限定 JTextArea 行數

import java.awt.*;
import javax.swing.*;
import java.awt.BorderLayout;
import java.awt.event.KeyEvent;
import java.awt.event.KeyListener;
import javax.swing.text.*;

public class Main extends JFrame implements KeyListener {
  private JPanel contentPane;
  private BorderLayout borderLayout1 = new BorderLayout();
  private JTextArea jTextArea1 = new JTextArea();
  private int rows = 3; // 限定行數

  public Main() {
    try {      
      setSize(new Dimension(400, 300));
      setDefaultCloseOperation(EXIT_ON_CLOSE);
      contentPane = (JPanel) getContentPane();
      contentPane.setLayout(borderLayout1);
      contentPane.add(jTextArea1, java.awt.BorderLayout.CENTER);
      jTextArea1.addKeyListener(this);
      setVisible(true);
    }
    catch (Exception e) {
      e.printStackTrace();
    }
  }

  public void keyTyped(KeyEvent e) {
    int lineCount = jTextArea1.getLineCount();
    // 限定 TextArea 行數
    if (rows < lineCount) {
      try {
        int lineStartOffset = jTextArea1.getLineStartOffset(lineCount - rows);
        jTextArea1.replaceRange("", 0, lineStartOffset);
      }
      catch (BadLocationException ex) {
        ex.printStackTrace();
      }
    }
  }

  public void keyPressed(KeyEvent e) {
  }

  public void keyReleased(KeyEvent e) {
  }
  
  public static void main(String[] args) {
    new Main();    
  }

}