Java Swing TextField Integer Value issue

Question

I'm making a GUI. In which user enter year in integers form and that year display on GUI. But the problem is i want to only enter 4 integers from user. If user enter 5 integers value then it show JOptionPane message please type again. But i don't know how to do it.

Code:

public class A extends JFrame{


    private JTextField tx;
    private JLabel year;
    private JButton bt;
    private JLabel at;


    public A(){
        super("Frame");


        getContentPane().setLayout(null);


        tx= new JTextField();
        tx.setBounds(150, 165, 128, 27);
        getContentPane().add(tx);   

        year= new JLabel("Enter Year :");
        year.setBounds(178, 133, 69, 27);
        getContentPane().add(year);


        at= new JLabel();
        at.setFont(new Font("Tahoma", Font.PLAIN, 17));
        at.setBounds(165, 295, 189, 27);
        getContentPane().add(at);


        bt= new JButton("Submit");
        bt.setBounds(178, 203, 84, 27);
        getContentPane().add(bt);
        bt.addActionListener(new ActionListener(){
            public void actionPerformed(ActionEvent arg0) {

                String w = tx.getText();
                int p = Integer.parseInt(w);

                 at.setText(""+p);

            }

        });

        setSize(450,450);
        setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        setLocationRelativeTo(null);
        setResizable(false);
        setVisible(true);


    }

}

Main

public class Main {

    public static void main(String[] args) {

        A obj = new A();


    }

} 

Answer 1

    String w = tx.getText();
            int p = Integer.parseInt(w);

            String e=String.valueOf(w);
    int m = e.length();
    System.out.println(m);

    if(e.length()==4){


             at.setText(""+p);
    }
    else{
        JOptionPane.showMessageDialog(null, "Wrong");
    }
        }   

    });

Guys does this thing is good for pratice