JFrame frame = new JFrame();

JTextPane jTextPane = new JTextPane();
jTextPane.setFont(new Font("Cambria Math", 0, 12));

JTextArea area = new JTextArea();
area.setBorder(BorderFactory.createLineBorder(Color.black));

jTextPane.getDocument().insertString(jTextPane.getDocument().getLength(), "a", null);
jTextPane.getDocument().insertString(jTextPane.getDocument().getLength(), "uD835uDD38", null);
jTextPane.getDocument().insertString(jTextPane.getDocument().getLength(), "b", null);

jTextPane.insertComponent(area);
frame.add(jTextPane);
frame.setSize(100, 100);
frame.setVisible(true);