Java text field for credit card number only showing last 4 digits

I have to do a supermarket app in Java for a class, one of the many requirements is that when a purchase is being made it asks for the credit/debit card data, and when the user fills the credit card number field it only displays the last four digits, something like XXXXXXXXXXXX1456

I am using Java Swing for the GUI and I have tried to use the formatted field, the password field and a normal field and I can´t seem to find a way to make it only display the last four digits when is being filled.

A JFormattedTextField has a formatter factory. I looked at the code of the default formatter factory – which is class DefaultFormatterFactory – and tried to adapt it to your requirements.

That class defines several formatter classes: one for when the JFormattedTextField is in edit mode, i.e. it has the keyboard focus and another for display mode, i.e. when the JFormattedTextField does not have keyboard focus.

In the below code I created a CreditCardFormatterFactory class as well as a CreditCardEditFormatter class and a CreditCardDisplayFormatter class.

Note that I may have misunderstood your requirements, so the below code may not be exactly what you require but I would like to think that it is good enough for you to be able to adapt it to your exact requirements.

import java.awt.BorderLayout;
import java.awt.EventQueue;
import java.text.ParseException;

import javax.swing.JButton;
import javax.swing.JFormattedTextField;
import javax.swing.JFormattedTextField.AbstractFormatter;
import javax.swing.JFrame;
import javax.swing.JLabel;
import javax.swing.JPanel;

public class FtfTest {
    private void buildAndDisplayGui() {
        JFrame frame = new JFrame();
        frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        frame.add(createFormattedTextField(), BorderLayout.PAGE_START);
        frame.add(createButton(), BorderLayout.PAGE_END);
        frame.pack();
        frame.setLocationByPlatform(true);
        frame.setVisible(true);
    }

    private JPanel createFormattedTextField() {
        JPanel panel = new JPanel();
        JLabel label = new JLabel("Credit Card Number");
        panel.add(label);
        JFormattedTextField creditCardFormattedTextField = new JFormattedTextField(
                                                                 new CreditCardFormatterFactory());
        creditCardFormattedTextField.setColumns(16);
        panel.add(creditCardFormattedTextField);
        return panel;
    }

    private JPanel createButton() {
        JPanel panel = new JPanel();
        JButton button = new JButton("Exit");
        button.addActionListener(e -> System.exit(0));
        panel.add(button);
        return panel;
    }

    public static void main(String[] args) {
        EventQueue.invokeLater(() -> new FtfTest().buildAndDisplayGui());
    }
}

class CreditCardFormatterFactory extends JFormattedTextField.AbstractFormatterFactory {

    /** {@code JFormattedTextField.AbstractFormatter} to use for display. */
    private JFormattedTextField.AbstractFormatter displayFormat;

    /** {@code JFormattedTextField.AbstractFormatter} to use for editing. */
    private JFormattedTextField.AbstractFormatter editFormat;

    /** {@code JFormattedTextField.AbstractFormatter} to use if the value is null. */
    private JFormattedTextField.AbstractFormatter nullFormat;

    public CreditCardFormatterFactory() {
        displayFormat = new CreditCardDisplayFormatter();
        editFormat = new CreditCardEditFormatter();
    }

    @Override
    public AbstractFormatter getFormatter(JFormattedTextField ftf) {
        JFormattedTextField.AbstractFormatter format = null;
        if (ftf != null) {
            if (ftf.hasFocus()) {
                format = getEditFormatter();
            }
            else {
                format = getDisplayFormatter();
            }
        }
        return format;
    }

    public JFormattedTextField.AbstractFormatter getDisplayFormatter() {
        return displayFormat;
    }

    public JFormattedTextField.AbstractFormatter getEditFormatter() {
        return editFormat;
    }

    public JFormattedTextField.AbstractFormatter getNullFormatter() {
        return nullFormat;
    }
}

class CreditCardDisplayFormatter extends JFormattedTextField.AbstractFormatter {

    @Override
    public Object stringToValue(String text) throws ParseException {
        System.out.printf("CreditCardDisplayFormatter.stringToValue( ^%s^ )%n", text);
        return text;
    }

    @Override
    public String valueToString(Object value) throws ParseException {
        System.out.printf("CreditCardDisplayFormatter.valueToString( ^%s^ )%n",
                          String.valueOf(value));
        String str = value == null ? "" : value.toString();
        if (str.length() > 0) {
            if (str.length() > 12) {
                str = "XXXXXXXXXXXX" + str.substring(12);
            }
            else {
                str = str.replaceAll("\\d", "X");
            }
        }
        return str;
    }
}

class CreditCardEditFormatter extends JFormattedTextField.AbstractFormatter {

    @Override
    public Object stringToValue(String text) throws ParseException {
        System.out.printf("CreditCardEditFormatter.stringToValue( ^%s^ )%n", text);
        return text;
    }

    @Override
    public String valueToString(Object value) throws ParseException {
        System.out.printf("CreditCardEditFormatter.valueToString( ^%s^ )%n",
                          String.valueOf(value));
        return value == null ? null : value.toString();
    }
}

String number = "6235761255231456";
String mask = number.replaceAll("\\d(?=\\d{4})", "X");
System.out.println(mask);
OutPut:
XXXXXXXXXXXX1456

here replace we are using replaceAll method which will take regular expression as parameter and and will replace every digit with X except last four digit the following regular expression is accepting all except last 4 digit.