Java 理解Android上下文:(空对象引用)
我想构建一个简单的静态方法,从当前手机中获取下一个预定闹钟。实现了非静态,在Java 理解Android上下文:(空对象引用),java,android,static,alarmmanager,android-context,Java,Android,Static,Alarmmanager,Android Context,我想构建一个简单的静态方法,从当前手机中获取下一个预定闹钟。实现了非静态,在Main_活动中一切正常,但现在在作为静态方法的单独类中,我在空对象引用上得到错误:“android.content.Context.getContentResolver()” 我想我对上下文的理解不够好。 我发现了这个:但是我认为这不是正确的方法,我想我只是错过了一些东西,有人能帮我吗 import android.os.Bundle; import android.provider.Settings; import
Main_活动中
一切正常,但现在在作为静态方法的单独类中,我在空对象引用上得到错误:“android.content.Context.getContentResolver()”
我想我对上下文的理解不够好。
我发现了这个:但是我认为这不是正确的方法,我想我只是错过了一些东西,有人能帮我吗
import android.os.Bundle;
import android.provider.Settings;
import android.support.v7.app.AppCompatActivity;
public class Controller extends AppCompatActivity {
private static Controller staticController = new Controller();
/**
* Finds out what the next user scheduled alarm is.
*
* @return (String) next time the user has scheduled an alarm on his device.
*/
protected static String nextAlarm() {
String nextAlarmTime = null;
// deprecated method will also detect non native alarm clocks!
nextAlarmTime = Settings.System.getString(staticController.getContentResolver(),
Settings.System.NEXT_ALARM_FORMATTED);
// fallback if deprecated method does not find valid alarm time!
// if (nextAlarmTime == null) {
// AlarmManager am = (AlarmManager) staticController.getSystemService(Context.ALARM_SERVICE);
// AlarmManager.AlarmClockInfo alarmInfo = am.getNextAlarmClock();
// Long alarm_next = alarmInfo.getTriggerTime();
// nextAlarmTime = (new Date(alarm_next)).toString();
// }
return nextAlarmTime;
}
// Do I need onCreate here ?
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
}
}
(我不知道这是否重要,但该控制器类未作为活动包含在清单文件中。我刚刚创建了一个新类,并从AppCompatActivity进行了扩展)这就是问题所在:
new Controller()代码>。切勿自己实例化活动
类(或从中派生的类)。只有系统应该这样做,从而初始化所有必需的字段。上面评论中提到的Commonware在这种情况下似乎是正确的
为什么不将上下文(或ContentResolver)作为参数传递
到nextAlarm()
以下是我将其更改为:
import android.app.AlarmManager;
import android.content.Context;
import android.provider.Settings;
import java.util.Date;
public class Controller extends { **//does not need to be a Activity any more**
/**
* Finds out what the next user scheduled alarm is.
*
* @return (String) next time the user has scheduled an alarm on his device.
*/
protected static String nextAlarm(Context context) { //**pass Context from other Activity**
String nextAlarmTime = null;
// deprecated method will also detect non native alarm clocks!
nextAlarmTime = Settings.System.getString(context.getContentResolver(), //**reference parameter here**
Settings.System.NEXT_ALARM_FORMATTED);
// fallback if deprecated method does not find valid alarm time!
if (nextAlarmTime == null) {
AlarmManager am = (AlarmManager) context.getSystemService(Context.ALARM_SERVICE); // **reference parameter again**
AlarmManager.AlarmClockInfo alarmInfo = am.getNextAlarmClock();
Long alarm_next = alarmInfo.getTriggerTime();
nextAlarmTime = (new Date(alarm_next)).toString();
}
return nextAlarmTime;
}
}
然后在某些活动中通过Controller.nextAlarm(this))
简单地调用它。什么是Controller
?为什么不将上下文
(或ContentResolver
)作为参数传递给nextAlarm()
?通过构造函数将上下文传递给simple类,并使用它实现以下目的: