CreditCardPanel

import java.awt.Dimension;

import javax.swing.BoxLayout;
import javax.swing.JPanel;
import javax.swing.JTextField;
import javax.swing.event.DocumentEvent;
import javax.swing.text.AttributeSet;
import javax.swing.text.BadLocationException;
import javax.swing.text.PlainDocument;


/**
 * Encapsulates the behaviour required
 * for credit card number entry, allowing
 * only digits to be entered and shifting
 * focus when one of the four fields is filled
 *
 * @author Charles Johnson
 * @version 1.0
 */
class CreditCardPanel extends JPanel {
    private JTextField[] fields = new JTextField[4];

    // Don't let the fields grow to an ugly size
    Dimension MAX_SIZE = new Dimension(1000, 25);

    public CreditCardPanel() {
        setLayout(new BoxLayout(this, BoxLayout.LINE_AXIS));

        for (int i = 0; i < fields.length; i++) {
            fields[i] = new JTextField(4);
            fields[i].setDocument(new FourDigitDocument(i));
            fields[i].setMaximumSize(MAX_SIZE);
            add(fields[i]);
        }
    }

    /**
     *
     * @return the credit card number
     */
    public String getNumber() {
        StringBuilder number = new StringBuilder(16);
        boolean allFilled = true;

        for (int i = 0; i < fields.length; i++) {
            if (!isFilled(fields[i])) {
                allFilled = false;

                break;
            } else {
                number.append(fields[i].getText());
            }
        }

        if (!allFilled) {
            throw new IllegalStateException("Credit card panel not filled");
        } else {
            return number.toString();
        }
    }

    private boolean isFilled(JTextField f) {
        String text = f.getText();

        return (text != null) && (text.length() == 4);
    }

    /**
     * Allows four digits only
     *
     * @author Charles Johnson
     * @version 1.0
      */
    private class FourDigitDocument extends PlainDocument {
        private int index;

        public FourDigitDocument(int index) {
            this.index = index;
        }

        /**
         *
         * @param offs the starting offset >= 0
         * @param s the string to insert; does nothing with null/empty strings
         * @param as the attributes for the inserted content
         *
         * @throws BadLocationException the given insert position is not a valid position within the document
         */
        @Override
        public void insertString(int offs, String s, AttributeSet as)
            throws BadLocationException {
            if ((s == null) || "".equals(s)) {
                return;
            }

            StringBuilder sb = new StringBuilder(getLength() + s.length());
            String newText = sb.append(getText(0, getLength())).insert(offs, s)
                               .toString();

            // Only allow four digits
            boolean valid = newText.matches("\\d{1,4}");

            if (valid) {
                super.insertString(offs, s, as);
            }

            // Jump to next field if full
            if ((newText.length() == 4) && valid && (index <= 2)) {
                fields[++index].requestFocus();
            }
        }
    }
}