How do I define column size?

Question

I have a query screen, and I would like to define the size of the columns, because some columns take up more space than necessary. I thought that if I made a comparison with the title of the column, although it might not be the most oriented way to do this, I could do several "if" defining the sizes I wanted.

To reinforce understanding, the problem would be this, I would have for example columns such as, Code, Parents, Acronym and Status. Code would not need to have a column larger than 50, while parents at least should be 80. symbol and status no more than 60.

Below I will put a very simple example, just for understanding.

Tela Sistema:

package telas;

import java.awt.event.ActionEvent;
import java.awt.event.ActionListener;
import javax.swing.JDesktopPane;
import javax.swing.JFrame;
import javax.swing.JMenu;
import javax.swing.JMenuBar;
import javax.swing.JMenuItem;

public class TelaSistema extends JFrame implements ActionListener {

    public static JDesktopPane jdp = new JDesktopPane();
    public JMenuBar jmb = new JMenuBar();
    public JMenu jmCadastros = new JMenu("Cadastros");
    public JMenuItem jmiE = new JMenuItem("Uma tela qualquer");

    public TelaSistema() {
        setExtendedState(MAXIMIZED_BOTH);
        setTitle("Sistema");
        getContentPane().add(jdp);
        setJMenuBar(jmb);
        jmb.add(jmCadastros);
        adicionaJMenuItem(jmCadastros, jmiE);
        setDefaultCloseOperation(EXIT_ON_CLOSE);
        setVisible(true);
    }

    private void adicionaJMenuItem(JMenu menu, JMenuItem item) {
        menu.add(item);
        item.addActionListener(this);
    }

    @Override
    public void actionPerformed(ActionEvent ae) {
        if (ae.getSource() == jmiE) {
            TelaCadastroA a = new TelaCadastroA();
            jdp.add(a);
        }
    }

    public static void main(String args[]) {
        TelaSistema telaSistema = new TelaSistema();
    }
}

Tela Cadastro:

package telas;

import java.awt.Component;
import java.awt.Dimension;
import java.awt.GridLayout;
import java.awt.Toolkit;
import java.awt.event.ActionEvent;
import java.awt.event.ActionListener;
import java.text.DecimalFormat;
import javax.swing.JButton;
import javax.swing.JInternalFrame;
import javax.swing.JPanel;
import javax.swing.JTable;
import javax.swing.SwingConstants;
import javax.swing.table.DefaultTableCellRenderer;

public class TelaCadastroA extends JInternalFrame implements ActionListener {

    public JButton jbConsultar = new JButton("Consultar");
    public JPanel jpBotoes = new JPanel();

    public TelaCadastroA() {
        super("");
        adicionaBotao(jbConsultar);
        setSize(500, 500);
        getContentPane().add("South", jpBotoes);
        jpBotoes.setLayout(new GridLayout(1, 1));
        Dimension tamanhoTela = Toolkit.getDefaultToolkit().getScreenSize();
        setLocation((tamanhoTela.width - getWidth()) / 2, ((tamanhoTela.height - getHeight())) / 2);
        setVisible(true);
    }

    private void adicionaBotao(JButton botao) {
        jpBotoes.add(botao);
        botao.addActionListener(this);
    }

    @Override
    public void actionPerformed(ActionEvent ae) {
        if (ae.getSource() == jbConsultar) {
            consultar();
        }
    }

    public void consultar() {
        TelaConsultar.getTela("Consulta", new String[]{
            "Código", "Pais", "Sigla", "Status"
        }, new DefaultTableCellRenderer[]{null, null, new CellRenderMonetario(), null}, this);
    }

    class CellRenderMonetario extends DefaultTableCellRenderer {

        private DecimalFormat df = new DecimalFormat(",##0.00");

        @Override
        public Component getTableCellRendererComponent(JTable tabela, Object valor, boolean isSelected, boolean temFocu, int linha, int coluna) {
            if (valor == null) {
                valor = 0;
            }

            if (valor instanceof String) {
                setText(df.format(Double.parseDouble((String) valor)) + " ");
            } else {
                setText(df.format(valor) + " ");
            }
            setHorizontalAlignment(SwingConstants.RIGHT);
            return this;
        }
    }
}

Query screen:

package telas;

import java.awt.Dimension;
import java.awt.Toolkit;
import java.awt.event.MouseEvent;
import java.awt.event.MouseListener;
import javax.swing.JInternalFrame;
import javax.swing.JScrollPane;
import javax.swing.JTable;
import javax.swing.event.InternalFrameEvent;
import javax.swing.event.InternalFrameListener;
import javax.swing.table.DefaultTableCellRenderer;
import javax.swing.table.DefaultTableModel;
import javax.swing.table.TableColumnModel;

public class TelaConsultar extends JInternalFrame implements InternalFrameListener, MouseListener {

    private static TelaConsultar telaConsultar = null;
    private String sql;
    private String[] titulos;
    private TelaCadastroA tela;
    private JTable tabela;
    private JScrollPane jsp;
    private DefaultTableModel dtm = new DefaultTableModel();
    private String proc;

    public TelaConsultar(String tituloJanela, String[] titulosColuna, DefaultTableCellRenderer[] renderizadores, TelaCadastroA tela) {
        super(tituloJanela, true, true, false, false);
        this.sql = sql;
        this.titulos = titulosColuna;
        this.tela = tela;
        tabela = new JTable() {
            @Override
            public boolean isCellEditable(int linha, int coluna) {
                return false;//Esta tabela não pode ser editada pois é uma consulta         
            }
        };
        tabela.setModel(dtm); //Define o modelo da tabela
        tabela.getTableHeader().setReorderingAllowed(false);
        for (int i = 0; i < titulosColuna.length; i++) {
            dtm.addColumn(titulosColuna[i]);
        }
        jsp = new JScrollPane(tabela);
        getContentPane().add(jsp);
        preencher();
        //pack();
        setSize(650, 400);
        setMinimumSize(new Dimension(getWidth(), getHeight()));
        centralizaTelaConsulta();
        setVisible(true);
        tabela.addMouseListener(this);
        addInternalFrameListener(this);

        TableColumnModel tableModel = tabela.getColumnModel();

        if (titulosColuna.equals("Código")) {
            for (int i = 0; i < titulosColuna.length; i++) {
                tableModel.getColumn(i).setMinWidth(30);
                tableModel.getColumn(i).setMaxWidth(50);
            }
        }

        else
        {
            for (int i = 0; i < titulosColuna.length; i++) {
                tableModel.getColumn(i).setMinWidth(100);
            }
        }

        for (int i = 0; i < tableModel.getColumnCount(); i++) {
            if (renderizadores[i] != null) {
                tableModel.getColumn(i).setCellRenderer(renderizadores[i]);
            }

        }
    }

    public static void getTela(String tituloJanela, String[] titulos, DefaultTableCellRenderer[] renderizadores, TelaCadastroA tela) {
        if (telaConsultar == null) {
            telaConsultar = new TelaConsultar(tituloJanela, titulos, renderizadores, tela);
            TelaSistema.jdp.add(telaConsultar);
        }
        TelaSistema.jdp.setSelectedFrame(telaConsultar);
        TelaSistema.jdp.moveToFront(telaConsultar);
    }

    private void preencher() {
        for (int i = 0; i < 10; i++) {
            dtm.addRow(new Object[]{"" + i, "Brasil - " + i});
        }

    }

    public void centralizaTelaConsulta() {
        Dimension tamanhoTela = Toolkit.getDefaultToolkit().getScreenSize();
        setLocation((tamanhoTela.width - getWidth()) / 2, ((tamanhoTela.height - getHeight())) / 2);
    }

    @Override
    public void mousePressed(MouseEvent e) {
    }

    @Override
    public void mouseReleased(MouseEvent e) {
    }

    @Override
    public void mouseEntered(MouseEvent e) {
    }

    @Override
    public void mouseExited(MouseEvent e) {
    }

    @Override
    public void internalFrameOpened(InternalFrameEvent e) {
    }

    @Override
    public void internalFrameClosing(InternalFrameEvent e) {
    }

    @Override
    public void internalFrameClosed(InternalFrameEvent e) {
        TelaSistema.jdp.remove(telaConsultar);
        telaConsultar = null;
    }

    @Override
    public void internalFrameIconified(InternalFrameEvent e) {
    }

    @Override
    public void internalFrameDeiconified(InternalFrameEvent e) {
    }

    @Override
    public void internalFrameActivated(InternalFrameEvent e) {

    }

    @Override
    public void internalFrameDeactivated(InternalFrameEvent e) {
    }

    @Override
    public void mouseClicked(MouseEvent me) {

    }
}

Answer 1

Based on that another answer, i made an adaptation within your code, add just below TableColumnModel tableModel = tabela.getColumnModel();:

    for (int i = 0; i < tableModel.getColumnCount(); i++) {
        String columnName = tableModel.getColumn(i).getHeaderValue().toString();

        switch (columnName) {

            case "Código":
                tableModel.getColumn(i).setMaxWidth(50);
                tableModel.getColumn(i).setPreferredWidth(50);
                tableModel.getColumn(i).setMinWidth(50);
                break;
            case "Pais":
                tableModel.getColumn(i).setMaxWidth(Integer.MAX_VALUE);
                tableModel.getColumn(i).setPreferredWidth(80);
                tableModel.getColumn(i).setMinWidth(80);
                break;
            default:
                tableModel.getColumn(i).setMaxWidth(60);
                tableModel.getColumn(i).setPreferredWidth(60);
                tableModel.getColumn(i).setMinWidth(60);

        }
    }

The result was:

Bearing in mind that the column Pais has minimum size but not a predefined maximum size, I set it to take the maximum size available on the screen.

Remembering that as you are not managing the Layoutmanager of the screen where the table was inserted, it will always tend to fully fill the width, because this is the behavior of LayoutManager standard of the internal panel of the JFrame, which is the BorderLayout.