Fragment其实就是一个小的Activity，他的生命周期和Activity差不多。Fragment概念的提出就是因为平板编程的需要，Fragment使得UI设计得以模块化，平板等大屏设备的UI设计得到了很好的技术支持。然而Fragment概念的提出是因为大屏UI设计的需求，但是使用并不局限于平板设计。下面这坨代码（的确是坨。。）实现的是一个类似微信的选项卡切换界面的功能，全局只有一个Activity，真正改变的只是Fragment。

Activity:

package com.example.myfragment;

import android.os.Bundle;
import android.app.Activity;
import android.app.FragmentManager;
import android.app.FragmentTransaction;
import android.view.View;
import android.view.View.OnClickListener;
import android.widget.TextView;

public class MainActivity extends Activity implements OnClickListener {
	private TextView tv1;
	private TextView tv2;
	private TextView tv3;
	private TextView tv4;

	private FragmentManager fm;
	private Fragment1 fragment1;
	private Fragment2 fragment2;
	private Fragment3 fragment3;
	private Fragment4 fragment4;

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

		tv1 = (TextView) findViewById(R.id.text1);
		tv2 = (TextView) findViewById(R.id.text2);
		tv3 = (TextView) findViewById(R.id.text3);
		tv4 = (TextView) findViewById(R.id.text4);

		tv1.setOnClickListener(this);
		tv2.setOnClickListener(this);
		tv3.setOnClickListener(this);
		tv4.setOnClickListener(this);

		fm = getFragmentManager();
		FragmentTransaction ft = fm.beginTransaction();

		fragment1 = new Fragment1();
		fragment2 = new Fragment2();
		fragment3 = new Fragment3();
		fragment4 = new Fragment4();

		ft.replace(R.id.content, fragment1);
		ft.commit();
	}

	@Override
	public void onClick(View v) {
		// TODO Auto-generated method stub
		FragmentTransaction ft = fm.beginTransaction();
		switch (v.getId()) {
		case R.id.text1:
			ft.replace(R.id.content, fragment1);
			break;
		case R.id.text2:
			ft.replace(R.id.content, fragment2);
			break;
		case R.id.text3:
			ft.replace(R.id.content, fragment3);
			break;
		case R.id.text4:
			ft.replace(R.id.content, fragment4);
			break;
		}
		ft.commit();
	}
}

activity xml:

<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:tools="http://schemas.android.com/tools"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    android:orientation="vertical" >

    <LinearLayout
        android:layout_width="fill_parent"
        android:layout_height="wrap_content"
        android:orientation="horizontal" >

        <TextView
            android:id="@+id/text1"
            android:layout_width="0dp"
            android:layout_height="wrap_content"
            android:layout_weight="1"
            android:gravity="center"
            android:text="Text1" />

        <TextView
            android:id="@+id/text2"
            android:layout_width="0dp"
            android:layout_height="wrap_content"
            android:layout_weight="1"
            android:gravity="center"
            android:text="Text2" />

        <TextView
            android:id="@+id/text3"
            android:layout_width="0dp"
            android:layout_height="wrap_content"
            android:layout_weight="1"
            android:gravity="center"
            android:text="Text3" />

        <TextView
            android:id="@+id/text4"
            android:layout_width="0dp"
            android:layout_height="wrap_content"
            android:layout_weight="1"
            android:gravity="center"
            android:text="Text4" />
    </LinearLayout>

    <RelativeLayout
        android:id="@+id/content"
        android:layout_width="fill_parent"
        android:layout_height="fill_parent" >
    </RelativeLayout>

</LinearLayout>

fragment1:

package com.example.myfragment;

import android.app.Fragment;
import android.os.Bundle;
import android.view.LayoutInflater;
import android.view.View;
import android.view.ViewGroup;

public class Fragment1 extends Fragment {
	@Override
	public View onCreateView(LayoutInflater inflater, ViewGroup container,
			Bundle savedInstanceState) {
		View view = inflater.inflate(R.layout.fragment1, null);
		return view;
	}
}

f1 xml:

<?xml version="1.0" encoding="utf-8"?>
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    android:orientation="vertical" >

    <TextView
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:text="Fragment1"
        android:textSize="20dp" >
    </TextView>

</LinearLayout>

fragment2到fragment4的定义方式和fragment1类似，这里就不贴出来了。