How to display a virtual list with thousands of items

Purpose. Display tens of thousands of items without creating one Component per row. The VirtualListView only creates/renders the visible views and recycles those that leave the screen (like Android’s RecyclerView). All rows have a fixed height (Relative itemHeight).

APIs used

• new VirtualListView(Relative itemHeight, DataAdapter adapter) — single constructor; the height is fixed for all rows.
• interface DataAdapter (nested in VirtualListView), with three methods:
  • int getCount() — total number of items.
  • Component createView() throws Exception — creates a blank view; called only when the recycling pool is empty.
  • void bindView(Component view, int index) throws Exception — populates a view (new or recycled) with the data at the index.
• VirtualListView.notifyDataChanged() — call after the dataset grows/changes under new indices (e.g., an appended page). It does not rebind rows already visible.
• VirtualListView.reloadData() — full reload: discards the view cache, recycles everything, scrolls to the top, and rebinds all visible slots. Use in search/filter/sort (content changes under the same indices).
• VirtualListView.scrollToIndex(int index) — scrolls to center the index.
• VirtualListView.getScrollY() — current vertical offset in pixels (useful for a collapsible header).

Complete example (copy, paste and run)

package com.exemplo;

import dev.crossfly.ui.*;
import dev.crossfly.ui.layout.*;
import dev.crossfly.ui.graphics.*;   // Relative (for VirtualListView)

/** Runnable main screen. Point the launcher (classname) and run: gradlew :lwjgl3:run */
public class MainWindow extends Window
{
   @Override
   public void onShow() throws Exception
   {
      super.onShow();
      setLayout(new VerticalLayout());

      final String[] dados = new String[50_000];
      for (int i = 0; i < dados.length; i++) dados[i] = "Registro " + i;

      Relative alturaItem = Relative.ofFont(3f);   // ~3 font heights per row (scales with the UI)

      VirtualListView lista = new VirtualListView(alturaItem, new VirtualListView.DataAdapter()
      {
         public int getCount()
         {
            return dados.length;
         }

         public Component createView() throws Exception
         {
            // reusable "empty" view — NO data here
            return new Label("");
         }

         public void bindView(Component view, int index) throws Exception
         {
            // populate the recycled view with the current index's data
            ((Label) view).setText(dados[index]);
         }
      });

      add(LAY_FILL, lista);
   }
}

The snippet below shows only the core. To run it, place it inside the MainWindow skeleton above.

Minimal snippet

// verified in source: VirtualListView.java (DataAdapter is a nested interface)
final String[] dados = new String[50_000];
for (int i = 0; i < dados.length; i++) dados[i] = "Registro " + i;

Relative alturaItem = Relative.ofFont(3f);     // ~3 font heights per row (scales with the UI)

VirtualListView lista = new VirtualListView(alturaItem, new VirtualListView.DataAdapter()
{
   public int getCount()
   {
      return dados.length;
   }

   public Component createView() throws Exception
   {
      // reusable "empty" view — NO data here
      return new Label("");
   }

   public void bindView(Component view, int index) throws Exception
   {
      // populate the recycled view with the current index's data
      ((Label) view).setText(dados[index]);
   }
});

layout.add(1f, lista);

Updating the data:

// Appended more items at the end (new indices) → notifyDataChanged
carregarMaisDados(dados);
lista.notifyDataChanged();

// Applied a filter/search (same index, different content) → reloadData
aplicarFiltro();
lista.reloadData();   // recycles, scrolls back to top and rebinds the visible items

Go deeper

• Critical rule: never keep references to views between binds — they are recycled for other indices.
• Row with several elements (icon + text + value)? Create a custom Component with a bind(...) method that stores the sub-references as fields: createView() returns new CardTarefa() and bindView does ((CardTarefa) view).bind(dados[index]) (see Thinking in components).
• Server data by page: see paginacao.md (uses PagedDataAdapter).
• Infinite scroll by append: see carregar-mais.md.
• Widget guide: VirtualListView · Best practices: Performance and large lists · Canonical source: Chapter 15