RadioGroup组与onCheckedChanged事件

//MainActivity.java

package com.example.test;

import android.os.Bundle;
import android.view.KeyEvent;
import android.view.View;
import android.app.Activity;
import android.view.Menu;
import android.widget.EditText;
import android.widget.RadioButton;
import android.widget.RadioGroup;
import android.widget.TextView;

public class MainActivity extends Activity {

    private TextView tv;
    private RadioGroup rg;
    private RadioButton rb1;
    private RadioButton rb2;
    @Override
    protected void onCreate(Bundle savedInstanceState)
    {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        tv=(TextView)findViewById(R.id.tv1);
        rg=(RadioGroup)findViewById(R.id.rg);
        rb1=(RadioButton)findViewById(R.id.rb1);
        rb2=(RadioButton)findViewById(R.id.rb2);
        rg.setOnCheckedChangeListener(mChangeRadio);
    }
    private RadioGroup.OnCheckedChangeListener mChangeRadio = new
            RadioGroup.OnCheckedChangeListener()
    {    
        @Override
        public void onCheckedChanged(RadioGroup group, int checkedId)
        {
            // TODO Auto-generated method stub
            if(checkedId==rb1.getId())
                tv.setText(rb1.getText());
            else if(checkedId==rb2.getId())
                tv.setText(rb2.getText());
        }
    };
}

//activity_main.xml

<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"
    tools:context=".MainActivity" >

     <TextView
         android:id="@+id/tv1"
         android:layout_width="wrap_content"
         android:layout_height="wrap_content"
         android:layout_alignParentLeft="true"
         android:layout_alignParentTop="true"
         android:layout_marginLeft="24dp"
         android:layout_marginTop="32dp"
         android:text="who are you?" />

     <RadioGroup
         android:id="@+id/rg"
         android:layout_width="wrap_content"
         android:layout_height="wrap_content"
         android:layout_alignLeft="@+id/tv1"
         android:layout_below="@+id/tv1"
         android:layout_marginTop="20dp" >

         <RadioButton
             android:id="@+id/rb1"
             android:layout_width="wrap_content"
             android:layout_height="wrap_content"
             android:layout_centerHorizontal="true"
             android:layout_centerVertical="true"
             android:text="boy" />

         <RadioButton
             android:id="@+id/rb2"
             android:layout_width="wrap_content"
             android:layout_height="wrap_content"
             android:layout_alignRight="@+id/rb1"
             android:layout_below="@+id/rb1"
             android:text="gril" />
     </RadioGroup>

</RelativeLayout>