ActionListener

public class CustomListener implements ActionListener {

    public void actionPerformed(ActionEvent e) {
        if (e.getSource() == b1) {
            //ВОТ ЭТО выводит мне одну цифру в поле а я хочу чтоб при нажатие был не 0 а 0000000
            // t1.setText("" + p);
            // Что мешает сделать так?
            t1.setText(buildYourString("p", 10));
        }
    }

    private String buildYourString(String pattern, int times) {
        return pattern.repeat(times);
    }
}

public static String repeatNTimes(String string, int times) {
    StringBuilder builder = new StringBuilder(string);
    if (times > 1) {
        for (int i = 0; i < times - 1; i++) {
            builder.append(string);
        }
    }

    return builder.toString();
}

StringBuilder builder = new StringBuilder();

public void actionPerformed(ActionEvent e) {
    if (e.getSource() == b1) {
        builder.append("0");
    }
    if (e.getSource() == b2) {
        builder.append("1");
    }
    // other appenders

    textArea.setText(builder.toString());
}

public class SimpleCalc extends JFrame implements ActionListener {

    private final JButton[] buttons = new JButton[16];
    private final JTextField textField = new JTextField();
    private final StringBuilder builder = new StringBuilder();

    public static void main(String[] args) {
        SwingUtilities.invokeLater(SimpleCalc::new);
    }

    public SimpleCalc() {
        setTitle("Calculator");
        setSize(400, 600);
        setDefaultCloseOperation(EXIT_ON_CLOSE);
        setLayout(new BorderLayout());

        textField.setEditable(false);
        textField.setFont(new Font("Monospaced", Font.PLAIN, 24));
        textField.setHorizontalAlignment(SwingConstants.RIGHT);
        textField.setBorder(BorderFactory.createEmptyBorder(10, 10, 10, 10));
        add(textField, BorderLayout.NORTH);

        JPanel buttonPanel = new JPanel(new GridLayout(4, 4, 5, 5));
        buttonPanel.setBorder(BorderFactory.createEmptyBorder(10, 10, 10, 10));

        String[] labels = {
                "7", "8", "9", "/",
                "4", "5", "6", "*",
                "1", "2", "3", "-",
                "0", ".", "=", "+"
        };

        for (int i = 0; i < buttons.length; i++) {
            buttons[i] = new JButton(labels[i]);
            buttons[i].setFont(new Font("Arial", Font.BOLD, 18));
            buttons[i].addActionListener(this);
            buttonPanel.add(buttons[i]);
        }

        add(buttonPanel, BorderLayout.CENTER);
        setVisible(true);
    }

    @Override
    public void actionPerformed(ActionEvent e) {
        String command = e.getActionCommand();
        if (command.equals("=")) {
            // calculation logic here
        } else {
            builder.append(command);
        }
        textField.setText(builder.toString());
    }
}