我正在使用android studio创建一个Android应用程序，并且最近尝试实现共享首选项，因为我需要从另一个活动中访问数据而不启动它。（这是一个数据库助手活动）。但是，当我尝试调试应用程序时，会显示：
java.lang.RuntimeException: Unable to start activity ComponentInfo{com.example.rcadit.foodgenie/com.example.rcadit.foodgenie.resultActivity}: java.lang.NullPointerException: Attempt to invoke virtual method 'java.lang.String android.content.Context.getPackageName()' on a null object reference
活动2 （我要访问数据的部分）
SharedPreferences prefs = PreferenceManager.getDefaultSharedPreferences(this);
String courseData = prefs.getString("courseData", "no id"); //no id: default value
String prepData = prefs.getString("prepData", "no id");
String hungerData = prefs.getString("hungerData", "no id");
String cuisineData = prefs.getString("cuisineData", "no id");
活动1 （我分配数据的部分）
public void onItemSelected(AdapterView<?> parent, View view, int position, long id) {
Spinner spinner = (Spinner) parent;
// On selecting a spinner item
String value = parent.getItemAtPosition(position).toString();
if (value.equals("Low") ) {
hungerValue = "1";
SharedPreferences prefs = PreferenceManager.getDefaultSharedPreferences(this);
SharedPreferences.Editor editor = prefs.edit();
editor.putString("hungerData", hungerValue); //InputString: from the EditText
editor.commit();
}
if (value.equals("Medium") ) {
hungerValue = "2";
SharedPreferences prefs = PreferenceManager.getDefaultSharedPreferences(this);
SharedPreferences.Editor editor = prefs.edit();
editor.putString("hungerData", hungerValue); //InputString: from the EditText
editor.commit();
}
if (value.equals("High") ) {
hungerValue = "3";
SharedPreferences prefs = PreferenceManager.getDefaultSharedPreferences(this);
SharedPreferences.Editor editor = prefs.edit();
editor.putString("hungerData", hungerValue); //InputString: from the EditText
editor.commit();
}
if (value.equals("Under 10 mins") ) {
prepValue = "< 10";
SharedPreferences prefs = PreferenceManager.getDefaultSharedPreferences(this);
SharedPreferences.Editor editor = prefs.edit();
editor.putString("prepData", prepValue); //InputString: from the EditText
editor.commit();
}
if (value.equals("No Preparation") ) {
prepValue = " = 0";
SharedPreferences prefs = PreferenceManager.getDefaultSharedPreferences(this);
SharedPreferences.Editor editor = prefs.edit();
editor.putString("prepData", prepValue); //InputString: from the EditText
editor.commit();
}
if (value.equals("Under 20 mins") ) {
prepValue = "< 20";
SharedPreferences prefs = PreferenceManager.getDefaultSharedPreferences(this);
SharedPreferences.Editor editor = prefs.edit();
editor.putString("prepData", prepValue); //InputString: from the EditText
editor.commit();
}
if (value.equals("Under 30 mins") ) {
prepValue = "< 30";
SharedPreferences prefs = PreferenceManager.getDefaultSharedPreferences(this);
SharedPreferences.Editor editor = prefs.edit();
editor.putString("prepData", prepValue); //InputString: from the EditText
editor.commit();
}
if (value.equals("Over 30 mins") ) {
prepValue = "> 30";
SharedPreferences prefs = PreferenceManager.getDefaultSharedPreferences(this);
SharedPreferences.Editor editor = prefs.edit();
editor.putString("prepData", prepValue); //InputString: from the EditText
editor.commit();
}
if(spinner.getId() == R.id.cuisine_spinner)
{
cuisineValue = value;
SharedPreferences prefs = PreferenceManager.getDefaultSharedPreferences(this);
SharedPreferences.Editor editor = prefs.edit();
editor.putString("cuisineData", cuisineValue); //InputString: from the EditText
editor.commit();
}
if(spinner.getId() == R.id.course_spinner)
{
courseValue = value;
SharedPreferences prefs = PreferenceManager.getDefaultSharedPreferences(this);
SharedPreferences.Editor editor = prefs.edit();
editor.putString("courseData", courseValue); //InputString: from the EditText
editor.commit();
}
Log.d(TAG, "onItemSelected: " + value + " " + courseValue+ " "+ cuisineValue + " "+ prepValue + " "+ hungerValue);
}
完整LogCat
08/02 17:00:08: Launching app
Cold swapped changes.
$ adb shell am start -n "com.example.rcadit.foodgenie/com.example.rcadit.foodgenie.SplashScreen" -a android.intent.action.MAIN -c android.intent.category.LAUNCHER -D
Connecting to com.example.rcadit.foodgenie
Connected to the target VM, address: 'localhost:8610', transport: 'socket'
I/art: Not late-enabling -Xcheck:jni (already on)
W/ActivityThread: Application com.example.rcadit.foodgenie is waiting for the debugger on port 8100...
I/System.out: Sending WAIT chunk
I/art: Debugger is active
I/System.out: Debugger has connected
I/System.out: waiting for debugger to settle...
I/System.out: waiting for debugger to settle...
I/System.out: waiting for debugger to settle...
I/System.out: waiting for debugger to settle...
I/System.out: waiting for debugger to settle...
I/System.out: waiting for debugger to settle...
I/System.out: waiting for debugger to settle...
I/System.out: waiting for debugger to settle...
I/System.out: debugger has settled (1387)
W/System: ClassLoader referenced unknown path: /data/app/com.example.rcadit.foodgenie-1/lib/x86
W/System: ClassLoader referenced unknown path: /data/app/com.example.rcadit.foodgenie-1/lib/x86
D/OpenGLRenderer: Use EGL_SWAP_BEHAVIOR_PRESERVED: true
[ 08-02 09:00:13.226 3744: 3744 D/ ]
HostConnection::get() New Host Connection established 0xaa9a5480, tid 3744
[ 08-02 09:00:13.300 3744: 3765 D/ ]
HostConnection::get() New Host Connection established 0xaa9a6020, tid 3765
I/OpenGLRenderer: Initialized EGL, version 1.4
D/android.widget.GridLayout: horizontal constraints: x2-x0>=792, x2-x1<=308, x1-x0<=325 are inconsistent; permanently removing: x2-x1<=308.
D/MyActivity: onItemSelected: Please Select null null null null
D/MyActivity: onItemSelected: Please Select null Please Select null null
D/MyActivity: onItemSelected: Please Select null Please Select null null
D/MyActivity: onItemSelected: Please Select Please Select Please Select null null
I/Choreographer: Skipped 30 frames! The application may be doing too much work on its main thread.
E/Surface: getSlotFromBufferLocked: unknown buffer: 0xaa9b7e30
D/MyActivity: onItemSelected: Low Please Select Please Select null 1
E/Surface: getSlotFromBufferLocked: unknown buffer: 0xb4016e50
D/OpenGLRenderer: endAllStagingAnimators on 0xa282d300 (ListPopupWindow$DropDownListView) with handle 0xa262fad0
D/MyActivity: onItemSelected: American Please Select American null 1
E/Surface: getSlotFromBufferLocked: unknown buffer: 0xb4016e50
D/OpenGLRenderer: endAllStagingAnimators on 0xa2830080 (ListPopupWindow$DropDownListView) with handle 0xa261f040
D/MyActivity: onItemSelected: Under 30 mins Please Select American < 30 1
E/Surface: getSlotFromBufferLocked: unknown buffer: 0xb4016e50
D/OpenGLRenderer: endAllStagingAnimators on 0xa269aa00 (ListPopupWindow$DropDownListView) with handle 0xa261f4b0
D/MyActivity: onItemSelected: Drinks Drinks American < 30 1
E/Surface: getSlotFromBufferLocked: unknown buffer: 0xaa9b7e30
W/art: Verification of void com.example.rcadit.foodgenie.resultActivity.onCreate(android.os.Bundle) took 436.369ms
D/AndroidRuntime: Shutting down VM
E/AndroidRuntime: FATAL EXCEPTION: main
Process: com.example.rcadit.foodgenie, PID: 3744
java.lang.RuntimeException: Unable to start activity ComponentInfo{com.example.rcadit.foodgenie/com.example.rcadit.foodgenie.resultActivity}: java.lang.NullPointerException: Attempt to invoke virtual method 'java.lang.String android.content.Context.getPackageName()' on a null object reference
at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2416)
at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2476)
at android.app.ActivityThread.-wrap11(ActivityThread.java)
at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1344)
at android.os.Handler.dispatchMessage(Handler.java:102)
at android.os.Looper.loop(Looper.java:148)
at android.app.ActivityThread.main(ActivityThread.java:5417)
at java.lang.reflect.Method.invoke(Native Method)
at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:726)
at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:616)
Caused by: java.lang.NullPointerException: Attempt to invoke virtual method 'java.lang.String android.content.Context.getPackageName()' on a null object reference
at android.content.ContextWrapper.getPackageName(ContextWrapper.java:133)
at android.preference.PreferenceManager.getDefaultSharedPreferencesName(PreferenceManager.java:375)
at android.preference.PreferenceManager.getDefaultSharedPreferences(PreferenceManager.java:370)
at com.example.rcadit.foodgenie.DatabaseAccess.<init>(DatabaseAccess.java:21)
at com.example.rcadit.foodgenie.DatabaseAccess.getInstance(DatabaseAccess.java:49)
at com.example.rcadit.foodgenie.resultActivity.onCreate(resultActivity.java:19)
at android.app.Activity.performCreate(Activity.java:6237)
at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1107)
at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2369)
at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2476)
at android.app.ActivityThread.-wrap11(ActivityThread.java)
at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1344)
at android.os.Handler.dispatchMessage(Handler.java:102)
at android.os.Looper.loop(Looper.java:148)
at android.app.ActivityThread.main(ActivityThread.java:5417)
at java.lang.reflect.Method.invoke(Native Method)
at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:726)
at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:616)
Disconnected from the target VM, address: 'localhost:8610', transport: 'socket'
我不知道我的错误在哪里，并且是android编程的新手。 ：（