java.lang.ClassCastException：java.lang.String无法转换为java.util.Set

Question

我是android的新手，正在尝试建立标志测验。但是当我运行程序时，出现此错误：

    java.lang.ClassCastException: java.lang.String cannot be cast to 
    java.util.Set

因为这是我的MainActivity中的这一行：

    Set<String> regions=sharedPreferences.getStringSet(REGIONS,null);

这是我的MainActivity：

    import android.content.Intent;
    import android.content.SharedPreferences;
    import android.content.pm.ActivityInfo;
    import android.content.res.Configuration;
    import android.os.Bundle;
    import android.preference.PreferenceFragment;
    import android.preference.PreferenceManager;
    import android.support.design.widget.FloatingActionButton;
    import android.support.design.widget.Snackbar;
    import android.support.v7.app.AppCompatActivity;
    import android.support.v7.widget.Toolbar;
    import android.view.View;
    import android.view.Menu;
    import android.view.MenuItem;
    import android.widget.Toast;
    import java.util.Set;

    public class MainActivity extends AppCompatActivity {

public static final String CHOICES="pref_numberOfChoices";
public static final String REGIONS="pref_regionsToInclude";

private boolean phoneDevice=true;
private boolean preferencesChanged=true;

@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_main);
    Toolbar toolbar = (Toolbar) findViewById(R.id.toolbar);
    setSupportActionBar(toolbar);

    int screenSize=getResources().getConfiguration().screenLayout &
            Configuration.SCREENLAYOUT_SIZE_MASK;
    if(screenSize==Configuration.SCREENLAYOUT_SIZE_LARGE ||
            screenSize==Configuration.SCREENLAYOUT_SIZE_XLARGE)
        phoneDevice=false;

    if(phoneDevice)
        setRequestedOrientation(ActivityInfo.SCREEN_ORIENTATION_PORTRAIT);

    PreferenceManager.setDefaultValues(this,R.xml.preferences,false);
    PreferenceManager.getDefaultSharedPreferences(this).
            registerOnSharedPreferenceChangeListener(preferencesChangeListener);
    SharedPreferences sharedPreferences=(SharedPreferences) PreferenceManager.getDefaultSharedPreferences(this);
    SharedPreferences.Editor editor=sharedPreferences.edit();
    Set<String> regions=sharedPreferences.getStringSet(REGIONS,null);
    if (regions.size() == 0) {
        regions.add(getString(R.string.default_region));
        editor.putStringSet(REGIONS, regions);
        editor.apply();
    }
}

@Override
protected void onStart()
{
    super.onStart();
    if(preferencesChanged)
    {
        MainActivityFragment quizFragment=(MainActivityFragment)
                getSupportFragmentManager().findFragmentById(R.id.quizFragment);
        quizFragment.updateGuessRows(PreferenceManager.getDefaultSharedPreferences(this));
        quizFragment.updateRegions(PreferenceManager.getDefaultSharedPreferences(this));
        quizFragment.resetQuiz();
        preferencesChanged=false;
    }
}

private SharedPreferences.OnSharedPreferenceChangeListener preferencesChangeListener=
        new SharedPreferences.OnSharedPreferenceChangeListener() {
            @Override
            public void onSharedPreferenceChanged(SharedPreferences sharedPreferences, String s) {
                preferencesChanged = true;
                MainActivityFragment quizFragment = (MainActivityFragment) getSupportFragmentManager().
                        findFragmentById(R.id.quizFragment);
                if (s.equals(REGIONS)) {
                    Set<String> regions = sharedPreferences.getStringSet(REGIONS, null);
                    if (regions.size() == 0) {
                        SharedPreferences.Editor editor = sharedPreferences.edit();
                        regions.add(getString(R.string.default_region));
                        editor.putStringSet(REGIONS, regions);
                        editor.apply();
                        Toast.makeText(MainActivity.this, R.string.default_region_message, Toast.LENGTH_SHORT).show();
                    }
                }
                Toast.makeText(MainActivity.this, R.string.restarting_quiz, Toast.LENGTH_SHORT).show();
            }
        };

@Override
public boolean onCreateOptionsMenu(Menu menu) {
    // Inflate the menu; this adds items to the action bar if it is present.
    int orientation=getResources().getConfiguration().orientation;
    if(orientation== Configuration.ORIENTATION_PORTRAIT) {
        getMenuInflater().inflate(R.menu.menu_main, menu);
        return true;
    }
    else
        return false;
}


@Override
public boolean onOptionsItemSelected(MenuItem item) {
    Intent preferencesIntent=new Intent(this,SettingsActivity.class);
    startActivity(preferencesIntent);
    return super.onOptionsItemSelected(item);
}}

这是preferences.xml：

    <PreferenceScreen
xmlns:android="http://schemas.android.com/apk/res/android">

<ListPreference
    android:entries="@array/guesses_list"
    android:entryValues="@array/guesses_list"
    android:key="pref_numberOfChoices"
    android:title="@string/number_of_choices"
    android:summary="@string/number_of_choices_description"
    android:persistent="true"
    android:defaultValue="4" />

<MultiSelectListPreference
    android:entries="@array/regions_list_for_settings"
    android:entryValues="@array/regions_list"
    android:key="pref_regionsToInclude"
    android:title="@string/world_regions"
    android:summary="@string/world_regions_description"
    android:persistent="true"
    android:defaultValue="@array/regions_list"/>

 </PreferenceScreen>

这是arrays.xml：

   <resources>

<string-array name="regions_list">
    <item>Africa</item>
    <item>Asia</item>
    <item>Europe</item>
    <item>North_America</item>
    <item>Oceania</item>
    <item>South_America</item>
</string-array>

<string-array name="regions_list_for_settings">
    <item>Africa</item>
    <item>Asia</item>
    <item>Europe</item>
    <item>North America</item>
    <item>Oceania</item>
    <item>South America</item>
</string-array>

<string-array name="guesses_list">
    <item>2</item>
    <item>4</item>
    <item>6</item>
    <item>8</item>
</string-array>

有什么建议吗？