public class LoadTxt extends JFrame {

    private static final String FILE_NAME = "C:\\Hello.java"; // 文件名，可修改
    JButton btnLoad;
    JTextArea txtArea;

    public LoadTxt() {
        this.setTitle("Text File Loader");

        // 按钮，放在JFrame下方（南部区域）
        btnLoad = new JButton("Click Here to Load File");
        btnLoad.addActionListener(new ActionListener() {
            public void actionPerformed(ActionEvent e) {
                System.out.println(e.getActionCommand());
                loadFile(FILE_NAME);
            }
        });
        this.add(btnLoad, BorderLayout.SOUTH);

        // 文本区，放在JFrame中部区域
        txtArea = new JTextArea();
        this.add(txtArea, BorderLayout.CENTER);
    }

    private void loadFile(String filename) {
        Scanner sc = null;
        try {
            sc = new Scanner(new File(filename));
            txtArea.setText(""); // 清空
            while (sc.hasNextLine()) {
                txtArea.append(sc.nextLine());
                txtArea.append("\n");
            }
        } catch (FileNotFoundException e) {
            e.printStackTrace();
        } finally {
            if (sc != null)
                sc.close();
        }
    }

    public static void main(String[] args) {
        LoadTxt wnd = new LoadTxt();
        wnd.setSize(600, 500);
        wnd.setVisible(true);
    }

}

------解决方案--------------------

示例参见 %JAVA_HOME%\demo\jfc\Notepad\Notepad.jar
代码参见 %JAVA_HOME%\demo\jfc\Notepad\src