private void textfield_bastidorKeyTyped (java.awt.event.KeyEvent evt) {
    if (textfield_bastidor.getText().length() >= 15) {
        evt.consume();
        JOptionPane.showMessageDialog(rootPane, "El bastidor tiene una longitud de 15 caracteres.");
    }
}

private void textfield_matriculaKeyTyped (java.awt.event.KeyEvent evt) {
    Pattern pat = Pattern.compile("^[0-9]{4}[a-zA-Z]{3}$");
    Matcher mat = pat.matcher(textfield_matricula.getText());
    if (mat.matches()) {
        System.out.println("codigo de que es correcto");
    } else {
        System.out.println("codigo de que es incorrecto");
    }
}

KeyListener keyListener2 = new KeyListener() {
    public void keyPressed(KeyEvent keyEvent) {

    }

    public void keyReleased(KeyEvent keyEvent) {
        Pattern pat = Pattern.compile("^\d{4}[A-Z]{3}");
        Matcher mat = pat.matcher(textField2.getText());
        if (mat.matches()) {
            System.out.println("codigo de que es correcto");
        } else {
            System.out.println("codigo de que es incorrecto");
        }
    }

    public void keyTyped(KeyEvent keyEvent) {

    }
};

textField2.addKeyListener(keyListener2);

JLabel myLabel = new JLabel("El bastidor debe tener 15 caracteres");

getContentPane().add(myLabel, BorderLayout.WEST);
KeyListener keyListener = new KeyListener() {
    public void keyPressed(KeyEvent keyEvent) {

    }

    public void keyReleased(KeyEvent keyEvent) {
        if (textField1.getText().length() == 15) {
            myLabel.setVisible(false);
            //JOptionPane.showMessageDialog(rootPane, "El bastidor tiene una longitud de 15 caracteres.");
        } else {
            myLabel.setVisible(true);
        }
    }

    public void keyTyped(KeyEvent keyEvent) {

    }
};
textField1.addKeyListener(keyListener);