Example: Adding Autocomplete to JTextField

Autocomplete can be very useful in just about any application, but its not trivial to implement. So here is a quick example of how you might do it in Java's Swing framework with JTextField (it should also work with JTextArea with only a few modifications). This example is a modified version of Oracle's example.

The Code

package com.example.myapp.console;

import java.awt.event.ActionEvent;
import java.util.Collections;
import java.util.List;

import javax.swing.AbstractAction;
import javax.swing.JTextField;
import javax.swing.SwingUtilities;
import javax.swing.event.DocumentEvent;
import javax.swing.event.DocumentListener;
import javax.swing.text.BadLocationException;

public class Autocomplete implements DocumentListener {

  private static enum Mode {
    INSERT,
    COMPLETION
  };

  private JTextField textField;
  private final List<String> keywords;
  private Mode mode = Mode.INSERT;

  public Autocomplete(JTextField textField, List<String> keywords) {
    this.textField = textField;
    this.keywords = keywords;
    Collections.sort(keywords);
  }

  @Override
  public void changedUpdate(DocumentEvent ev) { }

  @Override
  public void removeUpdate(DocumentEvent ev) { }

  @Override
  public void insertUpdate(DocumentEvent ev) {
    if (ev.getLength() != 1)
      return;

    int pos = ev.getOffset();
    String content = null;
    try {
      content = textField.getText(0, pos + 1);
    } catch (BadLocationException e) {
      e.printStackTrace();
    }

    // Find where the word starts
    int w;
    for (w = pos; w >= 0; w--) {
      if (!Character.isLetter(content.charAt(w))) {
        break;
      }
    }

    // Too few chars
    if (pos - w < 2)
      return;

    String prefix = content.substring(w + 1).toLowerCase();
    int n = Collections.binarySearch(keywords, prefix);
    if (n < 0 && -n <= keywords.size()) {
      String match = keywords.get(-n - 1);
      if (match.startsWith(prefix)) {
        // A completion is found
        String completion = match.substring(pos - w);
        // We cannot modify Document from within notification,
        // so we submit a task that does the change later
        SwingUtilities.invokeLater(new CompletionTask(completion, pos + 1));
      }
    } else {
      // Nothing found
      mode = Mode.INSERT;
    }
  }

  public class CommitAction extends AbstractAction {
    /**
     * 
     */
    private static final long serialVersionUID = 5794543109646743416L;

    @Override
    public void actionPerformed(ActionEvent ev) {
      if (mode == Mode.COMPLETION) {
        int pos = textField.getSelectionEnd();
        StringBuffer sb = new StringBuffer(textField.getText());
        sb.insert(pos, " ");
        textField.setText(sb.toString());
        textField.setCaretPosition(pos + 1);
        mode = Mode.INSERT;
      } else {
        textField.replaceSelection("\t");
      }
    }
  }

  private class CompletionTask implements Runnable {
    private String completion;
    private int position;

    CompletionTask(String completion, int position) {
      this.completion = completion;
      this.position = position;
    }

    public void run() {
      StringBuffer sb = new StringBuffer(textField.getText());
      sb.insert(position, completion);
      textField.setText(sb.toString());
      textField.setCaretPosition(position + completion.length());
      textField.moveCaretPosition(position);
      mode = Mode.COMPLETION;
    }
  }

}

Using the Code

private static final String COMMIT_ACTION = "commit";
JTextField mainTextField = new JTextField();

// Without this, cursor always leaves text field
mainTextField.setFocusTraversalKeysEnabled(false);
...
// Our words to complete
keywords = new ArrayList<String>(5);
        keywords.add("example");
        keywords.add("autocomplete");
        keywords.add("stackabuse");
        keywords.add("java");
Autocomplete autoComplete = new Autocomplete(mainTextField, keywords);
mainTextField.getDocument().addDocumentListener(autoComplete);

// Maps the tab key to the commit action, which finishes the autocomplete
// when given a suggestion
mainTextField.getInputMap().put(KeyStroke.getKeyStroke("TAB"), COMMIT_ACTION);
mainTextField.getActionMap().put(COMMIT_ACTION, autoComplete.new CommitAction());

If you want the list of keywords to change over time in your app (for example, if you're autocomplete-ing names of friends online in a chat app) then all you need to do is add a setter method to Autocomplete for List<String> keywords and update the keywords list with the list of Strings you want.