在片段和活动之间导航

Question

我在片段和活动之间导航时遇到问题，我使用了 Android studios 2.2.3 的自动生成模板并对其进行了自定义以在片段之间导航。

我添加了五个片段并使用了本教程Using Fragments with the Navigation Drawer Activity。

片段运行良好，一切都很好，所以我想使用相同的 导航抽屉 进行另一个活动，但它一直在崩溃，我使用 Intent。

public boolean onNavigationItemSelected(MenuItem item) {
    // Handle navigation view item clicks here.
    int id = item.getItemId();
    Fragment fragment = null;
    Class fragmentClass = null;
    if (id == R.id.nav_schedule) {
     fragmentClass = ScheduleFragment.class;
    } else if (id == R.id.nav_assingment) {
        fragmentClass = AssignmentsFragment.class;
    } else if (id == R.id.nav_teachers) {
        fragmentClass = TeachersFragment.class;
    } else if (id == R.id.nav_score) {
        fragmentClass = ScoreFragment.class;
    } else if (id == R.id.nav_events) {
        fragmentClass = EventsFragment.class;
    } else if (id == R.id.nav_setting) {

    }else if (id == R.id.nav_about) {
        Intent intent = new Intent(getApplicationContext(),AboutActivity.class);
        startActivity(intent);
    }else if (id == R.id.nav_logout) {
        Intent i = new Intent(getApplicationContext(),LoginActivity.class);
        i.addFlags(Intent.FLAG_ACTIVITY_CLEAR_TOP);
        i.addFlags(Intent.FLAG_ACTIVITY_CLEAR_TASK);
        startActivity(i);
    }
         try {
             try {
                 fragment = (Fragment) fragmentClass.newInstance();
             } catch (InstantiationException e) {
                 e.printStackTrace();
             } catch (IllegalAccessException e) {
                 e.printStackTrace();
             }
             FragmentManager fragmentManager = getSupportFragmentManager();
             fragmentManager.beginTransaction().replace(R.id.container, fragment).commit();
         }catch (android.app.Fragment.InstantiationException e){
             e.printStackTrace();
         }
    DrawerLayout drawer = (DrawerLayout) findViewById(R.id.drawer_layout);
    drawer.closeDrawer(GravityCompat.START);
    return true;
}

Logcat

E/AndroidRuntime: 致命异常: main 进程：com.zer0ll.demo.studentapp，PID：31638 java.lang.NullPointerException：尝试在空对象引用上调用虚拟方法“java.lang.Object java.lang.Class.newInstance()” 在 com.zer0ll.demo.studentapp.Activities.HomeActivity.onNavigationItemSelected(HomeActivity.java:95) 在 android.support.design.widget.NavigationView$1.onMenuItemSelected(NavigationView.java:156) 在 android.support.v7.view.menu.MenuBuilder.dispatchMenuItemSelected(MenuBuilder.java:822) 在 android.support.v7.view.menu.SubMenuBuilder.dispatchMenuItemSelected(SubMenuBuilder.java:88) 在 android.support.v7.view.menu.MenuItemImpl.invoke(MenuItemImpl.java:156) 在 android.support.v7.view.menu.MenuBuilder.performItemAction(MenuBuilder.java:969) 在 android.support.design.internal.NavigationMenuPresenter$1.onClick(NavigationMenuPresenter.java:342) 在 android.view.View.performClick(View.java:5156) 在 android.view.View$PerformClick.run(View.java:20755) 在 android.os.Handler.handleCallback(Handler.java:739) 在 android.os.Handler.dispatchMessage(Handler.java:95) 在 android.os.Looper.loop(Looper.java:145) 在 android.app.ActivityThread.main(ActivityThread.java:5835) 在 java.lang.reflect.Method.invoke（本机方法） 在 java.lang.reflect.Method.invoke(Method.java:372) 在 com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:1399) 在 com.android.internal.os.ZygoteInit.main(ZygoteInit.java:1194)

任何帮助将不胜感激。 谢谢。

Answer 1

您必须在清单中定义活动。

Answer 2

尝试在清单文件中添加您的活动，

<activity android:name="Packagename.AboutActivity">

        </activity>

    <activity android:name="Packagename.LoginActivity">

        </activity>

Answer 3

使用 Activity Context 不使用 Application Context 启动您的 Activity，并在启动 Activity 后返回 true，并且还使用正确的包名称在清单中声明，如下所示

 public boolean onNavigationItemSelected(MenuItem item) {
    // Handle navigation view item clicks here.
    int id = item.getItemId();
    Fragment fragment = null;
    Class fragmentClass = null;
    if (id == R.id.nav_schedule) {
        fragmentClass = ScheduleFragment.class;
    } else if (id == R.id.nav_assingment) {
        fragmentClass = AssignmentsFragment.class;
    } else if (id == R.id.nav_teachers) {
        fragmentClass = TeachersFragment.class;
    } else if (id == R.id.nav_score) {
        fragmentClass = ScoreFragment.class;
    } else if (id == R.id.nav_events) {
        fragmentClass = EventsFragment.class;
    } else if (id == R.id.nav_setting) {

    }else if (id == R.id.nav_about) {
        Intent intent = new Intent(classname.this,AboutActivity.class);
        startActivity(intent);
        return true;
    }else if (id == R.id.nav_logout) {
        Intent i = new Intent(classname.this,LoginActivity.class);
        i.addFlags(Intent.FLAG_ACTIVITY_CLEAR_TOP);
        i.addFlags(Intent.FLAG_ACTIVITY_CLEAR_TASK);
        startActivity(i);
        return true;
    }
    try {
        try {
            fragment = (Fragment) fragmentClass.newInstance();
        } catch (InstantiationException e) {
            e.printStackTrace();
        } catch (IllegalAccessException e) {
            e.printStackTrace();
        }
        FragmentManager fragmentManager = getSupportFragmentManager();
        fragmentManager.beginTransaction().replace(R.id.container, fragment).commit();
    }catch (android.app.Fragment.InstantiationException e){
        e.printStackTrace();
    }
    DrawerLayout drawer = (DrawerLayout) findViewById(R.id.drawer_layout);
    drawer.closeDrawer(GravityCompat.START);
    return true;
}