Question

我是Android新手。我一直收到这个错误：

错误：java.lang.NullPointerException：尝试在空对象引用上调用虚拟方法'android.view.ViewTreeObserver android.widget.GridView.getViewTreeObserver（）'

该程序的其他部分运行正常，但当我点击CalendarFragment时，应用程序崩溃，我收到此错误。

请帮助。感谢。

CalendarFragment的代码：

package app.pal.study.samplestudy;

import android.support.v4.app.Fragment;
import android.os.Bundle;
import android.view.LayoutInflater;
import android.view.View;
import android.view.ViewGroup;
import android.content.Intent;
import android.os.Build;
import android.os.Bundle;
import android.view.LayoutInflater;
import android.view.View;
import android.view.ViewGroup;
import android.view.ViewTreeObserver;
import android.widget.AdapterView;
import android.widget.GridView;
import java.util.Calendar;
import java.util.Date;
import java.util.List;

import static android.app.Activity.RESULT_OK;

import app.pal.study.samplestudy.monthgrid.GridItem;
import app.pal.study.samplestudy.monthgrid.MonthDisplayAdapter;

public class CalendarFragment extends Fragment {
private static final int REQUEST_CODE = 1;
private Calendar calendar;

@Override
public View onCreateView(LayoutInflater inflater, ViewGroup container,
                         Bundle savedInstanceState) {

    View rootView = inflater.inflate(R.layout.fragment_calendar, container, false);

    return rootView;
}
@Override
public void onResume() {
    super.onResume();

    if (calendar == null) {
        calendar = CalUtil.getCalendar(new Date());
    }
    refresh();
}

private void refresh() {
    final GridView calendarView = (GridView) getView().findViewById(R.id.calendar_grid_view);

    setTitle();
    initListAdapter(calendarView);

    calendarView.setOnItemClickListener(new AdapterView.OnItemClickListener() {
        public void onItemClick(AdapterView<?> parent, View v, int position, long id) {
            GridItem item = (GridItem) parent.getAdapter().getItem(position);
            if (item != null) {
                startListActivity(item);
            }
        }
    });
}

private void startListActivity(GridItem item) {
    calendar.set(Calendar.DATE, item.getDay());
    Intent intent = new Intent(getActivity(), EventListActivity.class);
    intent.putExtra(Constants.DATE_KEY, calendar.getTime());
    startActivityForResult(intent, REQUEST_CODE);
}

public void loadNextMonth() {
    calendar.setTime(CalUtil.addMonth(calendar.getTime(), 1));
    refresh();
}

public void loadLastMonth() {
    calendar.setTime(CalUtil.subtractMonth(calendar.getTime(), 1));
    refresh();
}

public void showAllEvents() {
    Intent intent = new Intent(getActivity(), AllEventsActivity.class);
    intent.putExtra(Constants.DATE_KEY, calendar.getTime());
    startActivityForResult(intent, REQUEST_CODE);
}

@Override
public void onActivityResult(int requestCode, int resultCode, Intent data) {
    super.onActivityResult(requestCode, resultCode, data);
    if (resultCode == RESULT_OK) {
        calendar.setTime((Date) data.getExtras().get(Constants.DATE_KEY));
    }
}

private void setTitle() {
    getActivity().setTitle(CalUtil.dateToFormattedString(calendar.getTime(), "MMMM, yyyy"));
}

private void initListAdapter(final GridView calendarView) {
    calendarView.getViewTreeObserver().addOnGlobalLayoutListener(new ViewTreeObserver.OnGlobalLayoutListener() {
        @Override
        public void onGlobalLayout() {
            if (Build.VERSION.SDK_INT < Build.VERSION_CODES.JELLY_BEAN) {
                calendarView.getViewTreeObserver().removeGlobalOnLayoutListener(this);
            } else {
                calendarView.getViewTreeObserver().removeOnGlobalLayoutListener(this);
            }

            CalendarEventDataSource dataSource = new CalendarEventDataSource(getActivity());
            dataSource.openReadOnlyDB();
            List<GridItem> gridList = CalUtil.getGridList(calendar.getTime(),
                    dataSource.getDaysWithEventsForMonth(calendar.getTime()));
            dataSource.close();

            calendarView.setAdapter(new MonthDisplayAdapter(
                            gridList, calendarView.getMeasuredHeight())
            );
        }
    });
}
}

Answer 1

尝试在OncreateView（）

中声明日历

public View onCreateView(LayoutInflater inflater, ViewGroup container,
                         Bundle savedInstanceState) {

   Calendar calendar;
    View rootView = inflater.inflate(R.layout.fragment_calendar, container, false);

    return rootView;
}

Answer 2

Try this 


package app.pal.study.samplestudy;

import android.support.v4.app.Fragment;
import android.os.Bundle;
import android.view.LayoutInflater;
import android.view.View;
import android.view.ViewGroup;
import android.content.Intent;
import android.os.Build;
import android.os.Bundle;
import android.view.LayoutInflater;
import android.view.View;
import android.view.ViewGroup;
import android.view.ViewTreeObserver;
import android.widget.AdapterView;
import android.widget.GridView;
import java.util.Calendar;
import java.util.Date;
import java.util.List;

import static android.app.Activity.RESULT_OK;

import app.pal.study.samplestudy.monthgrid.GridItem;
import app.pal.study.samplestudy.monthgrid.MonthDisplayAdapter;

public class CalendarFragment extends Fragment {
private static final int REQUEST_CODE = 1;
private Calendar calendar;
View rootView;

@Override
public View onCreateView(LayoutInflater inflater, ViewGroup container,
                         Bundle savedInstanceState) {

    rootView = inflater.inflate(R.layout.fragment_calendar, container, false);

    return rootView;
}
@Override
public void onResume() {
    super.onResume();

    if (calendar == null) {
        calendar = CalUtil.getCalendar(new Date());
    }
    refresh();
}

private void refresh() {
    final GridView calendarView = (GridView)rootView.findViewById(R.id.calendar_grid_view);

    setTitle();
    initListAdapter(calendarView);

    calendarView.setOnItemClickListener(new AdapterView.OnItemClickListener() {
        public void onItemClick(AdapterView<?> parent, View v, int position, long id) {
            GridItem item = (GridItem) parent.getAdapter().getItem(position);
            if (item != null) {
                startListActivity(item);
            }
        }
    });
}

private void startListActivity(GridItem item) {
    calendar.set(Calendar.DATE, item.getDay());
    Intent intent = new Intent(getActivity(), EventListActivity.class);
    intent.putExtra(Constants.DATE_KEY, calendar.getTime());
    startActivityForResult(intent, REQUEST_CODE);
}

public void loadNextMonth() {
    calendar.setTime(CalUtil.addMonth(calendar.getTime(), 1));
    refresh();
}

public void loadLastMonth() {
    calendar.setTime(CalUtil.subtractMonth(calendar.getTime(), 1));
    refresh();
}

public void showAllEvents() {
    Intent intent = new Intent(getActivity(), AllEventsActivity.class);
    intent.putExtra(Constants.DATE_KEY, calendar.getTime());
    startActivityForResult(intent, REQUEST_CODE);
}

@Override
public void onActivityResult(int requestCode, int resultCode, Intent data) {
    super.onActivityResult(requestCode, resultCode, data);
    if (resultCode == RESULT_OK) {
        calendar.setTime((Date) data.getExtras().get(Constants.DATE_KEY));
    }
}

private void setTitle() {
    getActivity().setTitle(CalUtil.dateToFormattedString(calendar.getTime(), "MMMM, yyyy"));
}

private void initListAdapter(final GridView calendarView) {
    calendarView.getViewTreeObserver().addOnGlobalLayoutListener(new ViewTreeObserver.OnGlobalLayoutListener() {
        @Override
        public void onGlobalLayout() {
            if (Build.VERSION.SDK_INT < Build.VERSION_CODES.JELLY_BEAN) {
                calendarView.getViewTreeObserver().removeGlobalOnLayoutListener(this);
            } else {
                calendarView.getViewTreeObserver().removeOnGlobalLayoutListener(this);
            }

            CalendarEventDataSource dataSource = new CalendarEventDataSource(getActivity());
            dataSource.openReadOnlyDB();
            List<GridItem> gridList = CalUtil.getGridList(calendar.getTime(),
                    dataSource.getDaysWithEventsForMonth(calendar.getTime()));
            dataSource.close();

            calendarView.setAdapter(new MonthDisplayAdapter(
                            gridList, calendarView.getMeasuredHeight())
            );
        }
    });
}
}

错误：java.lang.NullPointerException：尝试在空对象引用上调用虚方法

2 个答案: