Following example shows how to display text suggestions in a dropdown list.

Example

The Decorator

Following decorator can work with any kind of component which allows user to enter or type text.

public class SuggestionDropDownDecorator<C extends JComponent> {
  private final C invoker;
  private final SuggestionClient<C> suggestionClient;
  private JPopupMenu popupMenu;
  private JList<String> listComp;
  DefaultListModel<String> listModel;
  private boolean disableTextEvent;

  public SuggestionDropDownDecorator(C invoker, SuggestionClient<C> suggestionClient) {
      this.invoker = invoker;
      this.suggestionClient = suggestionClient;
  }

  public static <C extends JComponent> void decorate(C component, SuggestionClient<C> suggestionClient) {
      SuggestionDropDownDecorator<C> d = new SuggestionDropDownDecorator<>(component, suggestionClient);
      d.init();
  }

  public void init() {
      initPopup();
      initSuggestionCompListener();
      initInvokerKeyListeners();
  }

  private void initPopup() {
      popupMenu = new JPopupMenu();
      listModel = new DefaultListModel<>();
      listComp = new JList<>(listModel);
      listComp.setBorder(BorderFactory.createEmptyBorder(0, 2, 5, 2));
      listComp.setFocusable(false);
      popupMenu.setFocusable(false);
      popupMenu.add(listComp);
  }

  private void initSuggestionCompListener() {
      if (invoker instanceof JTextComponent) {
          JTextComponent tc = (JTextComponent) invoker;
          tc.getDocument().addDocumentListener(new DocumentListener() {
              @Override
              public void insertUpdate(DocumentEvent e) {
                  update(e);
              }

              @Override
              public void removeUpdate(DocumentEvent e) {
                  update(e);
              }

              @Override
              public void changedUpdate(DocumentEvent e) {
                  update(e);
              }

              private void update(DocumentEvent e) {
                  if (disableTextEvent) {
                      return;
                  }
                  SwingUtilities.invokeLater(() -> {
                      List<String> suggestions = suggestionClient.getSuggestions(invoker);
                      if (suggestions != null && !suggestions.isEmpty()) {
                          showPopup(suggestions);
                      } else {
                          popupMenu.setVisible(false);
                      }
                  });
              }
          });
      }//todo init invoker components other than text components
  }

  private void showPopup(List<String> suggestions) {
      listModel.clear();
      suggestions.forEach(listModel::addElement);
      Point p = suggestionClient.getPopupLocation(invoker);
      if (p == null) {
          return;
      }
      popupMenu.pack();
      listComp.setSelectedIndex(0);
      popupMenu.show(invoker, (int) p.getX(), (int) p.getY());
  }

  private void initInvokerKeyListeners() {
      //not using key inputMap cause that would override the original handling
      invoker.addKeyListener(new KeyAdapter() {
          @Override
          public void keyPressed(KeyEvent e) {
              if (e.getKeyCode() == VK_ENTER) {
                  selectFromList(e);
              } else if (e.getKeyCode() == VK_UP) {
                  moveUp(e);
              } else if (e.getKeyCode() == VK_DOWN) {
                  moveDown(e);
              } else if (e.getKeyCode() == VK_ESCAPE) {
                  popupMenu.setVisible(false);
              }
          }
      });
  }

  private void selectFromList(KeyEvent e) {
      if (popupMenu.isVisible()) {
          int selectedIndex = listComp.getSelectedIndex();
          if (selectedIndex != -1) {
              popupMenu.setVisible(false);
              String selectedValue = listComp.getSelectedValue();
              disableTextEvent = true;
              suggestionClient.setSelectedText(invoker, selectedValue);
              disableTextEvent = false;
              e.consume();
          }
      }
  }

  private void moveDown(KeyEvent keyEvent) {
      if (popupMenu.isVisible() && listModel.getSize() > 0) {
          int selectedIndex = listComp.getSelectedIndex();
          if (selectedIndex < listModel.getSize()) {
              listComp.setSelectedIndex(selectedIndex + 1);
              keyEvent.consume();
          }
      }
  }

  private void moveUp(KeyEvent keyEvent) {
      if (popupMenu.isVisible() && listModel.getSize() > 0) {
          int selectedIndex = listComp.getSelectedIndex();
          if (selectedIndex > 0) {
              listComp.setSelectedIndex(selectedIndex - 1);
              keyEvent.consume();
          }
      }
  }
}

The SuggestionClient interface

This is a hookup interface for the decorator. An implementation works on a specific component. This interface also allows the component on how it wants to display suggestions e.g. word by word or on the entire text etc.

public interface SuggestionClient<C extends JComponent> {

  Point getPopupLocation(C invoker);

  void setSelectedText(C invoker, String selectedValue);

  java.util.List<String> getSuggestions(C invoker);

}

The SuggestionClient Implementations

Following implementation is for any JTextComponent. It shows the suggestions on entire text.

/**
* Matches entire text instead of separate words
*/
public class TextComponentSuggestionClient implements SuggestionClient<JTextComponent> {

  private Function<String, List<String>> suggestionProvider;

  public TextComponentSuggestionClient(Function<String, List<String>> suggestionProvider) {
      this.suggestionProvider = suggestionProvider;
  }

  @Override
  public Point getPopupLocation(JTextComponent invoker) {
      return new Point(0, invoker.getPreferredSize().height);
  }

  @Override
  public void setSelectedText(JTextComponent invoker, String selectedValue) {
      invoker.setText(selectedValue);
  }

  @Override
  public List<String> getSuggestions(JTextComponent invoker) {
      return suggestionProvider.apply(invoker.getText().trim());
  }
}

Following implementation is also for a JTextComponent. It shows the suggestions on each words as we type.

/**
* Matches individual words instead of complete text
*/
public class TextComponentWordSuggestionClient implements SuggestionClient<JTextComponent> {
  private Function<String, List<String>> suggestionProvider;

  public TextComponentWordSuggestionClient(Function<String, List<String>> suggestionProvider) {
      this.suggestionProvider = suggestionProvider;
  }

  @Override
  public Point getPopupLocation(JTextComponent invoker) {
      int caretPosition = invoker.getCaretPosition();
      try {
          Rectangle2D rectangle2D = invoker.modelToView(caretPosition);
          return new Point((int) rectangle2D.getX(), (int) (rectangle2D.getY() + rectangle2D.getHeight()));
      } catch (BadLocationException e) {
          System.err.println(e);
      }
      return null;
  }

  @Override
  public void setSelectedText(JTextComponent tp, String selectedValue) {
      int cp = tp.getCaretPosition();
      try {
          if (cp == 0 || tp.getText(cp - 1, 1).trim().isEmpty()) {
              tp.getDocument().insertString(cp, selectedValue, null);
          } else {
              int previousWordIndex = Utilities.getPreviousWord(tp, cp);
              String text = tp.getText(previousWordIndex, cp - previousWordIndex);
              if (selectedValue.startsWith(text)) {
                  tp.getDocument().insertString(cp, selectedValue.substring(text.length()), null);
              } else {
                  tp.getDocument().insertString(cp, selectedValue, null);
              }
          }
      } catch (BadLocationException e) {
          System.err.println(e);
      }
  }

  @Override
  public List<String> getSuggestions(JTextComponent tp) {
      try {
          int cp = tp.getCaretPosition();
          if (cp != 0) {
              String text = tp.getText(cp - 1, 1);
              if (text.trim().isEmpty()) {
                  return null;
              }
          }
          int previousWordIndex = Utilities.getPreviousWord(tp, cp);
          String text = tp.getText(previousWordIndex, cp - previousWordIndex);
          return suggestionProvider.apply(text.trim());
      } catch (BadLocationException e) {
          System.err.println(e);
      }
      return null;
  }
}

Example main class

public class SuggestionExampleMain {
  public static void main(String[] args) {
      JFrame frame = createFrame();
      JTextField textField = new JTextField(10);
      SuggestionDropDownDecorator.decorate(textField,
              new TextComponentSuggestionClient(SuggestionExampleMain::getSuggestions));
      JTextPane textPane = new JTextPane();
      SuggestionDropDownDecorator.decorate(textPane,
              new TextComponentWordSuggestionClient(SuggestionExampleMain::getSuggestions));
      frame.add(textField, BorderLayout.NORTH);
      frame.add(new JScrollPane(textPane));
      frame.setVisible(true);
  }

  private static List<String> words =
          RandomUtil.getWords(2, 400).stream().map(String::toLowerCase).collect(Collectors.toList());

  private static List<String> getSuggestions(String input) {
      //the suggestion provider can control text search related stuff, e.g case insensitive match, the search  limit etc.
      if (input.isEmpty()) {
          return null;
      }
      return words.stream()
                  .filter(s -> s.startsWith(input))
                  .limit(20)
                  .collect(Collectors.toList());
  }

  private static JFrame createFrame() {
      JFrame frame = new JFrame("Suggestion Dropdown Example");
      frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
      frame.setSize(new Dimension(600, 300));
      return frame;
  }
}

Output

Example Project

Dependencies and Technologies Used:

• JDK 1.8
• Maven 3.3.9