这一步是添加FragmentTabHost的，大家可以在网上搜一下FragmentTabHost的用法，在这里添加FragmentTabHost，Tab是在MainTab.java枚举出来的，
 app /  ui /  MainTab.java，
  app /  widget /  MyFragmentTabHost.java
  MainActivity应该是这样：

第三讲
import android.annotation.SuppressLint;

import android.graphics.drawable.Drawable;
import android.os.Bundle;
import android.support.v4.app.Fragment;
import android.support.v4.widget.DrawerLayout;
import android.support.v7.app.ActionBar;
import android.support.v7.app.ActionBarActivity;
import android.view.LayoutInflater;
import android.view.Menu;
import android.view.MenuItem;
import android.view.MotionEvent;
import android.view.View;
import android.view.View.OnTouchListener;
import android.widget.TabHost;
import android.widget.TabHost.OnTabChangeListener;
import android.widget.TextView;
import com.xiaozhong.myapplication.smalfragment.NavigationDrawerFragment;


public class MainActivity extends ActionBarActivity implements
        NavigationDrawerFragment.NavigationDrawerCallbacks,
        OnTabChangeListener,  View.OnClickListener,
        OnTouchListener {
    //定义FragmentTabHost对象
    private MyFragmentTabHost mTabHost;
    /**
     * Fragment managing the behaviors, interactions and presentation of the
     * navigation drawer.
     */
    public NavigationDrawerFragment mNavigationDrawerFragment;

    private CharSequence mTitle;

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

        setContentView(R.layout.activity_main);

        initView();
        AppManager.getAppManager().addActivity(this);

    }
    /**
     * 初始化组件
     */
    public void initView() {
        mNavigationDrawerFragment = (NavigationDrawerFragment) getSupportFragmentManager()
                .findFragmentById(R.id.navigation_drawer);
        mTitle = getTitle();
        // Set up the drawer.
        mNavigationDrawerFragment.setUp(R.id.navigation_drawer,
                (DrawerLayout) findViewById(R.id.drawer_layout));
        //实例化TabHost对象，得到TabHost
        mTabHost = (MyFragmentTabHost)findViewById(R.id.tabhost);
        mTabHost.setup(this, getSupportFragmentManager(), R.id.realtabcontent);


        initTabs();
        mTabHost.setCurrentTab(0);
        mTabHost.setOnTabChangedListener(this);


    }
    /**
     * 初始化TabHost
     */
    private void initTabs() {
        //定义数组
        MainTab[] tabs = MainTab.values();
        final int size = tabs.length;
        for (int i = 0; i < size; i++) {
            MainTab mainTab = tabs[i];
            TabHost.TabSpec tab = mTabHost.newTabSpec(getString(mainTab.getResName()));
            // 实例化布局对象 为每一个Tab按钮设置图标、文字和内容
            View indicator = LayoutInflater.from(getApplicationContext())
                    .inflate(R.layout.tab_indicator, null);
            TextView title = (TextView) indicator.findViewById(R.id.tab_title);
            Drawable drawable = this.getResources().getDrawable(
                    mainTab.getResIcon());
            title.setCompoundDrawablesWithIntrinsicBounds(null, drawable, null,
                    null);
            if (i == 2) {
                indicator.setVisibility(View.INVISIBLE);
                mTabHost.setNoTabChangedTag(getString(mainTab.getResName()));
            }
            title.setText(getString(mainTab.getResName()));
            tab.setIndicator(indicator);
            tab.setContent(new TabHost.TabContentFactory() {

                @Override
                public View createTabContent(String tag) {
                    return new View(MainActivity.this);
                }
            });
            //将Tab按钮添加进Tab选项卡中
            mTabHost.addTab(tab, mainTab.getClz(), null);
            //设置Tab按钮的背景
            mTabHost.getTabWidget().getChildAt(i).setOnTouchListener(this);
        }
    }


    @Override
    protected void onDestroy() {
        super.onDestroy();
    }

    public void initData() {

    }

    @Override
    public void onNavigationDrawerItemSelected(int position) {
        // update the main content by replacing fragments
    }

    public void restoreActionBar() {
        ActionBar actionBar = getSupportActionBar();
        actionBar.setNavigationMode(ActionBar.NAVIGATION_MODE_STANDARD);
        actionBar.setDisplayShowTitleEnabled(true);
        actionBar.setTitle(mTitle);
    }


    @Override
    public boolean onCreateOptionsMenu(Menu menu) {
        getMenuInflater().inflate(R.menu.main_activity_menu, menu);
        if (!mNavigationDrawerFragment.isDrawerOpen()) {
            restoreActionBar();
            return true;
        }
        return super.onCreateOptionsMenu(menu);
    }
    @Override
    public boolean onOptionsItemSelected(MenuItem item) {
        return super.onOptionsItemSelected(item);
    }

    @Override
    public void onTabChanged(String tabId) {
        final int size = mTabHost.getTabWidget().getTabCount();
        for (int i = 0; i < size; i++) {
            View v = mTabHost.getTabWidget().getChildAt(i);
            if (i == mTabHost.getCurrentTab()) {
                v.setSelected(true);
            } else {
                v.setSelected(false);
            }
        }
        supportInvalidateOptionsMenu();
    }

    @Override
    public void onClick(View v) {

    }
    @SuppressLint("ClickableViewAccessibility")
    @Override
    public boolean onTouch(View v, MotionEvent event) {
        super.onTouchEvent(event);
        boolean consumed = false;
        // use getTabHost().getCurrentTabView to decide if the current tab is
        // touched again
        if (event.getAction() == MotionEvent.ACTION_DOWN
                && v.equals(mTabHost.getCurrentTabView())) {
            // use getTabHost().getCurrentView() to get a handle to the view
            // which is displayed in the tab - and to get this views context
            Fragment currentFragment = getCurrentFragment();
            if (currentFragment != null
                    && currentFragment instanceof OnTabReselectListener) {
                OnTabReselectListener listener = (OnTabReselectListener) currentFragment;
                listener.onTabReselect();
                consumed = true;
            }
        }
        return consumed;
    }
    private Fragment getCurrentFragment() {
        return getSupportFragmentManager().findFragmentByTag(
                mTabHost.getCurrentTabTag());
    }
}

cannot be cast to android.support.v4.app.Fragment: 你要么统一Fragament ，将所有Fragment加入一个list然后去get取出来，
要么Activity必须继承FragmentActivity。所以当出现这个错误的时候要查看import android.support.v4.app.Fragment;是否相同