.ActivityThread.main(ActivityThread.java:3683)
E/AndroidRuntime(3896): at java.lang.reflect.Method.invokeNative(Native Method)
E/AndroidRuntime(3896): at java.lang.reflect.Method.invoke(Method.java:507)
E/AndroidRuntime(3896): at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:839)
E/AndroidRuntime(3896): at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:597)
E/AndroidRuntime(3896): at dalvik.system.NativeStart.main(Native Method)
public class MainActivity extends Activity implements TextView.OnEditorActionListener {
public SharedPreferences myData;
public String mySharedData="com.samApp.project.samApp.ui";
public SharedPreferences.Editor editor;
protected void onCreate(Bundle savedBundle) {
super.onCreate(savedBundle);
myData = PreferenceManager.getDefaultSharedPreferences(this);
editor = myData.edit();
editor.commit();
attachfileButton.setOnClickListener(new View.OnClickListener() {
@Override
public void onClick(View v) {
Intent intent = new Intent(Intent.ACTION_GET_CONTENT);
intent.setType("image/*");
startActivityForResult(intent, PHOTO_SELECTED);
}
});
}
protected void onActivityResult(int requestCode, int resultCode,
Intent imageReturnedIntent) {
super.onActivityResult(requestCode, resultCode, imageReturnedIntent);
switch (requestCode) {
case PHOTO_SELECTED:
if (resultCode == RESULT_OK) {
Uri selectedImage = imageReturnedIntent.getData();
Log.d("URIsssss", String.valueOf(selectedImage));
AsynkTaskClass obj = new AsynkTaskClass ();
AsynkTaskClass.s3context=this;
obj.new UploadImage().execute(selectedImage);
}
}
}
}
protected void onResume() {
super.onResume();
editor.putString("name", "MeSSi");
editor.commit();
}
public void displaySharedData()
{
Log.d("SharedData",myData.getString("name","")); // line No : 52
}
}
public class AsynkTaskClass {
MainActivity mainActvtyObj = new MainActivity();
public static Context s3context;
protected void onPostExecute(S3TaskResult result) {
if(dialog != null)
dialog.dismiss();
if (result.getErrorMessage() != null) {
displayErrorAlert(
AsynkTaskClass.s3context.getString(R.string.upload_failure_title),
result.getErrorMessage());
}
mainActvtyObj.displaySharedData();
}
}
public class MainActivity extends Activity implements TextView.OnEditorActionListener {
public SharedPreferences myData;
public String mySharedData="com.samApp.project.samApp.ui";
public SharedPreferences.Editor editor;
protected void onCreate(Bundle savedBundle) {
super.onCreate(savedBundle);
myData = PreferenceManager.getDefaultSharedPreferences(this);
editor = myData.edit();
editor.commit();
attachfileButton.setOnClickListener(new View.OnClickListener() {
@Override
public void onClick(View v) {
Intent intent = new Intent(Intent.ACTION_GET_CONTENT);
intent.setType("image/*");
startActivityForResult(intent, PHOTO_SELECTED);
}
});
}
protected void onActivityResult(int requestCode, int resultCode,
Intent imageReturnedIntent) {
super.onActivityResult(requestCode, resultCode, imageReturnedIntent);
switch (requestCode) {
case PHOTO_SELECTED:
if (resultCode == RESULT_OK) {
Uri selectedImage = imageReturnedIntent.getData();
Log.d("URIsssss", String.valueOf(selectedImage));
AsynkTaskClass obj = new AsynkTaskClass ();
AsynkTaskClass.s3context=this;
obj.new UploadImage().execute(selectedImage);
}
}
}
}
protected void onResume() {
super.onResume();
editor.putString("name", "MeSSi");
editor.commit();
}
public void displaySharedData()
{
Log.d("SharedData",myData.getString("name","")); // line No : 52
}
}
public class AsynkTaskClass {
MainActivity mainActvtyObj = new MainActivity();
public static Context s3context;
protected void onPostExecute(S3TaskResult result) {
if(dialog != null)
dialog.dismiss();
if (result.getErrorMessage() != null) {
displayErrorAlert(
AsynkTaskClass.s3context.getString(R.string.upload_failure_title),
result.getErrorMessage());
}
mainActvtyObj.displaySharedData();
}
}
MainActivity mainActvtyObj = new MainActivity();
newonCreate（）onCreate（）myData相反，将对活动对象的引用作为参数传递给异步任务。最后我得到了一个可行的解决方案 我不知道它是否完美！但是效果很好 我替换了displaySharedData（）调用语句，即
maintactvtyobj.displaySharedData（）到以下位置：
（（MainActivity）s3context）.displaySharedData（）
现在它工作得很好 第45行是什么？您可以从任何地方访问首选项
，但您需要适当的上下文
来访问。为什么不给我们看一些代码呢？没有
MainActivity的代码，我们就帮不了您myDatamainActvtyObj