听着薛之谦的《初学者》刚好看到这篇老文章献给Android初学者。
Android Fragment是Android4.0以上才有的；而FragmentActivity是为了兼容4.0以下版本的Fragment使用的。
所以如果你想兼容4.0以下Android版本使用Fragment的话，框架Activity需要继承FragmentActivity，FragmentActivity这个类是在android.support.v4.app.FragmentActivity里的。
下面介绍2种用法：
1、继承Activity的。
（这个只针对4.0以上的Android平台使用Fragment）。
框架Activity：

import android.app.Activity;
import android.app.Fragment;
import android.app.FragmentManager;
import android.app.FragmentTransaction;
import android.os.Bundle;
import android.view.View;
import android.view.View.OnClickListener;
import android.widget.Button;
/**
 * 不兼容4.0以下模式Fragment
 */
public class Mt_Activity extends Activity implements OnClickListener {
    private Button btn_first, btn_second;
    private Fragment Fragment_first, Fragment_Second;
    private FragmentTransaction fragmentTransaction;
    private FragmentManager fragmentManager;
    @Override
    protected void onCreate(Bundle savedInstanceState) {
        // TODO Auto-generated method stub
        super.onCreate(savedInstanceState);
        setContentView(R.layout.fragment);
        initView();
        Fragment_Second = new Fragment_Second();
        fragmentManager = this.getFragmentManager();
        fragmentTransaction = fragmentManager.beginTransaction();
        fragmentTransaction.replace(R.id.main_fragment_layout, Fragment_Second,"second_Fragment");
        fragmentTransaction.commit();
    }
    private void initView() {
        btn_first = (Button) this.findViewById(R.id.btn_first);
        btn_second = (Button) this.findViewById(R.id.btn_second);
        btn_first.setOnClickListener(this);
        btn_second.setOnClickListener(this);
    }
    @Override
    public void onClick(View arg0) {
        switch (arg0.getId()) {
        case R.id.btn_first:
            // 加载不同的Fragment
            if (null == Fragment_first) {
                Fragment_first = new Fragment_first();
            }
            fragmentTransaction = fragmentManager.beginTransaction();
            fragmentTransaction.replace(R.id.main_fragment_layout, Fragment_first, "fist_Fragment");
            fragmentTransaction.commit();
            break;
        case R.id.btn_second:
            if (null == Fragment_Second) {
                Fragment_Second = new Fragment_Second();
            }
            fragmentTransaction = fragmentManager.beginTransaction();
            fragmentTransaction.replace(R.id.main_fragment_layout, Fragment_Second, "second_Fragment");
            fragmentTransaction.commit();
            break;
        default:
            break;
        }
    }
}

Fragment代码：

import android.app.Fragment;
import android.os.Bundle;
import android.view.LayoutInflater;
import android.view.View;
import android.view.ViewGroup;
public class Fragment_Second extends Fragment {
    private View rootView;
    @Override
    public View onCreateView(LayoutInflater inflater, ViewGroup container, Bundle savedInstanceState) {
        if(container==null)
           return null;
        rootView = inflater.inflate(R.layout.fragment_two, container,false);
        return rootView;
    }
}

2.继承FragmentActivity的（向下兼容4.0以下版本使用Fragment，导入的是android.support.v4包里的内容）
框架Activity：

import android.os.Bundle;
import android.support.v4.app.Fragment;
import android.support.v4.app.FragmentActivity;
import android.support.v4.app.FragmentManager;
import android.support.v4.app.FragmentTransaction;
import android.view.View;
import android.view.View.OnClickListener;
import android.widget.Button;
/**
 * 兼容4.0以下模式Fragment
 * 
 */
public class Mt_Activity extends FragmentActivity implements OnClickListener {
    private Button btn_first, btn_second;
    private Fragment Fragment_first, Fragment_Second;
    private FragmentTransaction fragmentTransaction;
    private FragmentManager fragmentManager;
    @Override
    protected void onCreate(Bundle savedInstanceState) {
        // TODO Auto-generated method stub
        super.onCreate(savedInstanceState);
        setContentView(R.layout.fragment);
        initView();
        Fragment_first = new Fragment_first();
        fragmentManager = this.getSupportFragmentManager();
        fragmentTransaction = fragmentManager.beginTransaction();
        fragmentTransaction.replace(R.id.main_fragment_layout, Fragment_first,"first_Fragment");
        fragmentTransaction.commit();
    }
    private void initView() {
        btn_first = (Button) this.findViewById(R.id.btn_first);
        btn_second = (Button) this.findViewById(R.id.btn_second);
        btn_first.setOnClickListener(this);
        btn_second.setOnClickListener(this);
    }
    @Override
    public void onClick(View arg0) {
        switch (arg0.getId()) {
        case R.id.btn_first:
            // 加载不同的Fragment
            if (null == Fragment_first) {
                Fragment_first = new Fragment_first();
            }
            fragmentTransaction = fragmentManager.beginTransaction();
            fragmentTransaction.replace(R.id.main_fragment_layout, Fragment_first, "fist_Fragment");
            fragmentTransaction.commit();
            break;
        case R.id.btn_second:
            if (null == Fragment_Second) {
                Fragment_Second = new Fragment_Second();
            }
            fragmentTransaction = fragmentManager.beginTransaction();
            fragmentTransaction.replace(R.id.main_fragment_layout, Fragment_Second, "second_Fragment");
            fragmentTransaction.commit();
            break;
        default:
            break;
        }
    }
}

Fragment代码：

import android.os.Bundle;
import android.support.v4.app.Fragment;
import android.view.LayoutInflater;
import android.view.View;
import android.view.ViewGroup;
public class Fragment_first extends Fragment {
    private View rootView;
    @Override
    public View onCreateView(LayoutInflater inflater, ViewGroup container, Bundle savedInstanceState) {
        if(container==null)
           return null;
        rootView = inflater.inflate(R.layout.fragment_first, container,false);
        return rootView;
    }
}

最后再说一句布局：

<RelativeLayout xmlns:android="http://schemas.android.com/apk/res/android"
    android:layout_width="match_parent"
    android:layout_height="match_parent" >
    <LinearLayout
        android:id="@+id/top_bar_layout"
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        android:layout_alignParentTop="true"
        android:orientation="horizontal" >
        <Button
            android:id="@+id/btn_one"
            android:layout_width="match_parent"
            android:layout_height="wrap_content"
            android:layout_weight="1"
            android:background="@drawable/top_bar_bg"
            android:text="按钮一" />
        <Button
            android:id="@+id/btn_two"
            android:layout_width="match_parent"
            android:layout_height="wrap_content"
            android:layout_weight="1"
            android:background="@drawable/top_bar_bg"
            android:text="按钮二" />
    </LinearLayout>
    <LinearLayout
        android:id="@+id/fragment_replace_layout"
        android:layout_width="match_parent"
        android:layout_height="match_parent"
        android:layout_below="@id/top_bar_layout"
        android:background="#ff0000" >
    </LinearLayout>
</RelativeLayout>

布局类似这种布局即可，并不是一定非要FrameLayout

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