我有1个主片段容器和4个片段，我希望通过底部导航栏切换。我不希望每个片段在每次需要显示时都被重新加载（使用replace方法）。我尝试使用下面的代码加载一个片段（如果它以前没有加载）或者显示它（如果它已经加载）（并隐藏前一个）。出于某种原因，隐藏和展示似乎不起作用。

navigationView.setOnItemSelectedListener(item -> {

           int itemId = item.getItemId();

           Fragment fragment = getFragment(currentFragment);

           if (itemId == R.id.groupChatsIconItem) {
               if (fragment == null || fragment.getClass() != GroupChatFragment.class)
                   loadFragment(GROUP_CHAT_FRAGMENT);
           } else if (itemId == R.id.friendsIconItem) {
               if (fragment == null || fragment.getClass() != FriendsFragment.class)
                   loadFragment(FRIENDS_FRAGMENT);
           } else if (itemId == R.id.myProfileActivityIconItem) {
               if (fragment == null || fragment.getClass() != MyProfileFragment.class)
                   loadFragment(MY_PROFILE_FRAGMENT);
           } else if (itemId == R.id.directChatIconItem) {
               if (fragment == null || fragment.getClass() != DirectChatFragment.class)
                   loadFragment(DIRECT_CHAT_FRAGMENT);
           }
           return true;
       });

   }

public void loadFragment(String tag) {

       //if that's the first fragment - not adding to back stack to make sure the navigation view isn't being displayed alone.
       if (currentFragment == null) {
           currentFragment = tag;

           getSupportFragmentManager()
                   .beginTransaction()
                   .add(R.id.mainFragmentContainerView, getFragment(tag), tag)
                   .setReorderingAllowed(true)
                   .commit();
       }

       //The desired fragment isn't loaded yet.  loading it
       else if (getSupportFragmentManager().findFragmentByTag(tag) == null) {
           getSupportFragmentManager()
                   .beginTransaction()
                   .hide(getFragment(currentFragment))
                   .add(mainFragmentContainerView.getId(), getFragment(tag), tag)
                   .addToBackStack("yes")
                   .setReorderingAllowed(true)
                   .commit();
           currentFragment = tag;
       }
       //fragment was already loaded - hide the current one and show the new one
       else {
           Log.d("banana","desired fragment is already loaded");
           getSupportFragmentManager()
                   .beginTransaction()
                   .hide(getFragment(currentFragment))
                   .show(getFragment(tag))
                   .commit();
           currentFragment = tag;
       }
   }


private Fragment getFragment(String tag) {
       if (tag == null)
           return null;
       if (tag.equals(GROUP_CHAT_FRAGMENT))
           return new GroupChatFragment();
       if (tag.equals(FRIENDS_FRAGMENT))
           return new FriendsFragment();
       if (tag.equals(MY_PROFILE_FRAGMENT))
           return new MyProfileFragment();
       return new DirectChatFragment();
   }

我不太清楚为什么隐藏和显示似乎不工作-每当我试图点击加载一个片段的第一次它被正确加载，但一旦我试图点击一个片段，已经加载，它只是不改变。我也试过把hide和load/show功能分离成两个不同的功能，但没有成功。任何帮助都会非常感谢：）