Java Swing How to - Enable scroll bars when JTextArea exceeds certain amount of lines

Question

We would like to know how to enable scroll bars when JTextArea exceeds certain amount of lines.

Answer

import java.awt.BorderLayout;
import java.awt.Color;
import java.awt.Dimension;
//  w  ww. j a  v a2s.co m
import javax.swing.JFrame;
import javax.swing.JPanel;
import javax.swing.JScrollPane;
import javax.swing.JTextArea;
import javax.swing.border.EmptyBorder;
import javax.swing.event.DocumentEvent;
import javax.swing.event.DocumentListener;

public class Main {

  public static void main(String[] args) {
    JPanel topPanel = new JPanel();
    topPanel.setPreferredSize(new Dimension(200, 200));
    topPanel.setBackground(Color.WHITE);

    JTextArea chatArea = new JTextArea();
    JScrollPane scrollPane = new JScrollPane(chatArea);

    JPanel mainPanel = new JPanel(new BorderLayout(5, 5));
    mainPanel.setBorder(new EmptyBorder(5, 5, 5, 5));
    mainPanel.add(topPanel, BorderLayout.CENTER);
    mainPanel.add(scrollPane, BorderLayout.SOUTH);

    chatArea.getDocument().addDocumentListener(new DocumentListener() {

      @Override
      public void insertUpdate(DocumentEvent e) {
        updateLineCount();
      }

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

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

      private void updateLineCount() {
        int lineCount = chatArea.getLineCount();
        if (lineCount <= 4) {
          chatArea.setRows(lineCount);
          mainPanel.revalidate();
        }
      }
    });

    JFrame f = new JFrame();
    f.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
    f.add(mainPanel);
    f.pack();
    f.setVisible(true);
  }
}