Android DatePickerDialog用法

<?xml version="1.0" encoding="utf-8"?>
<RelativeLayout xmlns:android="http://schemas.android.com/apk/res/android"
    android:layout_width="fill_parent"
    android:layout_height="wrap_content"
    android:orientation="vertical" >
    <TextView
        android:id="@+id/text01"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:text="查询日期:"
        android:textSize="18sp"
        />
    <TextView 
        android:id="@+id/tvDate"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_below="@id/text01"
        android:layout_alignLeft="@id/text01"
        />
    <Button 
        android:id="@+id/btnDatePicker"
        android:text="编辑"
        android:layout_toRightOf="@id/tvDate"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:onClick="show"
        android:layout_alignTop="@id/tvDate"
        />
</RelativeLayout>

package com.turboradio;

import java.text.SimpleDateFormat;
import java.util.Calendar;

import android.app.Activity;
import android.app.DatePickerDialog;
import android.os.Bundle;
import android.view.View;
import android.widget.DatePicker;
import android.widget.TextView;

public class DatePickerActivity extends Activity {
	private TextView tvDate;
	private Calendar cal = Calendar.getInstance();
	private DatePickerDialog.OnDateSetListener listener = new DatePickerDialog.OnDateSetListener() {
		
		@Override
		public void onDateSet(DatePicker view, int year, int monthOfYear,int dayOfMonth) {
			cal.set(Calendar.YEAR, year);
			cal.set(Calendar.MONTH, monthOfYear);
			cal.set(Calendar.DAY_OF_MONTH, dayOfMonth);
			updateDate();
		}
	};
	@Override
	protected void onCreate(Bundle savedInstanceState) {
		super.onCreate(savedInstanceState);
		setContentView(R.layout.date_picker_dialog);
		tvDate = (TextView)findViewById(R.id.tvDate);
		
	}
	public void show(View v){
		new DatePickerDialog(DatePickerActivity.this,listener,
				cal.get(Calendar.YEAR),
				cal.get(Calendar.MONTH),
				cal.get(Calendar.DAY_OF_MONTH)
				).show();
	}
	private void updateDate(){
		SimpleDateFormat simpleDateFormat = new SimpleDateFormat("yyyy-MM-dd");
		tvDate.setText(simpleDateFormat.format(cal.getTime()));
	}
}