MultiSelectListPreference

MainActivity.java

  private Set getHiddenUnits = new HashSet<>();
    List list;
  @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
    getHiddenUnits = App.getHiddenUnits();

        if(!getHiddenUnits.isEmpty())
        {
            Toast.makeText(this, getHiddenUnits.toString(), Toast.LENGTH_SHORT).show();
            for(Object object : getHiddenUnits) {
                String element = (String) object;
                hideElement(element);
                Toast.makeText(this, element, Toast.LENGTH_SHORT).show();
            }
        }
        else
        {
            setAdapterFirstStart();
        }
    }

// Generic function to convert set to list
    public static <T> List<T> convertSetToList(Set<T> set)
    {
        return new ArrayList<>(set);
    }

    //TODO: get real element and not the sorted list one
    void hideElement(String position)
    {
        int element = Integer.parseInt(position);
        removeItem(element);
        adapter.notifyItemChanged(element);
    }

    boolean hiddenCheck(int position)
    {
        String item = String.valueOf(position);
        return getHiddenUnits.contains(item);
    }

    boolean hiddenListCheck(int position)
    {
        return list.contains(position);
    }

    boolean hiddenCheck(Conversion position)
    {
        return getHiddenUnits.contains(position.getNumber());
    }

    //returns active list for settings
    public static ArrayList<Conversion> getConversionList() {
        return conversionList;
    }


 public ArrayList<Conversion> setList() {
        boolean hidden = false;
        boolean test = true;
        if (conversionList.isEmpty()) {
            Conversion length = new Conversion(R.drawable.length, R.string.length, Constants.LENGTH, ConversionActivity.class, hiddenCheck(Constants.LENGTH));
            addConversionToList(length);
        }
   
     private void addConversionToList(Conversion conversion) {
        if (!conversion.getIsHidden()) {
            conversionList.add(conversion);
        }
       
         public void removeItem(int position) {
        conversionList.remove(position);
        adapter.notifyItemRemoved(position);
    }

    public void setAdapterFirstStart() {
        if (adapter == null) {
            array = setList(false); //ArrayList won't be changing much, so I decided to load it into adapter earlier
            adapter = new ConversionAdapter(array, this, this);
            recyclerView.setAdapter(adapter);
        } else {
            adapter.notifyDataSetChanged();
        }
    }

    public void setAdapter() {
        if (adapter == null) {
            array = setList(); //ArrayList won't be changing much, so I decided to load it into adapter earlier
            adapter = new ConversionAdapter(array, this, this);
            recyclerView.setAdapter(adapter);
        } else {
            adapter.notifyDataSetChanged();
        }
    }

prefs.java

   Preference hidden = findPreference(getString(R.string.key_unit_hidden));
            if (hidden != null) {
                hidden.setOnPreferenceClickListener(new Preference.OnPreferenceClickListener() {
                    public boolean onPreferenceClick(Preference preference) {

                        return true;
                    }
                });
            }

prefs.xml

<androidx.preference.MultiSelectListPreference
            android:key="@string/key_unit_hidden"
            android:summary="@string/hide_units_summary"
            android:entries="@array/units"
            android:entryValues="@array/unit_numbers"
            android:defaultValue="@array/unit_defaults"
            android:title="@string/hide_categories_title" />