我的代码没有错误，但是当我启动时，我在主 XML 页面中收到一个致命错误，内容如下：
03-21 22:00:31.849: E/AndroidRuntime(606): java.lang.RuntimeException: 无法实例化活动 ComponentInfo{countrycabin.ist236/countrycabin.ist236.Main}: java.lang.NullPointerException
我不确定是什么导致了这个问题。正如我所说，我没有错误。我制作了一个与此非常相似的程序，运行良好，我只是将 TimePicker 添加到此程序中。
我想做的事情非常简单。我正在启动程序，当我单击按钮时，会显示一个对话框，其中显示 DatePicker 或 TimePicker，当有人选择日期时，文字会显示在底部的 TextView 中。然而，我什至无法测试这一点，因为它甚至不会启动。我的模拟器告诉我它不会响应。
正如教科书中所述，我正在使用 Honeycomb 来实现此目的。
这是我的全部代码。
主要清单 XML
<?xml version="1.0" encoding="utf-8"?>
<manifest xmlns:android="http://schemas.android.com/apk/res/android"
package="countrycabin.ist236"
android:versionCode="1"
android:versionName="1.0" >
<uses-sdk
android:minSdkVersion="8"
android:targetSdkVersion="11" />
<application
android:allowBackup="true"
android:icon="@drawable/ic_launcher"
android:label="@string/app_name"
android:theme="@style/AppTheme" >
<activity
android:name="countrycabin.ist236.Main"
android:label="@string/app_name" >
<intent-filter>
<action android:name="android.intent.action.MAIN" />
<category android:name="android.intent.category.LAUNCHER" />
</intent-filter>
</activity>
</application>
</manifest>
main XML
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
xmlns:tools="http://schemas.android.com/tools"
android:id="@+id/LinearLayout1"
android:layout_width="match_parent"
android:layout_height="match_parent"
android:orientation="horizontal"
tools:context=".Main" >
<ImageView
android:id="@+id/imgCabin"
android:layout_width="282dp"
android:layout_height="match_parent"
android:contentDescription="@string/strCabin"
android:src="@drawable/cabin" />
<TableLayout
android:layout_width="fill_parent"
android:layout_height="fill_parent">
<TableRow>
<TextView
android:id="@+id/txtTitle"
android:layout_width="wrap_content"
android:background="#666FFF"
android:padding="50dp"
android:text="@string/strTitle"
android:textSize="40sp"
android:typeface="serif" />
</TableRow>
<TableRow>
<TextView
android:id="@+id/txtDesc"
android:layout_width="wrap_content"
android:padding="25dp"
android:text="@string/strDesc"
android:textSize="30sp" />
</TableRow>
<RadioGroup
android:id="@+id/radio"
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:layout_weight="0.20"
android:contentDescription="@string/strRad" >
<RadioButton
android:id="@+id/radCabin1"
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:layout_marginBottom="5dp"
android:layout_marginLeft="250dp"
android:checked="true"
android:text="@string/strCabinOne"
android:textSize="25sp" />
<RadioButton
android:id="@+id/radCabin2"
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:layout_marginLeft="250dp"
android:text="@string/strCabinTwo"
android:textSize="25sp" />
</RadioGroup>
<TableRow>
<Button
android:id="@+id/btnDate"
android:layout_width="50dp"
android:padding="20sp"
android:text="@string/strDate"
android:textSize="25sp" />
</TableRow>
<TableRow>
<Button
android:id="@+id/btnTime"
android:layout_width="50dp"
android:padding="20sp"
android:text="@string/strTime"
android:textSize="25sp" />
</TableRow>
<TableRow>
<TextView
android:id="@+id/txtDate"
android:layout_gravity="center"
android:padding="20dp"
android:textSize="25sp" />
</TableRow>
<TableRow>
<TextView
android:id="@+id/txtTime"
android:layout_gravity="center"
android:padding="20dp"
android:textSize="25sp" />
</TableRow>
</TableLayout>
</LinearLayout>
主类（Java代码）
package countrycabin.ist236;
import java.util.Calendar;
import android.os.Bundle;
import android.app.Activity;
import android.app.DatePickerDialog;
import android.app.Dialog;
import android.app.TimePickerDialog;
import android.view.Menu;
import android.view.View;
import android.widget.Button;
import android.widget.DatePicker;
import android.widget.RadioButton;
import android.widget.TextView;
import android.widget.TimePicker;
public class Main extends Activity {
private int currentYear;
private int currentMonth;
private int currentDay;
static final int DATE_DIALOG_ID = 0;
static final int TIME_DIALOG_ID = 1;
int hour;
int minute;
String cabin1 = "The Wooden Castle";
String cabin2 = "Cozy Little Spot";
private Button btDate;
private Button btTime;
private TextView timeDisplay;
private TextView dateDisplay;
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.main);
timeDisplay = (TextView) findViewById(R.id.txtTime);
dateDisplay = (TextView) findViewById(R.id.txtDate);
btDate = (Button) findViewById(R.id.btnDate);
btTime = (Button) findViewById(R.id.btnTime);
btDate.setOnClickListener(new View.OnClickListener(){
@Override
public void onClick(View v) {
showDialog(DATE_DIALOG_ID);
}
});
final Calendar c = Calendar.getInstance();
currentYear = c.get(Calendar.YEAR);
currentMonth = c.get(Calendar.MONTH);
currentDay = c.get(Calendar.DAY_OF_MONTH);
btTime.setOnClickListener(new View.OnClickListener(){
@Override
public void onClick(View v){
showDialog(TIME_DIALOG_ID);
}
});
final Calendar d = Calendar.getInstance();
hour = d.get(Calendar.HOUR_OF_DAY);
minute = d.get(Calendar.MINUTE);
}
protected Dialog onCreateDialog(int id) {
switch(id){
case DATE_DIALOG_ID:
return new DatePickerDialog(this, reservationDate, currentYear, currentMonth, currentDay);
case TIME_DIALOG_ID:
return new TimePickerDialog(this, timeDate, hour, minute, false);
}
return null;
}
private DatePickerDialog.OnDateSetListener reservationDate = new DatePickerDialog.OnDateSetListener() {
final RadioButton c1 = (RadioButton) findViewById(R.id.radCabin1);
final RadioButton c2 = (RadioButton) findViewById(R.id.radCabin2);
@Override
public void onDateSet(DatePicker view, int year, int month, int day){
if(c1.isChecked()){
dateDisplay.setText("Your rental time is set for " + (month + 1) + "-" + day + "-" + year + " to " + (month + 1) + "-" + (day + 3) + "-" + year + " in " + cabin1 + ".");
}
if(c2.isChecked()){
dateDisplay.setText("Your rental time is set for " + (month + 1) + "-" + day + "-" + year + " to " + (month + 1) + "-" + (day + 3) + "-" + year + " in " + cabin2 + ".");
}
}
};
private TimePickerDialog.OnTimeSetListener timeDate = new TimePickerDialog.OnTimeSetListener() {
@Override
public void onTimeSet(TimePicker view, int hours, int minutes) {
timeDisplay.setText("Your arrival time will be at " + hours + ":" + minutes + ".");
}
};
@Override
public boolean onCreateOptionsMenu(Menu menu) {
// Inflate the menu; this adds items to the action bar if it is present.
getMenuInflater().inflate(R.menu.main, menu);
return true;
}
}
