Reset JTable depending on content (number of rows)

Question

I have a JTable inside a JPanel which in turn is in a Jframe. The JTable loads users from a table (database in MySQL).

I have a search engine using DNIs in which key-to-key, with a KeyTyped event, is updating the contacts in the table, and show only those that meet the browser pattern (JTextField). If there are only 2-3-4 clients, the table is not resized to the customer size, but fills the rest of the table with a gray background. (see image). How could the lower bound of the table be reset?

Code:

public class Listado_clientes1 extends javax.swing.JFrame{
    private TableRowSorter<DefaultTableModel> TRSFiltro;
    
    public Listado_clientes1() {
        this.getContentPane().setBackground(Color.orange);
        panel_top.setBackground(Color.orange);
        tabla_clientes.setPreferredScrollableViewportSize(
                new Dimension(tabla_clientes.getPreferredSize().width, tabla_clientes.getRowHeight()*20)
        );

        try {
            DefaultTableModel modelo = new DefaultTableModel();
            tabla_clientes.setModel(modelo);

            PreparedStatement ps = null;
            ResultSet rs = null;
            Connection con = Conexiones.conexion_a_BBDD("agenda");

            String sql = "SELECT dni, nombre, apellidos, telefono, direccion, ciudad, email FROM clientes";
            ps = con.prepareStatement(sql);
            rs = ps.executeQuery();

            ResultSetMetaData rsMd = (ResultSetMetaData) rs.getMetaData();
            int cantidadColumnas = rsMd.getColumnCount();

            modelo.addColumn("DNI");
            modelo.addColumn("NOMBRE");
            modelo.addColumn("APELLIDOS");
            modelo.addColumn("TLFONO");
            modelo.addColumn("DIRECCION");
            modelo.addColumn("CIUDAD");
            modelo.addColumn("EMAIL");
            
            while (rs.next()) {
                Object[] filas = new Object[cantidadColumnas];
                for (int i=0; i<cantidadColumnas; i++) {
                    filas[i] = rs.getObject(i+1);
                }
                modelo.addRow(filas);
            }
        } catch (SQLException ex) {
            System.err.println(ex.toString());
        }
    }

    /**
     * Filtrar: Buscar por DNI.
     */
    public void filtrar_dni() {
        int columna = 0; //Es la fila del DNI.
        TRSFiltro.setRowFilter(RowFilter.regexFilter(textfield_buscar.getText(), columna));
    }
    
    
    private void textfield_buscarKeyTyped(java.awt.event.KeyEvent evt) {                                          
        Character letra = evt.getKeyChar();
        evt.setKeyChar(Character.toUpperCase(letra));

        textfield_buscar.addKeyListener(new KeyAdapter(){
        public void keyReleased(final KeyEvent e){
            String texto = (textfield_buscar.getText());
            textfield_buscar.setText(texto);
            filtrar_dni();
        }
        });
        
        TRSFiltro = new TableRowSorter<DefaultTableModel>((DefaultTableModel) tabla_clientes.getModel());
        tabla_clientes.setRowSorter(TRSFiltro);
    }
}

Outline / Scheme:

Answer 1

Any time the number of rows in the view of the table changes you need to recalculate the preferred scrollable viewport size of the table. Once this size is recalculated you need to invoke the layout manager of the panel containing the table:

A reusable method would be something like:

public void resetTablePreferredScrollableViewportSize(JTable table, int maxRows)
{
    Dimension size = table.getPreferredSize();
    int displayRows = Math.min(table.getRowCount(), maxRows);
    size.height = displayRows * table.getRowHeight();

    table.setPreferredScrollableViewportSize( size );

    Container parent = SwingUtilities.getAncestorOfClass(JPanel.class, table);
    parent.revalidate();
    parent.repaint();
}

So after setting the row filter you could use:

tabla_clientes.setRowSorter(TRSFiltro);
resetTablePreferredScrollableViewportSize(tabla_clientes, 10);

Now the scroll pane should be sized to display up to 10 rows after which the scrollbar of the scroll pane will appear.

Edit:

I don't know how to put a reproducible example

Then you don't understand what your problem is.

Your question is that you want to:

Reset JTable (size) depending on number of rows

So the data is irrelevant, only the number of rows is relevant.

It is easy to test this because you can create a DefaultTableModel with a specified number of rows.

Below is a simple MRE. To test you:

1. enter a number in the text field
2. press enter

The table will be resized to display the specified number of rows. If the number is greater that 10 the table size will be fixed and a scrollbar will appear:

import java.awt.*;
import javax.swing.*;
import java.awt.event.*;
import javax.swing.table.*;

public class SSCCE extends JPanel
{
    JTextField textField;
    JTable table;

    public SSCCE()
    {
        setBackground(Color.YELLOW);

        textField = new JTextField(5);
        add(textField);

        table = new JTable(new DefaultTableModel(0, 4));
        table.setPreferredScrollableViewportSize(table.getPreferredSize());
        add( new JScrollPane(table) );

        textField.addActionListener((e) ->
        {
            int rows = Integer.parseInt( textField.getText() );

            table.setModel( new DefaultTableModel(rows, 4) );
            resetTablePreferredScrollableViewportSize(table, 10);
        });
    }

    public void resetTablePreferredScrollableViewportSize(JTable table, int maxRows)
    {
        Dimension size = table.getPreferredSize();
        int displayRows = Math.min(table.getRowCount(), maxRows);
        size.height = displayRows * table.getRowHeight();

        table.setPreferredScrollableViewportSize( size );

        Container parent = SwingUtilities.getAncestorOfClass(JPanel.class, table);
        parent.revalidate();
        parent.repaint();
    }

    private static void createAndShowGUI()
    {
        JFrame frame = new JFrame("SSCCE");
        frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        frame.add(new SSCCE());
        frame.setDefaultCloseOperation(WindowConstants.EXIT_ON_CLOSE);
//      frame.pack();
        frame.setSize(500, 250);
        frame.setLocationByPlatform( true );
        frame.setVisible( true );
    }

    public static void main(String[] args) throws Exception
    {
        java.awt.EventQueue.invokeLater( () -> createAndShowGUI() );
    }
}