Backspace stopped working in any JTextField in Java Swing after adding the custom class for IP text field with KeyListener added to it and only to it

After adding the below code, Backspace key stops working at any JTextFields, when I press Backspace, nothing happens.

import javax.swing.*;
import javax.swing.text.DefaultEditorKit;
import java.awt.*;
import java.awt.event.*;
import java.net.InetAddress;
import java.net.UnknownHostException;

public class JIp4AddressInput extends JTextField
{
    private final char[] buff = "  0.  0.  0.  0".toCharArray();

    private int bpos;

    private void putnum (int num, int offset)
    {
        int a = num/100;
        num -= a*100;
        int b = num/10;
        num -= b*10;
        buff[offset] = (char)('0'+a);
        buff[offset+1] = (char)('0'+b);
        buff[offset+2] = (char)('0'+num);
    }

    private void align (int base)
    {
        int end = base+3;
        StringBuffer sb = new StringBuffer();
        for (int s=base; s<end; s++)
        {
            if (buff[s] != ' ')
                sb.append(buff[s]);
        }
        while (sb.length() > 1 && sb.charAt(0) == '0')
            sb.delete(0,1);
        while (sb.length() < 3)
            sb.insert(0, ' ');
        try
        {
            int num = Integer.parseInt(sb.toString().trim());
            if (num > 255)
                sb = new StringBuffer("255");
            if (num < 0)
                sb = new StringBuffer("  0");
        }
        catch (NumberFormatException e)
        {
            sb = new StringBuffer("  0");
        }
        for (int s=base; s<end; s++)
        {
            buff[s] = sb.charAt(s-base);
        }
    }

    private void alignAll()
    {
        align(0);
        align (4);
        align(8);
        align (12);
    }

    private void fwd ()
    {
        bpos = bpos == 15 ? bpos : bpos +1;
    }

    private void back ()
    {
        bpos = bpos == 0 ? bpos : bpos -1;
    }

    private void backspace()
    {
        back();
        if (bpos == 3 || bpos == 7 || bpos == 11)
        {
            return;
        }
        if (bpos < 15)
            buff[bpos] = ' ';
    }

    private void setChar (char c)
    {
        if (bpos == 3 || bpos == 7 || bpos == 11)
        {
            fwd();
        }
        if (bpos < 15)
            buff[bpos] = c;
        fwd();
    }

    public JIp4AddressInput()
    {
        super();
        setPreferredSize(new Dimension(110, 30));
        setEditable(false);

        Action beep = getActionMap().get(DefaultEditorKit.deletePrevCharAction);
        beep.setEnabled (false);

        setText (new String (buff));

        addFocusListener(new FocusListener()
        {
            @Override
            public void focusGained(FocusEvent e)
            {
                setText (new String (buff));
                setCaretPosition(0);
                getCaret().setVisible(true);
            }

            @Override
            public void focusLost(FocusEvent e)
            {
                alignAll();
                setText(new String(buff));
            }
        });

        addKeyListener(new KeyAdapter()
        {
            @Override
            public void keyTyped (KeyEvent e)
            {
                bpos = getCaretPosition();
                char c = e.getKeyChar();
                if ((c>= '0' && c<= '9') || c == ' ')
                {
                    setChar (c);
                }
                else if (c == KeyEvent.VK_BACK_SPACE)
                {
                    backspace();
                }
                else if (c == KeyEvent.VK_ENTER)
                {
                    alignAll();
                }
                setText(new String(buff));
                setCaretPosition(bpos);
            }
        });
    }

    ////////////////////////////////////////////////////////////////////////////////////////

    public InetAddress getAddress()
    {
        String[] parts = new String(buff).split("\\.");
        byte[] adr = new byte[4];
        for (int s=0; s<4; s++)
            adr[s] = (byte)Integer.parseInt(parts[s].trim());
        try {
            return InetAddress.getByAddress(adr);
        } catch (UnknownHostException e) {
            return null;
        }
    }

    public void putAddress (InetAddress in)
    {
        byte[] adr = in.getAddress();
        putnum(adr[0]&0xff, 0);
        putnum(adr[1]&0xff, 4);
        putnum(adr[2]&0xff, 8);
        putnum(adr[3]&0xff, 12);
        alignAll();
        setText (new String(buff));
    }
}

It is strange, as KeyListener in the code is applied to specific field and not other JTextField. With debugger I have checked, that the KeyListener inner code isn't triggered when in other fields I press Backspace. But anyway it isn't executed and not applied to that fields, just ignores it.

Java 11.

Probably you are deactivating the backspace with these lines:

Action beep = getActionMap().get(DefaultEditorKit.deletePrevCharAction);
beep.setEnabled (false);

Since the modification happens on the DefaultEditorKit it is likely global, not just for the JTextField that you are looking at.