Android在活动中加载碎片的问题

在一个活动中设计一个button 点击以后加载一个PreferenceFragment
调试的时候发现加载以后活动页面和碎片页面重合了 不知道是怎么回事 有没有高手帮忙看一下？

代码如下：
============================================================

package com.example.UsingPreference;

import android.os.Bundle;
import android.app.Activity;
import android.view.Menu;
import android.preference.*;

public class AppPreferenceActivity extends PreferenceFragment {

	@Override
	public void onCreate(Bundle savedInstanceState) {
		super.onCreate(savedInstanceState);
		addPreferencesFromResource(R.xml.myapppreferences);
	}
}


===========================================================
package com.example.UsingPreference;

import android.os.Bundle;
import android.content.*;
import android.app.Activity;
import android.app.FragmentManager;
import android.app.FragmentTransaction;
import android.view.*;

public class UsingPreferenceActivity extends Activity {

	FragmentManager fM = getFragmentManager();
	FragmentTransaction fT = fM.beginTransaction();
	
	@Override
	protected void onCreate(Bundle savedInstanceState) {
		super.onCreate(savedInstanceState);
		setContentView(R.layout.main);
	}

	@Override
	public boolean onCreateOptionsMenu(Menu menu) {
		// Inflate the menu; this adds items to the action bar if it is present.
		getMenuInflater().inflate(R.menu.main, menu);
		return true;
	}
	
	public void onClickLoad(View view)
	{
		AppPreferenceActivity f1 = new AppPreferenceActivity();
		fT.replace(android.R.id.content, f1);
		fT.addToBackStack(null);
		fT.commit();
	}

}

====================================
<?xml version="1.0" encoding="utf-8"?>
<PreferenceScreen xmlns:android="http://schemas.android.com/apk/res/android">
    <PreferenceCategory android:title="Category 1">
        <CheckBoxPreference
            android:title="Checkbox"
            android:defaultValue="false"
            android:summary="True or False"
            android:key="checkboxPref" />
        </PreferenceCategory>>
    
    <PreferenceCategory android:title="Category 2">
        <EditTextPreference
            android:summary="Enter a string"
            android:defaultValue="[Enter a string here]"
            android:title="Edit Text"
            android:key="editTextPref" />
        <RingtonePreference
            android:summary="Select a ringtone"
            android:title="Ringtone"
            android:key="ringtonePref" />
        <PreferenceScreen
            android:title="Second Preference Screen"
            android:summary="Click here to go to the second Preference Screen"
            android:key="secondPrefScreenPref" >
            <EditTextPreference
              android:summary="Enter a string"
              android:title="Edit Text[second Screen]"
              android:key="secondEditTextPref" />
        </PreferenceScreen>
    </PreferenceCategory>>
</PreferenceScreen>