How to pass a local variable to an anonymous class?

Question

I need to pass the control variable of a for, as a parameter for a method. The problem, is that I am trying to do this according to the JRadioButton which was selected, I am creating them according to my for, and set events for them then.

The point is, inside the ActionEvent I cannot use the control variable for, unless it is final, however, this is not possible, since it can change "size" according to the columns of a table.

(Remark: on the if, in e.getActionCommand().equals(tituloColuna[i]) is an attempt to have the action applied only to the radioButton that has the same name that gives it condition).

Does anyone have any suggestions ?

I made a very simple example, just to illustrate the situation.

import java.awt.Dimension;
import java.awt.EventQueue;
import java.awt.event.ActionEvent;
import javax.swing.JComponent;
import javax.swing.JFrame;
import javax.swing.JPanel;
import javax.swing.JRadioButton;
import javax.swing.JTable;
import javax.swing.table.DefaultTableModel;

public class TesteFor extends JFrame {

    public static void main(String[] args) {
        EventQueue.invokeLater(()
                -> {
            TesteFor ts = new TesteFor();
            ts.setVisible(true);
        });
    }

    public TesteFor() {
        setTitle("Teste");
        add(componentesTela());
        setPreferredSize(new Dimension(375, 300));
        pack();
        setLocationRelativeTo(null);
        setDefaultCloseOperation(EXIT_ON_CLOSE);
    }

    private JComponent componentesTela() {
        JPanel jp = new JPanel();

        String tituloColuna[] = {"Coluna 01", "Coluna 02"};

        //Tabela apenas para indicar de onde veio os valores, não estou adicionando ela no exemplo.
        JTable tabela = new JTable();
        tabela.setModel(new DefaultTableModel(
                new Object[][]{
                    {null, null, null},
                    {null, null, null},
                    {null, null, null}
                },
                new String[]{
                    "", ""
                }
        ));

        int numeroColunas = tabela.getColumnModel().getColumnCount();

        for (int i = 0; i < numeroColunas; i++) {
            JRadioButton radio = new JRadioButton("Ocultar " + tituloColuna[i]);
            jp.add(radio);
            radio.setActionCommand(tituloColuna[i]); //e.getActionCommand() vai retornar o nome do radio clicado

            radio.addActionListener((ActionEvent e)
                    -> {
                //ERRO
                /*if (e.getActionCommand().equals(tituloColuna[i])) {
                    alterarTabela(i);
                    System.out.println("Peguei a " + e.getActionCommand());
                }*/
            });
        }
        return jp;
    }

    private void alterarTabela(int indiceColuna) {
        //alterar colunas de acordo com o indice.
    }
}

Answer 1

There are two ways to solve this scope problem: either you find a way to pass a final local variable to the anonymous class or you increase the scope of the variable at class level. I think the second option is not so useful, since you will apparently only use this variable to iterate the loop and nothing else.

It is also possible to circumvent this by creating a class that extends the Listener you want to apply, passing the value to this new class, but I believe that it is also not necessary to increase the complexity to this level according to the given example.

Then we can solve with the first form, which is to create a way to pass the loop iteration index to the anonymous class, so that the iteration can be incremented and the loop iteration variable does not need to be final. To do this, just make a copy of the current index inside the loop and pass this copy to the anonymous class, because it can be final, since we only care about its value until the end of the current iteration:

for (int i = 0; i < numeroColunas; i++) {
    JRadioButton radio = new JRadioButton("Ocultar " + tituloColuna[i]);
    jp.add(radio);
    radio.setActionCommand(tituloColuna[i]); //e.getActionCommand() vai retornar o nome do radio clicado
    final int currentIndex = i;
    radio.addActionListener(e -> {
            alterarTabela(currentIndex);
            System.out.println("Peguei a " + e.getActionCommand());
    });
}

Thus, changes to your code are minimal, solving the scope problem.

Within the example given in the question, this is the least laborious suggestion and that will least change your code.