我正在使用两个片段之间的actionbarnavigator开发一个应用程序，这意味着我有两个不同的用户视图，他们可以在它们之间滑动。一个片段有一个按钮，它打开另一个活动以显示listview中数据库的信息。但当我点击按钮时，我成了一个nullpointerexception。我不知道为什么。我希望s.o.可以帮我。
这是主要活动
public class MainActivity extends ActionBarActivity implements ActionBar.TabListener {
SectionsPagerAdapter mSectionsPagerAdapter;
Debtor debtor;
Creditor creditor;
public DataBaseHandler myDBHandler;
ViewPager mViewPager;
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
final ActionBar actionBar = getActionBar();
actionBar.setNavigationMode(ActionBar.NAVIGATION_MODE_TABS);
mSectionsPagerAdapter = new SectionsPagerAdapter(getSupportFragmentManager());
mViewPager = (ViewPager) findViewById(R.id.pager);
mViewPager.setAdapter(mSectionsPagerAdapter);
mViewPager.setOnPageChangeListener(new ViewPager.SimpleOnPageChangeListener() {
@Override
public void onPageSelected(int position) {
actionBar.setSelectedNavigationItem(position);
}
});
for (int i = 0; i < mSectionsPagerAdapter.getCount(); i++) {
actionBar.addTab(actionBar.newTab()
.setText(mSectionsPagerAdapter.getPageTitle(i))
.setTabListener(this));
}
}
@Override
protected void onResume() {
super.onResume();
initFragments();
openDB();
}
@Override
protected void onDestroy() {
super.onDestroy();
closeDB();
}
private void initFragments() {
debtor = (Debtor)mSectionsPagerAdapter.getItem(0);
creditor = (Creditor)mSectionsPagerAdapter.getItem(1);
}
private void openDB() {
myDBHandler = new DataBaseHandler(this);
myDBHandler.open();
}
private void closeDB() {
myDBHandler.close();
}
@Override
public boolean onCreateOptionsMenu(Menu menu) {
getMenuInflater().inflate(R.menu.main, menu);
return true;
}
@Override
public void onTabSelected(ActionBar.Tab tab, FragmentTransaction fragmentTransaction) {
mViewPager.setCurrentItem(tab.getPosition());
}
@Override
public void onTabUnselected(ActionBar.Tab tab, FragmentTransaction fragmentTransaction) {
}
@Override
public void onTabReselected(ActionBar.Tab tab, FragmentTransaction fragmentTransaction) {
}
}
片段类
public class Debtor extends Fragment {
private Button showDebtorsHistory;
MainActivity mainActivity;
public static final String ARG_SECTION_NUMBER = "section_number";
public Debtor(int position) {
}
@Override
public View onCreateView(LayoutInflater inflater, ViewGroup container, Bundle savedInstanceState) {
mainActivity = (MainActivity) getActivity();
View v = inflater.inflate(R.layout.debtor, container, false);
showDebtorsHistory = (Button)v.findViewById(R.id.button1);
showDebtorsHistory.setOnClickListener(new OnClickListener() {
@Override
public void onClick(View v) {
Intent i = new Intent(getActivity(), DebtorHistory.class);
startActivityForResult(i, 100);
}
});
return v;
}
@Override
public void onAttach(Activity activity) {
super.onAttach(activity);
try {
mainActivity = (MainActivity) activity;
}
catch (ClassCastException e) {
throw new ClassCastException(activity.toString() + " Fehler: Keine MainActivity-Instanz!");
}
}
}
和单击按钮时必须打开的活动
public class DebtorHistory extends Activity {
Cursor cursor;
private ListView mainList;
MainActivity mainActivity;
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.debtor_history);
mainList = (ListView)findViewById(R.id.listView_Debtors);
populateListViewFromDB();
}
@SuppressWarnings("deprecation")
public void populateListViewFromDB() {
cursor = mainActivity.myDBHandler.getAllDifferentNames1();
String[] colmn = new String[] {"n", "i", "d"};
int[] tolist = new int[] {R.id.txt_name,
R.id.list_userImage,
R.id.datum };
SimpleCursorAdapter myCursorAdapter
= new SimpleCursorAdapter
(this, R.layout.history_listrow, cursor, colmn, tolist);
mainList.setAdapter(myCursorAdapter);
}
}
logcat消息
01-28 15:19:31.967: E/AndroidRuntime(1205): java.lang.RuntimeException: Unable to start activity ComponentInfo{***.debtmanagerpro/***.debtmanagerpro.DebtorHistory}: java.lang.NullPointerException
01-28 15:19:31.967: E/AndroidRuntime(1205): at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2073)
01-28 15:19:31.967: E/AndroidRuntime(1205): at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2098)
01-28 15:19:31.967: E/AndroidRuntime(1205): at android.app.ActivityThread.access$600(ActivityThread.java:138)
01-28 15:19:31.967: E/AndroidRuntime(1205): at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1204)
01-28 15:19:31.967: E/AndroidRuntime(1205): at android.os.Handler.dispatchMessage(Handler.java:99)
01-28 15:19:31.967: E/AndroidRuntime(1205): at android.os.Looper.loop(Looper.java:137)
01-28 15:19:31.967: E/AndroidRuntime(1205): at android.app.ActivityThread.main(ActivityThread.java:4872)
01-28 15:19:31.967: E/AndroidRuntime(1205): at java.lang.reflect.Method.invokeNative(Native Method)
01-28 15:19:31.967: E/AndroidRuntime(1205): at java.lang.reflect.Method.invoke(Method.java:511)
01-28 15:19:31.967: E/AndroidRuntime(1205): at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:790)
01-28 15:19:31.967: E/AndroidRuntime(1205): at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:557)
01-28 15:19:31.967: E/AndroidRuntime(1205): at dalvik.system.NativeStart.main(Native Method)
01-28 15:19:31.967: E/AndroidRuntime(1205): Caused by: java.lang.NullPointerException
01-28 15:19:31.967: E/AndroidRuntime(1205): at ***.debtmanagerpro.DebtorHistory.populateListViewFromDB(DebtorHistory.java:30)
01-28 15:19:31.967: E/AndroidRuntime(1205): at ***.debtmanagerpro.DebtorHistory.onCreate(DebtorHistory.java:23)
01-28 15:19:31.967: E/AndroidRuntime(1205): at android.app.Activity.performCreate(Activity.java:5191)
01-28 15:19:31.967: E/AndroidRuntime(1205): at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1082)
01-28 15:19:31.967: E/AndroidRuntime(1205): at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2037)
01-28 15:19:31.967: E/AndroidRuntime(1205): ... 11 more
DebtorHistory中的第23行是 populateListViewFromDB（）; ，30是 cursor = mainActivity.myDBHandler.getAllDifferentNames1（）;
提前感谢。
修改
public class DebtorHistory extends Activity {
Cursor cursor;
private ListView mainList;
DataBaseHandler myDBHandler;
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.debtor_history);
mainList = (ListView)findViewById(R.id.listView_Debtors);
populateListViewFromDB();
}
@Override
protected void onResume() {
super.onResume();
myDBHandler = new DataBaseHandler(this);
myDBHandler.open();
}
@SuppressWarnings("deprecation")
public void populateListViewFromDB() { //MainList mit Werten aus der db befüllen
try {
startManagingCursor(cursor = myDBHandler.getAllDifferentNames1());
} catch (Exception e) {
if(cursor == null) {
Log.e("#################", "is null :(");
}
}
;
String[] colm= new String[] {"n", "i", "d"};
int[] tolist= new int[] {R.id.txt_name, R.id.list_userImage, R.id.datum };
SimpleCursorAdapter myCursorAdapter
= new SimpleCursorAdapter
(this, R.layout.history_listrow, cursor, colm, tolist);
mainList.setAdapter(myCursorAdapter);
}
}
答案 0 :(得分：1)
我认为问题出在populateListViewFromDb（）方法中。也许你在尝试获取光标时会得到一个nullpointer异常。调试该方法，看看你得到了什么。
答案 1 :(得分：0)
您不能简单地从其他活动中访问活动。
mainActivity永远不会在DebtorActivity中初始化，因此当您在NullPointerException中调用mainActivity的函数时，populateListViewFromDb是合理的：
cursor = mainActivity.myDBHandler.getAllDifferentNames1();
您必须找到另一种方法将数据库传递给第二个Activity。
我建议为数据库Access构建一个帮助程序类，并在Activities中使用它。