java – 为什么settings.xml布局与ActionBar / Toolbar重叠？

在SettingsActivity.java&中添加settings.xml之后运行应用程序,活动显示如下：

这是SettingsActivity.java文件的代码：

public class SettingsActivity extends AppCompatActivity {

    Toolbar toolbar;

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

        SpannableString s = new SpannableString("Settings");
        s.setSpan(new TypefaceSpan(this,"Roboto-Medium.ttf"),s.length(),Spannable.SPAN_EXCLUSIVE_EXCLUSIVE);

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

        getSupportActionBar().setDisplayHomeAsUpEnabled(true);
        getSupportActionBar().setTitle(s);

        // Display the fragment as the main content.
        getFragmentManager().beginTransaction()
                .replace(android.R.id.content,new SettingsFragment())
                .commit();

    }

    public static class SettingsFragment extends PreferenceFragment implements SharedPreferences.OnSharedPreferenceChangeListener {

        public static final String SHARE_APP_PREFERENCE_KEY = "pref_key_share_app";

        @Override
        public void onCreate(Bundle savedInstanceState) {
            super.onCreate(savedInstanceState);

            // Load the preferences from an XML resource
            addPreferencesFromResource(R.xml.settings);

        }

        public void onSharedPreferenceChanged(SharedPreferences sharedPreferences,String key) {
            if (key.equals(SHARE_APP_PREFERENCE_KEY)) {
                // do something
            }
        }

    }

}

这是activity_settings.xml文件的代码：

<RelativeLayout xmlns:android="http://schemas.android.com/apk/res/android"
                xmlns:tools="http://schemas.android.com/tools"
                xmlns:app="http://schemas.android.com/apk/res-auto"
                android:layout_width="match_parent"
                android:layout_height="match_parent"
                tools:context="com.abc.xxx.SettingsActivity">

    <android.support.v7.widget.Toolbar
        android:id="@+id/toolbar"
        android:layout_width="match_parent"
        android:layout_height="?attr/actionBarSize"
        android:background="?attr/colorPrimary"
        app:layout_scrollFlags="scroll|enterAlways"
        android:theme="@style/ThemeOverlay.AppCompat.Dark.ActionBar"
        app:popupTheme="@style/ThemeOverlay.AppCompat.Light"
        android:elevation="4dp"/>

</RelativeLayout>

这是settings.xml文件的代码：

<?xml version="1.0" encoding="utf-8"?>
<PreferenceScreen xmlns:android="http://schemas.android.com/apk/res/android" >

    <PreferenceCategory android:title="@string/pref_notification" >
        <CheckBoxPreference
            android:defaultValue="true"
            android:key="prefNotification"
            android:summary="@string/pref_notify_summary" >
        </CheckBoxPreference>
    </PreferenceCategory>

</PreferenceScreen>

我不知道为什么settings.xml布局与ActionBar / Toolbar重叠！

请告诉我.

提前致谢.