第二次活動呼叫導致空指針異常

Question

這裏是活動類和第二活動類。 secnd類不按按鈕 顯示空指針異常。其中有活動類和第二個活動類。所述secnd類不被按鈕 工作示出了空指針例外在FirstActivity.java

showevent（）

public void ShowEvent(View view){ 
     Intent intent = new Intent(this, Getclicker.class); 
     date = (TextView)findViewById(R.id.textView2); 
     String datevalue = date.getText().toString(); 
     userSelection=(TextView)findViewById(R.id.textView1); 
     String Userselectvalue = userSelection.getText().toString(); 

     intent.putExtra(EXTRA_MESSAGE, datevalue); 
     intent.putExtra(EXTRA_MESSAGE1, Userselectvalue); 

     startActivity(intent); 
    } 

Getclicker.java

package example.events1; 

import android.app.ListActivity; 
import android.content.Intent; 
import android.database.Cursor; 
import android.os.Bundle; 
import android.support.v4.widget.SimpleCursorAdapter; 
import android.widget.ListView; 

public class Getclicker extends ListActivity { 

    classdbOpenHelper eventsData1; 
    Cursor cursor1; 
    ListView listView ; 

    @Override 
    public void onCreate(Bundle savedInstanceState) { 
     Intent intent = getIntent(); 
     String datevalue = intent.getStringExtra(FirstActivity.EXTRA_MESSAGE); 
     String Userselectvalue = intent.getStringExtra(FirstActivity.EXTRA_MESSAGE1); 
     super.onCreate(savedInstanceState); 
     setContentView(R.layout.activity_second);      /*String datevalue = date.getText().toString(); 
     String Userselectvalue = userSelection.getText().toString(); 

     String showevent = eventsData.getContact(datevalue,Userselectvalue); 
     show.setText(showevent);*/ 

     cursor1 = eventsData1.getContact(datevalue,Userselectvalue); 
     String[] fromColumns = {classdbOpenHelper.KEY_EVENT}; 
     int[] toViews = {R.id.event}; 
     SimpleCursorAdapter adapter = new SimpleCursorAdapter(this,R.layout.events, cursor1, fromColumns, toViews,0); 
      listView = (ListView) findViewById(R.id.list); 
     listView.setAdapter(adapter); 
     } 

    public void onDestroy() { 
     eventsData1.close(); 
     } 

} 

logcat的

08-07 11:40:40.795: E/AndroidRuntime(2544): FATAL EXCEPTION: main 
08-07 11:40:40.795: E/AndroidRuntime(2544): java.lang.RuntimeException: Unable to start activity ComponentInfo{example.events1/example.events1.Getclicker}: java.lang.NullPointerException 
08-07 11:40:40.795: E/AndroidRuntime(2544):  at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2180) 
08-07 11:40:40.795: E/AndroidRuntime(2544):  at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2230) 
08-07 11:40:40.795: E/AndroidRuntime(2544):  at android.app.ActivityThread.access$600(ActivityThread.java:141) 
08-07 11:40:40.795: E/AndroidRuntime(2544):  at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1234) 
08-07 11:40:40.795: E/AndroidRuntime(2544):  at android.os.Handler.dispatchMessage(Handler.java:99) 
08-07 11:40:40.795: E/AndroidRuntime(2544):  at android.os.Looper.loop(Looper.java:137) 
08-07 11:40:40.795: E/AndroidRuntime(2544):  at android.app.ActivityThread.main(ActivityThread.java:5041) 
08-07 11:40:40.795: E/AndroidRuntime(2544):  at java.lang.reflect.Method.invokeNative(Native Method) 
08-07 11:40:40.795: E/AndroidRuntime(2544):  at java.lang.reflect.Method.invoke(Method.java:511) 
08-07 11:40:40.795: E/AndroidRuntime(2544):  at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:793) 
08-07 11:40:40.795: E/AndroidRuntime(2544):  at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:560) 
08-07 11:40:40.795: E/AndroidRuntime(2544):  at dalvik.system.NativeStart.main(Native Method) 
08-07 11:40:40.795: E/AndroidRuntime(2544): Caused by: java.lang.NullPointerException 
08-07 11:40:40.795: E/AndroidRuntime(2544):  at example.events1.Getclicker.onCreate(Getclicker.java:28) 
08-07 11:40:40.795: E/AndroidRuntime(2544):  at android.app.Activity.performCreate(Activity.java:5104) 
08-07 11:40:40.795: E/AndroidRuntime(2544):  at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1080) 
08-07 11:40:40.795: E/AndroidRuntime(2544):  at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2144) 
08-07 11:40:40.795: E/AndroidRuntime(2544):  ... 11 more 

Answer 1

在你的代碼的問題是在這裏

cursor1 = eventsData1.getContact(datevalue,Userselectvalue); 

eventData1未初始化任何東西。