向上导航的空指针android
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【中文标题】向上导航的空指针android【英文标题】：Nullpointer at Upnavigation android 【发布时间】：2014-10-02 21:42:12 【问题描述】：当点击操作栏的向上导航时，我想通过意图发送两个额外内容。在清单上声明父活动，在 OnCreate 处设置第二个活动getActionBar().setDisplayHomeAsUpEnabled(true);。但是父活动需要一个意图值才能工作（citySave）。我已经尝试了很多，但我没有得到它的工作，向上导航将意图值发送到父活动（R.id.home）。
我没有从第二个活动中添加整个代码，因为它会出现太多迹象。我在第二个活动中使用了 2 个片段，它们被定义为静态的。我不知道，这是否与问题有关。
第二个活动：
public class RouteView extends ActionBarActivity
Integer citySave;
String cityTitle;
Integer routeSave;
String routeTitel;
private static int NUM_ITEMS = 2;
SectionsPagerAdapter mSectionsPagerAdapter;
ListView listView;
ArrayList<RowItem> pois;
/**
* The @link ViewPager that will host the section contents.
*/
ViewPager mViewPager;
@Override
protected void onCreate(Bundle savedInstanceState)
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_route_view);
Bundle b = getIntent().getExtras();
citySave = b.getInt("stadt");
cityTitle = b.getString("titel_stadt");
routeSave = b.getInt("route");
routeTitel = b.getString("titel_route");
b.putString("titel_route",routeTitel);
b.putInt("route", routeSave);
getActionBar().setDisplayHomeAsUpEnabled(true);
//RouteItemFragment fragobj = new RouteItemFragment();
//fragobj.setArguments(b);
final ActionBar actionBar = getSupportActionBar();
actionBar.setNavigationMode(ActionBar.NAVIGATION_MODE_TABS);
setTitle("Route:" + " " + routeTitel);
public void onBackPressed()
super.onBackPressed();
Intent intent = new Intent();
intent.setClass(RouteView.this, RouteChooseActivity.class);
Bundle b = new Bundle();
b.putInt("stadt", citySave);
b.putString("titel_stadt", cityTitle);
intent.putExtras(b);
startActivity(intent);
finish();
public void onPause()
super.onPause();
Bundle b = new Bundle();
b.putInt("stadt", citySave);
b.putInt("route", routeSave);
b.putString("route_titel",routeTitel);
finish();
public void onResume()
super.onResume();
Bundle b = getIntent().getExtras();
citySave = b.getInt("stadt");
routeSave = b.getInt("route");
routeTitel = b.getString("route_titel");
public void onStop()
super.onStop();
finish();
@Override
public boolean onOptionsItemSelected(MenuItem item)
switch (item.getItemId())
case R.id.home:
NavUtils.navigateUpFromSameTask(this);
Intent upIntent = NavUtils.getParentActivityIntent(this);
Bundle b = new Bundle();
b.putInt("stadt", citySave);
b.putString("titel_stadt", cityTitle);
upIntent.putExtras(b);
//startActivity(upIntent);
return true;
case R.id.action_stadt:
setContentView(R.layout.activity_stadt);
Intent stadt = new Intent(RouteView.this, StadtActivity.class);
startActivity(stadt);
return true;
case R.id.action_route:
setContentView(R.layout.activity_route_choose);
Intent route = new Intent(RouteView.this, RouteChooseActivity.class);
route.putExtra("stadt", citySave);
route.putExtra("titel_stadt", cityTitle);
startActivity(route);
return true;
case R.id.action_help:
setContentView(R.layout.activity_help);
Intent help = new Intent(RouteView.this, Help.class);
startActivity(help);
return true;
case R.id.action_exit:
moveTaskToBack(true);
System.exit(0);
return true;
default:
return super.onOptionsItemSelected(item);
家长活动：
public class RouteChooseActivity extends ActionBarActivity implements OnItemClickListener
Integer citySave;
Integer routeSave;
String cityTitle;
ListView listView;
List<RowItem> cities;
CityParser parser = new CityParser();
@Override
protected void onCreate(Bundle savedInstanceState)
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_route_choose);
Bundle b = getIntent().getExtras();
citySave = b.getInt("stadt");
cityTitle = b.getString("titel_stadt");
TextView headingRoute = (TextView) findViewById(R.id.routeTitel);
headingRoute.setText(String.format(getResources().getString(R.string.route_text)) + " " + cityTitle + " aus:");
listView = (ListView) findViewById(R.id.listRoute);
if (citySave.equals(1))
try
cities = parser.parse(getAssets().open("route_passau.xml"));
CityListViewAdapter adapter = new CityListViewAdapter(this, R.layout.row_items, (ArrayList<RowItem>) cities);
Collections.sort(cities, new Comparator<RowItem>()
public int compare(RowItem s1, RowItem s2)
return s1.getTitle().compareTo(s2.getTitle());
);
listView.setAdapter(adapter);
catch (IOException e)
e.printStackTrace();
else if (citySave.equals(2))
try
cities = parser.parse(getAssets().open("route_bamberg.xml"));
CityListViewAdapter adapter = new CityListViewAdapter(this, R.layout.row_items, (ArrayList<RowItem>) cities);
Collections.sort(cities, new Comparator<RowItem>()
public int compare(RowItem s1, RowItem s2)
return s1.getTitle().compareTo(s2.getTitle());
);
listView.setAdapter(adapter);
catch (IOException e)
e.printStackTrace();
else if (citySave.equals(3))
try
cities = parser.parse(getAssets().open("route_augsburg.xml"));
CityListViewAdapter adapter = new CityListViewAdapter(this, R.layout.row_items, (ArrayList<RowItem>) cities);
Collections.sort(cities, new Comparator<RowItem>()
public int compare(RowItem s1, RowItem s2)
return s1.getTitle().compareTo(s2.getTitle());
);
listView.setAdapter(adapter);
catch (IOException e)
e.printStackTrace();
else
try
TextView errorMessage = (TextView) findViewById(R.id.routeTitel);
errorMessage.setText(R.string.errorroute);
catch (NullPointerException e)
Context context = getApplicationContext();
CharSequence text = getResources().getString(R.string.errorroute);
int duration = Toast.LENGTH_LONG;
Toast toast = Toast.makeText(context, text, duration);
toast.setGravity(Gravity.BOTTOM | Gravity.CENTER_HORIZONTAL, 0, 0);
toast.show();
listView.setOnItemClickListener(this);
final ActionBar actionBar = getSupportActionBar();
setTitle("Routen in" + " " + cityTitle);
@Override
public void onItemClick(AdapterView<?> parent, View view, int position, long id)
Intent intent = new Intent();
intent.setClass(RouteChooseActivity.this, RouteView.class);
RowItem cities = (RowItem) parent.getItemAtPosition(position);
Bundle b = new Bundle();
b.putInt("stadt", citySave);
b.putString("titel_stadt", cityTitle);
b.putInt("route", cities.getID());
b.putString("titel_route", cities.getTitle());
intent.putExtras(b);
startActivity(intent);
@Override
protected void onSaveInstanceState (Bundle outState)
super.onSaveInstanceState(outState);
outState.putInt("stadt", citySave);
outState.getString("titel_stadt", cityTitle);
@Override
public void onRestoreInstanceState(Bundle savedInstanceState)
super.onRestoreInstanceState(savedInstanceState);
// Restore UI state from the savedInstanceState.
// This bundle has also been passed to onCreate.
citySave = savedInstanceState.getInt("stadt");
Log.i("debug", "saved data: " + citySave);
@Override
public void onResume()
super.onResume();
Bundle b = getIntent().getExtras();
citySave = b.getInt("stadt");
cityTitle = b.getString("titel_stadt");
@Override
public void onPause()
super.onPause();
Bundle b = new Bundle();
b.putInt("stadt", citySave);
b.putString("titel_stadt", cityTitle);
unbindDrawables(findViewById(R.id.Bild));
System.gc();
@Override
public void onStop()
super.onStop();
Bundle b = new Bundle();
b.putInt("stadt", citySave);
b.putString("titel_stadt", cityTitle);
finish();
@Override
protected void onDestroy()
super.onDestroy();
unbindDrawables(findViewById(R.id.Bild));
System.gc();
@Override
public void onRestart()
super.onRestart();
Bundle b = getIntent().getExtras();
citySave = b.getInt("stadt");
cityTitle = b.getString("titel_stadt");
private void unbindDrawables(View view)
if (view.getBackground() != null)
view.getBackground().setCallback(null);
if (view instanceof ViewGroup && !(view instanceof AdapterView))
for (int i = 0; i < ((ViewGroup) view).getChildCount(); i++)
unbindDrawables(((ViewGroup) view).getChildAt(i));
((ViewGroup) view).removeAllViews();
@Override
public boolean onCreateOptionsMenu(Menu menu)
getMenuInflater().inflate(R.menu.route, menu);
return true;
@Override
public boolean onOptionsItemSelected(MenuItem item)
switch (item.getItemId())
case R.id.action_stadt:
setContentView(R.layout.activity_stadt);
Intent stadt = new Intent(RouteChooseActivity.this, StadtActivity.class);
startActivity(stadt);
return true;
case R.id.action_help:
setContentView(R.layout.activity_help);
Intent help = new Intent(RouteChooseActivity.this, Help.class);
startActivity(help);
return true;
case R.id.action_exit:
moveTaskToBack(true);
System.exit(0);
return true;
default:
return super.onOptionsItemSelected(item);
Logcat：
java.lang.RuntimeException: Unable to start activity ComponentInfode.cityknight.app/de.cityknight.app.RouteChooseActivity: java.lang.NullPointerException
at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2195)
at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2245)
at android.app.ActivityThread.access$800(ActivityThread.java:135)
at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1196)
at android.os.Handler.dispatchMessage(Handler.java:102)
at android.os.Looper.loop(Looper.java:136)
at android.app.ActivityThread.main(ActivityThread.java:5017)
at java.lang.reflect.Method.invokeNative(Native Method)
at java.lang.reflect.Method.invoke(Method.java:515)
at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:779)
at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:595)
at dalvik.system.NativeStart.main(Native Method)
Caused by: java.lang.NullPointerException
at de.cityknight.app.RouteChooseActivity.onCreate(RouteChooseActivity.java:44)
at android.app.Activity.performCreate(Activity.java:5231)
at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1087)
at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2159)
at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2245)
at android.app.ActivityThread.access$800(ActivityThread.java:135)
at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1196)
at android.os.Handler.dispatchMessage(Handler.java:102)
at android.os.Looper.loop(Looper.java:136)
at android.app.ActivityThread.main(ActivityThread.java:5017)
at java.lang.reflect.Method.invokeNative(Native Method)
at java.lang.reflect.Method.invoke(Method.java:515)
at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:779)
at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:595)
at dalvik.system.NativeStart.main(Native Method)
【问题讨论】：
检查Bundle b是否为空。 它不能为空，因为 OnBackPressed 正在工作。 查看pastebin.com/N0fgWWYw 并告诉我发生了什么 您的 NullPointerException 发生在 RouteChooseActivity 的第 44 行，其余代码在哪里？ 我已经添加了整个代码，但是第 44 行是 citySave = b.getInt("stadt"); 【参考方案1】：在 Activity 的 onCreate() 方法中，Bundle 将具有以下值：（来自 official documentation）
如果活动在之前关闭后重新初始化 向下，则此 Bundle 包含它最近提供的数据 onSaveInstanceState（捆绑）。注意：否则为空。
getIntent().getExtras() 也是如此。所以你应该检查这个值以避免 NullPointerException。
Bundle b = getIntent().getExtras();
if (b != null)
citySave = b.getInt("stadt");
cityTitle = b.getString("titel_stadt");
【讨论】：
谢谢，我会尽快尝试。如果我理解正确，我必须设置 onRestoreInstanceState？ 已编辑答案。您必须检查该值是否为空。 谢谢，否则是什么条件？我已经测试过： else citySave = savedInstanceState.getInt("stadt"); cityTitle = savedInstanceState.getString("titel_stadt"); 但 NullPointer。没有其他条件 NullPointer 在 citySave.equals... 如果Bundle为null，表示没有数据。所以不要尝试处理数据，因为会有 NullPointerException。 如你所见，我是一个该死的初学者。好的，我明白了。如何获取数据？【参考方案2】：在 *** 上找到正确答案：https://***.com/a/17342137/3762248
“android 活动的“标准”行为是，每次有新意图时，都会创建一个新的活动实例（请参阅此处的 launchMode-docu）。因此，您的附加功能似乎走吧，如果你调用 navigateUpTo。
在你的情况下，我建议使用
android:launchMode="singleTop"
AndroidManifest.xml 中的父活动。这样，您将返回到您现有的活动（只要它位于任务的后堆栈的顶部）。这样您的额外内容将被保留。
我也是，不明白为什么您引用的 Google 文档中没有提到这一点，因为这似乎是使用向上导航时所期望的行为。”
【讨论】：
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