java - 无法启动 Activity : Attempt to invoke virtual method (NullPointerException)

Question

我的 java android 应用程序有问题:当用我的 SettingsFragment 替换fragment_container 时，我不断收到此错误消息:

java.lang.NullPointerException: Attempt to invoke virtual method 'void android.support.v7.widget.RecyclerView.setAdapter(android.support.v7.widget.RecyclerView$Adapter)' on a null object reference.

首先，我想从 MainActivity 的 NavigationViewer 开始打开我的 SettingsActivity。然后用我的preferences.xml替换该 fragment

我已将框架 ID“fragment_container”设置为 Activity_settings.xml，我想将其替换为我的 SettingsFragment 类/preferences.xml。

// MainActivity.java

    public class MainActivity extends AppCompatActivity
            implements NavigationView.OnNavigationItemSelectedListener, SettingsFragment.OnFragmentInteractionListener {
    [..] 
    public boolean onNavigationItemSelected(MenuItem item) {
            // Handle navigation view item clicks here.
            switch (item.getItemId()) {
                case  R.id.nav_settings:
                    Intent intent = new Intent(this, SettingsActivity.class);
                    startActivity(intent);
                    break;
                }

            DrawerLayout drawer = findViewById(R.id.drawer_layout);
            drawer.closeDrawer(GravityCompat.START);
            return true;

//activity_settings.xml

<android.support.design.widget.CoordinatorLayout xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:app="http://schemas.android.com/apk/res-auto"
    xmlns:tools="http://schemas.android.com/tools"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    tools:context=".SettingsActivity">

    <LinearLayout
        android:layout_width="match_parent"
        android:layout_height="match_parent"
        android:orientation="vertical">
        <FrameLayout
            android:id="@+id/fragment_container"
            android:layout_width="match_parent"
            android:layout_height="match_parent" />

    </LinearLayout>

    <android.support.design.widget.AppBarLayout
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        android:theme="@style/AppTheme.AppBarOverlay">

        <android.support.v7.widget.Toolbar
            android:id="@+id/toolbar"
            android:layout_width="match_parent"
            android:layout_height="?attr/actionBarSize"
            android:background="?attr/colorPrimary"
            app:popupTheme="@style/AppTheme.PopupOverlay" />

    </android.support.design.widget.AppBarLayout>

    <include layout="@layout/content_settings" />


</android.support.design.widget.CoordinatorLayout>

// SettingsActivity.java

    public class SettingsActivity extends AppCompatActivity  implements  SettingsFragment.OnFragmentInteractionListener{

        @Override
        public void onFragmentInteraction(Uri uri){
            //you can leave it empty
        }

        @Override
        protected void onCreate(Bundle savedInstanceState) {
            super.onCreate(savedInstanceState);
            setContentView(R.layout.activity_settings);


            /*Toolbar toolbar = findViewById(R.id.toolbar);
            setSupportActionBar(toolbar);
            */

            getSupportFragmentManager()
                    .beginTransaction()
                    .replace(R.id.fragment_container, new SettingsFragment())
                    .commit();

        }

    }

//activity_settings.xml

<android.support.design.widget.CoordinatorLayout
    xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:app="http://schemas.android.com/apk/res-auto"
    xmlns:tools="http://schemas.android.com/tools"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    tools:context=".MainActivity">

    <android.support.design.widget.AppBarLayout
        android:layout_height="wrap_content"
        android:layout_width="match_parent"
        android:theme="@style/AppTheme.AppBarOverlay">

        <android.support.v7.widget.Toolbar
            android:id="@+id/toolbar"
            android:layout_width="match_parent"
            android:layout_height="?attr/actionBarSize"
            android:background="?attr/colorPrimary"
            app:popupTheme="@style/AppTheme.PopupOverlay" />

    </android.support.design.widget.AppBarLayout>


</android.support.design.widget.CoordinatorLayout>

//SettingsFragment.java

public class SettingsFragment extends PreferenceFragmentCompat  {

    @Override
    public void onCreatePreferences(Bundle savedInstanceState, String rootKey) {
        setPreferencesFromResource(R.xml.preferences, rootKey);
    }

错误消息:

    E/AndroidRuntime: FATAL EXCEPTION: main
    Process: com.example.homeautomation, PID: 30127
    java.lang.RuntimeException: Unable to start activity ComponentInfo{com.example.homeautomation/com.example.homeautomation.SettingsActivity}: java.lang.NullPointerException: Attempt to invoke virtual method 'void android.support.v7.widget.RecyclerView.setAdapter(android.support.v7.widget.RecyclerView$Adapter)' on a null object reference
        at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2957)
        at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:3032)
        at android.app.ActivityThread.-wrap11(Unknown Source:0)
        at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1696)
        at android.os.Handler.dispatchMessage(Handler.java:105)
        at android.os.Looper.loop(Looper.java:164)
        at android.app.ActivityThread.main(ActivityThread.java:6944)
        at java.lang.reflect.Method.invoke(Native Method)
        at com.android.internal.os.Zygote$MethodAndArgsCaller.run(Zygote.java:327)
        at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:1374)
        at de.robv.android.xposed.XposedBridge.main(XposedBridge.java:108)
     Caused by: java.lang.NullPointerException: Attempt to invoke virtual method 'void android.support.v7.widget.RecyclerView.setAdapter(android.support.v7.widget.RecyclerView$Adapter)' on a null object reference
        at android.support.v7.preference.PreferenceFragmentCompat.bindPreferences(PreferenceFragmentCompat.java:535)
        at android.support.v7.preference.PreferenceFragmentCompat.onViewCreated(PreferenceFragmentCompat.java:340)
        at android.support.v4.app.FragmentManagerImpl.moveToState(FragmentManager.java:1471)
        at android.support.v4.app.FragmentManagerImpl.moveFragmentToExpectedState(FragmentManager.java:1784)
        at android.support.v4.app.FragmentManagerImpl.moveToState(FragmentManager.java:1852)
        at android.support.v4.app.BackStackRecord.executeOps(BackStackRecord.java:802)
        at android.support.v4.app.FragmentManagerImpl.executeOps(FragmentManager.java:2625)
        at android.support.v4.app.FragmentManagerImpl.executeOpsTogether(FragmentManager.java:2411)
        at android.support.v4.app.FragmentManagerImpl.removeRedundantOperationsAndExecute(FragmentManager.java:2366)
        at android.support.v4.app.FragmentManagerImpl.execPendingActions(FragmentManager.java:2273)
        at android.support.v4.app.FragmentManagerImpl.dispatchStateChange(FragmentManager.java:3273)
        at android.support.v4.app.FragmentManagerImpl.dispatchActivityCreated(FragmentManager.java:3229)
        at android.support.v4.app.FragmentController.dispatchActivityCreated(FragmentController.java:201)
        at android.support.v4.app.FragmentActivity.onStart(FragmentActivity.java:620)
        at android.support.v7.app.AppCompatActivity.onStart(AppCompatActivity.java:178)
        at android.app.Instrumentation.callActivityOnStart(Instrumentation.java:1340)
        at android.app.Activity.performStart(Activity.java:7200)
        at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2920)
        at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:3032) 
        at android.app.ActivityThread.-wrap11(Unknown Source:0) 
        at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1696) 
        at android.os.Handler.dispatchMessage(Handler.java:105) 
        at android.os.Looper.loop(Looper.java:164) 
        at android.app.ActivityThread.main(ActivityThread.java:6944) 
        at java.lang.reflect.Method.invoke(Native Method) 
        at com.android.internal.os.Zygote$MethodAndArgsCaller.run(Zygote.java:327) 
        at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:1374) 
        at de.robv.android.xposed.XposedBridge.main(XposedBridge.java:108) 

有人知道导致此问题的原因以及如何解决此问题吗？我将不胜感激任何形式的帮助:)

Answer 1

我已经找到了解决方案，而且......如果我之前检查过我的 SettingsFragment.java 类，我可能会更早发现此错误......

此刻我想显示SettingsFragment

getSupportFragmentManager().beginTransaction().replace(R.id.fragment_container, new SettingsFragment()).commit();

SettingsFragment.class 中存在一些错误。由于android studio生成了一些模板代码，这些代码的标题明显是“//TODO”，因此由于代码不完整，它无法构建Fragment。

一旦我删除了这个不完整的使用参数的示例代码，一切就再次正常工作了。