我试图在我的应用程序中实现首选项片段，所以我设置了preferences.xml文件，应该显示首选项的片段以及交易片段。当我尝试显示SettingsFragment时，应用程序崩溃并显示以下错误日志：
java.lang.NullPointerException: Attempt to invoke virtual method 'java.lang.Class java.lang.Object.getClass()' on a null object reference
at android.support.v4.app.BackStackRecord.doAddOp(BackStackRecord.java:431)
at android.support.v4.app.BackStackRecord.replace(BackStackRecord.java:481)
at android.support.v4.app.BackStackRecord.replace(BackStackRecord.java:472)
at com.ikurek.pwr.MainActivity.onNavigationItemSelected(MainActivity.java:145)
at android.support.design.widget.NavigationView$1.onMenuItemSelected(NavigationView.java:153)
at android.support.v7.view.menu.MenuBuilder.dispatchMenuItemSelected(MenuBuilder.java:810)
at android.support.v7.view.menu.MenuItemImpl.invoke(MenuItemImpl.java:152)
at android.support.v7.view.menu.MenuBuilder.performItemAction(MenuBuilder.java:957)
at android.support.design.internal.NavigationMenuPresenter$1.onClick(NavigationMenuPresenter.java:328)
at android.view.View.performClick(View.java:4780)
at android.view.View$PerformClick.run(View.java:19866)
at android.os.Handler.handleCallback(Handler.java:739)
at android.os.Handler.dispatchMessage(Handler.java:95)
at android.os.Looper.loop(Looper.java:135)
at android.app.ActivityThread.main(ActivityThread.java:5254)
at java.lang.reflect.Method.invoke(Native Method)
at java.lang.reflect.Method.invoke(Method.java:372)
at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:903)
at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:698)
为什么此对象为null？我还应该如何设置设置片段？我知道解决此问题的方法可能非常简单，但是我无法以某种方式自己解决这个问题。这是我的代码：
从MainActivity调用片段事务的部分：
public boolean onNavigationItemSelected(MenuItem item) {
Fragment fragment = null;
Class fragmentClass = null;
int id = item.getItemId();
if (id == R.id.nav_news) {
fragmentClass = NewsFragment.class;
} else if (id == R.id.nav_map) {
fragmentClass = CatFragment.class;
} else if (id == R.id.nav_buildings) {
fragmentClass = BuildingsFragment.class;
} else if (id == R.id.nav_settings) {
fragmentClass = SettingsFragment.class;
} else if (id == R.id.nav_info) {
fragmentClass = AppInfoFragment.class;
} else if (id == R.id.nav_bugreport) {
fragmentClass = ContactFragment.class;
}
try {
fragment = (Fragment) fragmentClass.newInstance();
} catch (Exception e) {
e.printStackTrace();
}
FragmentManager fragmentManager = getSupportFragmentManager();
fragmentManager.beginTransaction().replace(R.id.frameLayoutForFragments, fragment).commit();
DrawerLayout drawer = (DrawerLayout) findViewById(R.id.drawer_layout);
drawer.closeDrawer(GravityCompat.START);
return true;
}
我的preferences.xml：
<PreferenceScreen xmlns:android="http://schemas.android.com/apk/res/android">
<CheckBoxPreference
android:defaultValue="false"
android:key="saveLastPosition"
android:summary="1st check box"
android:title="Check"/>
<CheckBoxPreference
android:defaultValue="true"
android:key="keepScreenOn"
android:summary="2nd check box"
android:title="Box"/>
</PreferenceScreen>
和我的SettingsFragment：
public class SettingsFragment extends PreferenceFragment {
public SettingsFragment() {
// Required empty public constructor
}
public static SettingsFragment newInstance() {
SettingsFragment fragment = new SettingsFragment();
return fragment;
}
@Override
public void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
addPreferencesFromResource(R.xml.preferences);
}
}
最佳答案
好吧，据我所知，您正在将一个空片段传递给片段管理器。您会注意到：
Fragment fragment = null;
Class fragmentClass = SettingsFragment.class;
FragmentManager fragmentManager = getSupportFragmentManager();
fragmentManager.beginTransaction().replace(R.id.frameLayoutForFragments, fragment).commit();
你有：
Fragment fragment = null;
您实际上从未将其声明为SettingsFragment。您应该具有：
Fragment fragment = new SettingsFragment();
或在您的replace语句中声明。
Class fragmentClass = SettingsFragment.class;
FragmentManager fragmentManager = getSupportFragmentManager();
fragmentManager.beginTransaction().replace(R.id.frameLayoutForFragments, new SettingsFragment()).commit();