Getting ClassCastException on PreferenceManager.setDefaultValues()

Question

I am getting this exception

Caused by: java.lang.ClassCastException: androidx.preference.PreferenceScreen cannot be cast to android.preference.GenericInflater$Parent

on doing PreferenceManager.setDefaultValues(this, R.xml.root_preferences, false);
in the main activity.

According to this SO post, the above error can happen if in the manifest file the activity and its parent is same but that is not the case with me. I just don't get why this exception is thrown.

AndroidManifest.xml

         <activity
                android:name=".SettingsActivity"
                android:label="@string/title_activity_settings"
                android:parentActivityName=".MainActivity">
                <meta-data
                    android:name="android.support.PARENT_ACTIVITY"
                    android:value="com.example.android.droidcafeinput.MainActivity" />
            </activity>
        <activity
            android:name=".MainActivity"
            android:label="@string/app_name"
            android:theme="@style/AppTheme.NoActionBar">
            <intent-filter>
                <action android:name="android.intent.action.MAIN" />

                <category android:name="android.intent.category.LAUNCHER" />
            </intent-filter>
        </activity>

root_preferences.xml

<androidx.preference.PreferenceScreen xmlns:app="http://schemas.android.com/apk/res-auto">

    <PreferenceCategory app:title="@string/messages_header">

        <EditTextPreference
            app:key="signature"
            app:title="@string/signature_title"
            app:useSimpleSummaryProvider="true" />

        <ListPreference
            app:defaultValue="reply"
            app:entries="@array/reply_entries"
            app:entryValues="@array/reply_values"
            app:key="reply"
            app:title="@string/reply_title"
            app:useSimpleSummaryProvider="true" />

    </PreferenceCategory>

    <PreferenceCategory app:title="@string/sync_header">

        <SwitchPreferenceCompat
            app:key="sync"
            app:title="@string/sync_title" />

        <SwitchPreferenceCompat
            app:dependency="sync"
            app:key="attachment"
            app:summaryOff="@string/attachment_summary_off"
            app:summaryOn="@string/attachment_summary_on"
            app:title="@string/attachment_title" />

    </PreferenceCategory>

</androidx.preference.PreferenceScreen>

SettingsActivity.java

public class SettingsActivity extends AppCompatActivity {

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.settings_activity);
        getSupportFragmentManager()
                .beginTransaction()
                .replace(R.id.settings, new SettingsFragment())
                .commit();
        ActionBar actionBar = getSupportActionBar();
        if (actionBar != null) {
            actionBar.setDisplayHomeAsUpEnabled(true);
        }
    }

    public static class SettingsFragment extends PreferenceFragmentCompat {
        @Override
        public void onCreatePreferences(Bundle savedInstanceState, String rootKey) {
            setPreferencesFromResource(R.xml.root_preferences, rootKey);
        }
    }
}

Stacktrace

Caused by: Java.lang.RuntimeException: Unable to start activity ComponentInfo{com.example.android.droidcafeinput/com.example.android.droidcafeinput.MainActivity}: java.lang.ClassCastException: androidx.preference.PreferenceScreen cannot be cast to android.preference.GenericInflater$Parent
        at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2817)
        at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2892)
        at android.app.ActivityThread.-wrap11(Unknown Source:0)
        at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1593)
        at android.os.Handler.dispatchMessage(Handler.java:105)
        at android.os.Looper.loop(Looper.java:164)
        at android.app.ActivityThread.main(ActivityThread.java:6541)
        at java.lang.reflect.Method.invoke(Native Method)
        at com.android.internal.os.Zygote$MethodAndArgsCaller.run(Zygote.java:240)
        at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:767)
     Caused by: java.lang.ClassCastException: androidx.preference.PreferenceScreen cannot be cast to android.preference.GenericInflater$Parent
        at android.preference.GenericInflater.inflate(GenericInflater.java:321)
        at android.preference.GenericInflater.inflate(GenericInflater.java:264)
        at android.preference.PreferenceManager.inflateFromResource(PreferenceManager.java:324)
        at android.preference.PreferenceManager.setDefaultValues(PreferenceManager.java:650)
        at android.preference.PreferenceManager.setDefaultValues(PreferenceManager.java:609)
        at com.example.android.droidcafeinput.MainActivity.onCreate(MainActivity.java:74)
        at android.app.Activity.performCreate(Activity.java:6975)
        at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1213)
        at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2770)
        at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2892) 
        at android.app.ActivityThread.-wrap11(Unknown Source:0) 
        at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1593) 
        at android.os.Handler.dispatchMessage(Handler.java:105) 
        at android.os.Looper.loop(Looper.java:164) 
        at android.app.ActivityThread.main(ActivityThread.java:6541) 
        at java.lang.reflect.Method.invoke(Native Method) 
        at com.android.internal.os.Zygote$MethodAndArgsCaller.run(Zygote.java:240) 
        at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:767)

Answer 1

Looks like you are trying to use android.preference.PreferenceManager with the AndroidX library. These libraries are incompatible: since you are using the AndroidX library elsewhere, make sure you don't have any android.preference imports. Also, in the AndroidX library you don't need to call setDefaultValues since the default values are read and used automatically - it should work if you just remove that line.