Невозможно создать экземпляр активности. Информация о компоненте… не может быть преобразована в android.app.Activity

Я не могу понять, почему я org.example.sudoku.Prefs cannot be cast to android.app.Activity этот org.example.sudoku.Prefs cannot be cast to android.app.Activity error. Я использую эту книгу Hello, Android от Ed Burnette, и я пытаюсь создать простое меню настроек. Я искал всюду для ответа, и либо я недостаточно понимаю, чтобы понять это, либо я идиот. У меня есть все, что было в моих настройках XML, и манифест, но он каждый раз нажимает на мои настройки в меню параметров. Вот мой код и сообщения об ошибках:

Prefs.java:

   package org.example.sudoku;

    import android.os.Bundle;
    import android.preference.PreferenceFragment;
    //import android.preference.PreferenceActivity;


    public class Prefs extends PreferenceFragment {
        public void onCreate(Bundle saveInstanceState) {
            super.onCreate(saveInstanceState);
            addPreferencesFromResource(R.xml.settings); 
        }

    }

Sudoku.java:

       package org.example.sudoku;
        import android.app.Activity;
        import android.os.Bundle;

        import android.content.Intent;
        import android.view.View;
        import android.view.View.OnClickListener;

        import android.view.Menu;
        import android.view.MenuInflater;
        import android.view.MenuItem;


        public class Sudoku extends Activity implements OnClickListener{

            @Override
            public void onCreate(Bundle savedInstanceState) {
                super.onCreate(savedInstanceState);
                setContentView(R.layout.main);

                // Set up click listeners for all the buttons

                View continueButton = findViewById(R.id.continue_button);
                    continueButton.setOnClickListener(this);
                View newButton = findViewById(R.id.new_button);
                    newButton.setOnClickListener(this);
                View aboutButton = findViewById(R.id.about_button);
                    aboutButton.setOnClickListener(this);
                View exitButton = findViewById(R.id.exit_button);
                    exitButton.setOnClickListener(this);
            }

            public void onClick(View v) {
                switch (v.getId())
                {
                case R.id.about_button:
                    Intent i = new Intent(this, About.class);
                    startActivity(i);
                    break;
                }
        }

            @Override
        public boolean onCreateOptionsMenu(Menu menu) {
            super.onCreateOptionsMenu(menu);
            MenuInflater inflater = getMenuInflater();
            inflater.inflate(R.menu.menu, menu);
            return true;    

        }


        @Override
        public boolean onOptionsItemSelected(MenuItem item) {
            switch(item.getItemId()) {
            case R.id.settings:
                //startActivity(new Intent(this, Prefs.class));
                Intent intent = new Intent(this, Prefs.class);
                startActivity(intent);
                return true;
            }
            return false;


        }
        }

AndroidManifest.xml:

    <?xml version="1.0" encoding="UTF-8"?>
        <menu xmlns:android="http://schemas.android.com/apk/res/android">
            <item android:id="@+id/settings"
                android:title="@string/settings_label"
                android:alphabeticShortcut="@string/settings_shortcut" />



        </menu>

        <?xml version="1.0" encoding="utf-8"?>
    <manifest xmlns:android="http://schemas.android.com/apk/res/android"
        package="org.example.sudoku"
        android:versionCode="1"
        android:versionName="1.0" >

        <uses-sdk android:minSdkVersion="15" />

        <application
            android:icon="@drawable/ic_launcher"
            android:label="@string/app_name" >
            <activity
                android:name=".Sudoku"
                android:label="@string/app_name" >
                <intent-filter>
                    <action android:name="android.intent.action.MAIN" />

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

            <activity android:name=".About"
                    android:label="@string/about_title"
                    android:theme="@android:style/Theme.Dialog">

            </activity>
            <activity android:name=".Prefs"
                    android:label="@string/settings_title">

            </activity>
        </application>

    </manifest>

strings.xml:

    <?xml version="1.0" encoding="utf-8"?>
    <resources>

        <string name="hello">Hello World, Sudoku!</string>
        <string name="app_name">Sudoku Game</string>
        <string name="continue_label">Continue</string>
        <string name="new_game_label">New Game</string>
        <string name="about_label">About</string>
        <string name="exit_label">Exit</string>
        <string name="about_text">Sudoku is a logic-based number placement puzzle. Starting with a partially completed 9x9 grid so that each row, each column, and each of the 3x3 boxes(also called <i>block</i>) contains the digits 1 to 9 exactly once</string>
        <string name="about_title">About Android Sudoku</string>

        <string name="settings_label">Settings
            </string>
        <string name="settings_title">Sudoku Settings</string>
        <string name="settings_shortcut">s</string>
        <string name="music_title">Music</string>
        <string name="music_summary">Play Background Music</string>
        <string name="hints_title">Hints</string>
        <string name="hints_summary">Show hints during play</string>


        </resources>

settings.xml:

    <code>
    <?xml version="1.0" encoding="utf-8"?>
        <PreferenceScreen
        xmlns:android="http://schemas.android.com/apk/res/android">
        <PreferenceCategory
                android:title="@string/settings_title"> 
        <CheckboxPreference
            android:key="music"
            android:title="@string/music_title"
            android:summary="@string/music_summary"
            android:defaultValue="true" />
        <CheckBoxPreference
            android:key="hints"
            android:title="@string/hints_title"
            android:summary="@string/hints_summary"
            android:defaultValue="true" />
        </PreferenceCategory>
        </PreferenceScreen>

main.xml:

  <TextView android:layout_width="wrap_content" android:layout_height="wrap_content" android:text="@string/app_name" android:gravity="center" android:layout_marginBottom="20dip" android:textSize="24.5sp"/> <TableLayout android:layout_height="wrap_content" android:layout_width="wrap_content" android:layout_gravity="center" android:stretchColumns="*"> <TableRow> <Button android:id="@+id/continue_button" android:text="@string/continue_label"/> <Button android:id="@+id/new_button" android:text="@string/new_game_label"/> </TableRow> <TableRow > <Button android:id="@+id/about_button" android:text="@string/about_label"/> <Button android:id="@+id/exit_button" android:text="@string/exit_label"/> </TableRow> </TableLayout> </LinearLayout>

Вот все сообщения об ошибках, которые я получаю:

    05-18 12:37:16.781: E/AndroidRuntime(623): FATAL EXCEPTION: main
    05-18 12:37:16.781: E/AndroidRuntime(623): java.lang.RuntimeException: Unable to instantiate activity ComponentInfo{org.example.sudoku/org.example.sudoku.Prefs}: java.lang.ClassCastException: org.example.sudoku.Prefs cannot be cast to android.app.Activity
    05-18 12:37:16.781: E/AndroidRuntime(623):  at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:1880)
    05-18 12:37:16.781: E/AndroidRuntime(623):  at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:1981)
    05-18 12:37:16.781: E/AndroidRuntime(623):  at android.app.ActivityThread.access$600(ActivityThread.java:123)
    05-18 12:37:16.781: E/AndroidRuntime(623):  at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1147)
    05-18 12:37:16.781: E/AndroidRuntime(623):  at android.os.Handler.dispatchMessage(Handler.java:99)
    05-18 12:37:16.781: E/AndroidRuntime(623):  at android.os.Looper.loop(Looper.java:137)
    05-18 12:37:16.781: E/AndroidRuntime(623):  at android.app.ActivityThread.main(ActivityThread.java:4424)
    05-18 12:37:16.781: E/AndroidRuntime(623):  at java.lang.reflect.Method.invokeNative(Native Method)
    05-18 12:37:16.781: E/AndroidRuntime(623):  at java.lang.reflect.Method.invoke(Method.java:511)
    05-18 12:37:16.781: E/AndroidRuntime(623):  at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:784)
    05-18 12:37:16.781: E/AndroidRuntime(623):  at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:551)
    05-18 12:37:16.781: E/AndroidRuntime(623):  at dalvik.system.NativeStart.main(Native Method)
    05-18 12:37:16.781: E/AndroidRuntime(623): Caused by: java.lang.ClassCastException: org.example.sudoku.Prefs cannot be cast to android.app.Activity
    05-18 12:37:16.781: E/AndroidRuntime(623):  at android.app.Instrumentation.newActivity(Instrumentation.java:1023)
    05-18 12:37:16.781: E/AndroidRuntime(623):  at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:1871)
    05-18 12:37:16.781: E/AndroidRuntime(623):  ... 11 more