JTable中可编辑JComboBox的占位符

Question

I have a JTable in which one column (Column 1) is a JComboBox that allows options to be made from a list, and to which new options can be entered. MWE:

import javax.swing.DefaultCellEditor;
import javax.swing.JComboBox;
import javax.swing.JFrame;
import javax.swing.JPanel;
import javax.swing.JScrollPane;
import javax.swing.JTable;
import javax.swing.table.TableColumn;

import java.awt.Dimension;
import java.awt.GridLayout;
import java.awt.event.ActionEvent;
import java.awt.event.ActionListener;
import java.util.Arrays;
import java.util.HashSet;

public class ComboTableDemo extends JPanel {

    public ComboTableDemo() {
        super(new GridLayout(1,0));
        final String[] headings = {"Name", "Option"};
        final String string1 = "Foo";
        final String string2 = "Bar";
        Object[][] data = {
                {"Albert", string1},
                {"Bob", null},
                {"Clare", null},
                {"David", null}
        };

        final JTable table = new JTable(data, headings);
        table.setPreferredScrollableViewportSize(new Dimension(300, 100));
        table.setFillsViewportHeight(true);

        final String[] optionsInit = new String[] {string1, string2};
        HashSet<String> options = new HashSet<String>(Arrays.asList(optionsInit));
        JComboBox<String> optionsCombo = new JComboBox<String>(optionsInit);

        optionsCombo.addActionListener(new ActionListener() {

            @Override
            public void actionPerformed(ActionEvent ev) {
                String newSelection = (String)optionsCombo.getSelectedItem();
                if(!options.contains(newSelection)) {
                    options.add(newSelection);
                    optionsCombo.addItem(newSelection);
                }
            }

        });
        optionsCombo.setEditable(true);
        TableColumn column = table.getColumnModel().getColumn(1);
        column.setCellEditor(new DefaultCellEditor(optionsCombo));

        add(new JScrollPane(table));
    }

    public static void main(String[] args) {
        javax.swing.SwingUtilities.invokeLater(new Runnable() {
            public void run() {
                JFrame frame = new JFrame("ComboTableDemo");
                frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
                ComboTableDemo pane = new ComboTableDemo();
                pane.setOpaque(true);
                frame.setContentPane(pane);
                frame.pack();
                frame.setVisible(true);
            }
        });
    }
}

In the table it isn't obvious that the user can and should enter values for the empty entries, so I'd like to include placeholder text to make this clear. I've seen elsewhere that a custom ListCellRenderer can be provided with setRenderer in the case of uneditable combos, but in the case of editable combos (as explained in the tutorial ) it appears that a ComboBoxEditor must be supplied using setEditor . Is there a simple implementation for this, or perhaps even a better way to achieve the same ends?

Answer 1

Actually, input components within a JTable present a special case, since they are only active when the cell is edited. So as well as setting the editor to control how a value is edited, you also need to change the renderer to control how the chosen value is presented, with

    // As before:
    TableColumn column = table.getColumnModel().getColumn(1);
    column.setCellEditor(new DefaultCellEditor(optionsCombo));
    // Additional line to set renderer:
    column.setCellRenderer(new PlaceholderRenderer("<choose or add option>"));

Here the PlaceholderRenderer should be a TableCellRenderer implementation that displays the placeholder string when no value is selected. Eg:

import java.awt.Component;
import javax.swing.JTable;
import javax.swing.table.DefaultTableCellRenderer;

public class PlaceholderRenderer extends DefaultTableCellRenderer {

    final private String placeholder;

    public PlaceholderRenderer(String placeholder) {
        super();
        this.placeholder = placeholder;
    }

    @Override
    public Component getTableCellRendererComponent(JTable table,
            Object value,
            boolean isSelected,
            boolean hasFocus,
            int row,
            int column) {
        if ((value == null) || (value.equals(""))) { 
            return super.getTableCellRendererComponent(table, this.placeholder, isSelected, hasFocus, row, column);  
        } else { 
            return super.getTableCellRendererComponent(table, value, isSelected, hasFocus, row, column);  
        }
    }

}