谁能帮我看看这个actionbar tab为什么不能切换页面呐

Question

.java

package com.m.myandroid;

import android.support.v4.app.FragmentActivity;
import android.support.v4.app.FragmentTransaction;
import android.support.v7.app.ActionBar;
import android.support.v7.app.ActionBar.Tab;
import android.support.v7.app.ActionBarActivity;
import android.support.v4.app.Fragment;
import android.content.Intent;
import android.os.Bundle;
import android.view.Menu;
import android.view.MenuItem;
import android.view.View;
import android.widget.Button;

public class SecondActivity extends ActionBarActivity {

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

    Button bt1 = (Button) findViewById(R.id.bottom1);
    bt1.setOnClickListener(new View.OnClickListener() {

        public void onClick(View v) {
            // TODO 自动生成的方法存根
            Intent intent = new Intent(SecondActivity.this,
                    FirstActivity.class);
            startActivity(intent);
        }
    });

    Button bt2 = (Button) findViewById(R.id.bottom2);
    bt2.setBackgroundResource(R.drawable.copy_img2);

    Button bt3 = (Button) findViewById(R.id.bottom3);
    bt3.setOnClickListener(new View.OnClickListener() {

        public void onClick(View v) {
            // TODO 自动生成的方法存根
            Intent intent = new Intent(SecondActivity.this,
                    ThirdActivity.class);
            startActivity(intent);
        }
    });

    ActionBar bar = getSupportActionBar();
    bar.setNavigationMode(ActionBar.NAVIGATION_MODE_TABS);

    ActionBar.Tab tab1 = bar.newTab().setText("智能排序");
    ActionBar.Tab tab2 = bar.newTab().setText("离我最近");

    Fragment fragment1 = new Second_Tab1();
    Fragment fragment2 = new Second_Tab2();

    tab1.setTabListener(new MyTabsListener(fragment1, this));
    tab2.setTabListener(new MyTabsListener(fragment2, this));
    bar.addTab(tab1);
    bar.addTab(tab2);
}

protected class MyTabsListener implements ActionBar.TabListener {

    private Fragment fragment;
    private FragmentActivity mActivity;
    FragmentActivity fmt;

    public MyTabsListener(Fragment fragment, FragmentActivity activity) {
        this.fragment = fragment;
        this.mActivity = activity;

    }

    @Override
    public void onTabReselected(Tab arg0, FragmentTransaction arg1) {
        // TODO 自动生成的方法存根

    }

    @Override
    public void onTabSelected(Tab tab, FragmentTransaction ft) {
        // TODO 自动生成的方法存根
    FragmentTransaction fmt = mActivity.getSupportFragmentManager()                 .beginTransaction();
    fmt.add(R.id.fragment_place2, fragment);
    fmt.commit();
    }

    @Override
    public void onTabUnselected(Tab arg0, FragmentTransaction arg1) {
        // TODO 自动生成的方法存根

    FragmentTransaction fmt = mActivity.getSupportFragmentManager()
                .beginTransaction();
    fmt.remove(fragment);
        fmt.commit();
    }
}

@Override
public boolean onCreateOptionsMenu(Menu menu) {
    // Inflate the menu; this adds items to the action bar if it is present.
    getMenuInflater().inflate(R.menu.first, menu);
    return true;
}

// 点击search按钮对应操作
@Override
public boolean onOptionsItemSelected(MenuItem item) {

    // switch (item.getItemId()){
    // case R.id.first_search:
    // }
    return super.onOptionsItemSelected(item);
}

}

.xml

xmlns:tools="http://schemas.android.com/tools"
android:layout_width="fill_parent"
android:layout_height="fill_parent"
android:orientation="vertical">

<LinearLayout
    android:id="@+id/fragment_place2"
    android:layout_width="wrap_content"
    android:layout_height="wrap_content" 
    android:orientation="vertical">