Android UI control series: RadioButton (radio button)

package org.loulijun.radio;

import android.app.Activity;
import android.os.Bundle;
import android.view.Gravity;
import android.widget.RadioButton;
import android.widget.RadioGroup;
import android.widget.TextView;
import android.widget.Toast;

public class RadioTest extends Activity {
    /** Called when the activity is first created. */
        TextView textview;
        RadioGroup radiogroup;
        RadioButton radio1,radio2,radio3,radio4;
    @Override
    public void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.main);
        textview=(TextView)findViewById(R.id.textview1);
        radiogroup=(RadioGroup)findViewById(R.id.radiogroup1);
        radio1=(RadioButton)findViewById(R.id.radiobutton1);
        radio2=(RadioButton)findViewById(R.id.radiobutton2);
        radio3=(RadioButton)findViewById(R.id.radiobutton3);
        radio4=(RadioButton)findViewById(R.id.radiobutton4);

        radiogroup.setOnCheckedChangeListener(new RadioGroup.OnCheckedChangeListener() {

                        @Override
                        public void onCheckedChanged(RadioGroup group, int checkedId) {
                                // TODO Auto-generated method stub
                                if(checkedId==radio2.getId())
                                {
                                        DisplayToast("正确答案："+radio2.getText()+"，恭喜你，回答正确！");
                                }else
                                {
                                        DisplayToast("请注意，回答错误！");
                                }
                        }
                });
    }
    public void DisplayToast(String str)
    {
            Toast toast=Toast.makeText(this, str, Toast.LENGTH_LONG);
            toast.setGravity(Gravity.TOP,0,220);
            toast.show();
    }
}

<?xml version="1.0" encoding="utf-8"?>  
<resources>  
    <string name="hello">哪个城市美女多？</string>  
    <string name="app_name">单选按钮测试</string>  
    <string name="radiobutton1">杭州</string>  
    <string name="radiobutton2">成都</string>  
    <string name="radiobutton3">重庆</string>  
    <string name="radiobutton4">苏州</string>  
</resources>

<?xml version="1.0" encoding="utf-8"?>
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
    android:orientation="vertical"
    android:layout_width="fill_parent"
    android:layout_height="fill_parent"
    >
<TextView
    android:layout_width="fill_parent"
    android:layout_height="wrap_content"
    android:text="@string/hello"
    android:id="@+id/textview1"
    />
    <RadioGroup
            android:id="@+id/radiogroup1"
            android:layout_width="wrap_content"
            android:layout_height="wrap_content"
            android:orientation="vertical"
            android:layout_x="3px"
    >
            <RadioButton
                    android:id="@+id/radiobutton1"
                    android:layout_width="wrap_content"
                    android:layout_height="wrap_content"
                    android:text="@string/radiobutton1"
            />
            <RadioButton
                    android:id="@+id/radiobutton2"
                    android:layout_width="wrap_content"
                    android:layout_height="wrap_content"
                    android:text="@string/radiobutton2"
            />
            <RadioButton
                    android:id="@+id/radiobutton3"
                    android:layout_width="wrap_content"
                    android:layout_height="wrap_content"
                    android:text="@string/radiobutton3"
            />
            <RadioButton
                    android:id="@+id/radiobutton4"
                    android:layout_width="wrap_content"
                    android:layout_height="wrap_content"
                    android:text="@string/radiobutton4"
            />
    </RadioGroup>
</LinearLayout>