How can I show a value in a jTextField when selecting a Jcheckbox at runtime?

Question

private void btCalcularActionPerformed(java.awt.event.ActionEvent evt) {                                           
    double salario = 0, irrf = 0, inss = 0, valorTotal = 0;

    FolhaPagamento pagamento = new FolhaPagamento(Double.parseDouble(txtValorSalario.getText()));

    salario = pagamento.getSalario();

    if(cbIRRF.isSelected()){
        irrf = salario * 0.17;
        txtIRRF.setText(String.valueOf(irrf));
    }
    else txtIRRF.setText("");

    if(cbINSS.isSelected()){
        inss = salario * 0.05;
        txtINSS.setText(String.valueOf(inss));
    }
    else txtINSS.setText("");

    valorTotal = salario + irrf + inss;
    txtValorTotal.setText(String.valueOf(valorTotal));

}

The user enters any numerical value related to the employee’s salary and can click on the checkboxes (JCheckBox) relating to incidental contributions. If the IRRF box is checked, the value of the Withheld Income Tax should appear in the corresponding Source, that is, 17% of the salary value. Similarly, if the INSS box is checked, the corresponding INSS value should appear (5%).

Answer 1

Just add an event to your checkbox, an actionPerformed is ideal, see the example below.

    JFrame frame = new JFrame("teste");
    JPanel painel = new JPanel(new GridLayout(1, 2));
    JTextField field = new JTextField();
    JCheckBox checkBox = new JCheckBox("Me selecione");
    checkBox.addActionListener(((e) -> {
        if (checkBox.isSelected()) {
            field.setText("Voce clicou o Checkbox Parabens!");
        } else {
            field.setText("");
        }

    }));
    painel.add(checkBox);
    painel.add(field);
    frame.add(painel);
    frame.setSize(500, 50);
    frame.setVisible(true);
    frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);