错误:尝试调用虚拟方法的java.lang。在空对象引用上的对象android.content.Context.getSystemService(java.lang.String)'。

public class AlarmTask implements Runnable{
// The date selected for the alarm
private final Calendar date;
// The android system alarm manager
private final AlarmManager am;
// Your context to retrieve the alarm manager from
private final Context context;

public AlarmTask(Context context, Calendar date) {

    this.context = context;
    this.am = (AlarmManager) context.getSystemService(Context.ALARM_SERVICE); //error for this line
    this.date = date;
}
public class AlarmTask implements Runnable{
//