public class MainActivity extends Activity {

   @Override
   protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);   //调用父类的onCreate()方法
        setContentView(R.layout.activity_main);    //调用布局文件
       }
}

<?xml version="1.0" encoding="utf-8"?>
<manifest xmlns:android="http://schemas.android.com/apk/res/android"
    package="com.example.fist"                             //应用程序所在包名
    android:versionCode="1"                                //程序版本
    android:versionName="1.0" >                            //显示给用户的信息

    <uses-sdk                                              //Android程序使用的最低级别
        android:minSdkVersion="17"
        android:targetSdkVersion="17" />

    <application                                          //应用程序的配置
        android:allowBackup="true"
        android:icon="@drawable/ic_launcher"              //配置整个应用程序的图标
        android:label="@string/app_name"                  //配置标签显示信息，从strings.xml中读取
        android:theme="@style/AppTheme" >
        <activity                                         //配置要使用的Activity程序
            android:name=".MainActivity"                  //Activity程序类名
            android:label="@string/app_name" >            //从资源程序读取出类的名称
            <intent-filter>                               //应用程序已运行就执行此Activity
                <action android:name="android.intent.action.MAIN" />
                <category android:name="android.intent.category.LAUNCHER" />
            </intent-filter>
        </activity>
    </application>
</manifest>

<RelativeLayout 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:paddingBottom="@dimen/activity_vertical_margin"
    android:paddingLeft="@dimen/activity_horizontal_margin"
    android:paddingRight="@dimen/activity_horizontal_margin"
    android:paddingTop="@dimen/activity_vertical_margin"
    tools:context="com.example.fist.MainActivity" >

    <TextView                                                      //定义文本显示主件
        android:layout_width="wrap_content"                       //组件宽度为文字高度
        android:layout_height="wrap_content"
        android:text="@string/hello_world" />                     //组件ID，程序中使用
</RelativeLayout>

<?xml version="1.0" encoding="utf-8"?>
<resources>
    <string name="app_name">fist</string>        //id名为app_name
    <string name="hello_world">Hello world!</string>
    <string name="action_settings">Settings</string>
</resources>

package com.example.fist;

public final class R {
    public static final class attr {
    }
    
    public static final class drawable {
        public static final int ic_launcher=0x7f020000;
    }
    public static final class id {
        public static final int action_settings=0x7f080002;
    }
    public static final class layout {
        public static final int activity_main=0x7f030000;
    }
    public static final class menu {
        public static final int main=0x7f070000;
    }
    public static final class string {
        public static final int action_settings=0x7f050002;
        public static final int app_name=0x7f050000;
        public static final int hello_world=0x7f050001;
    }
}