在控件TextView中添加自己的文字时遇到的问题

这个问题就是，我已经给myTextView建立了一个ID，但是在使用findViewById查找时遇到问题，将函数findViewById的返回值强行转化为TextView类，代码为TextView myTextView=(TextView)findViewById(R.id.myTextView);
这行代码的两个TextView都显示TextView不能被解析成一个变量，我的SRC源文件如下
package hi.hello;

import android.app.Activity;
import android.os.Bundle;

public class HiActivity extends Activity {
   /** Called when the activity is first created. */
   @Override
   public void onCreate(Bundle savedInstanceState) {
   super.onCreate(savedInstanceState);
   setContentView(R.layout.main);
   TextView myTextView=(TextView)findViewById(R.id.myTextView);//从这里出现问题
   myTextView.setText("我的第一个TextView");
   }
}
Main.xml如下
<?xml version="1.0" encoding="utf-8"?>
<LinearLayout xmlns:android="http://schemas.
   android:layout_width="fill_parent"
   android:layout_height="fill_parent"
   android:orientation="vertical" >

   <TextView
   android:layout_width="fill_parent"
   android:layout_height="wrap_content"
   android:id="@+id/myTextView"/>

</LinearLayout>
R.JAVA如下
/* AUTO-GENERATED FILE. DO NOT MODIFY.
  *
  * This class was automatically generated by the
  * aapt tool from the resource data it found. It
  * should not be modified by hand.
  */

package hi.hello;

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 myTextView=0x7f050000;
   }
   public static final class layout {
   public static final int main=0x7f030000;
   }
   public static final class string {
   public static final int app_name=0x7f040001;
   public static final int hello=0x7f040000;
   }
}
不知问题出在哪里，我初学