LordFarquaad LordFarquaad - 3 months ago 8
Java Question

Why does setPreferredSize() sometimes take precedent over setMinimumSize()?

I had an error yesterday where I added a

JTable
and a
JPanel
(with a
JButton
in it) to a
JScrollPane
. The
JButton
was fixed to the bottom of the table, and it added a row to the
JTable
when clicked.

The problem was if the table ever got bigger than the
JScrollPane
, it would only allow you to scroll to the bottom of the
JTable
; you couldn't get to the
JButton
anymore. Today, I made an MCVE to try and get help, but first I monkeyed with it a bit more and ended up fixing my problem, but in a way that left me with more questions than answers... Here's the MCVE I had prepared:

import java.awt.Color;
import java.awt.Dimension;
import java.awt.GridBagConstraints;
import java.awt.GridBagLayout;
import java.awt.event.*;
import java.util.*;

import javax.swing.*;
import javax.swing.table.DefaultTableModel;

public class MCVE extends JFrame{

private JButton addRow;
private MCVEModel tableModel;
private JTable table;
private JScrollPane pane;
private JPanel scrollPanel, panel;

public static void main (String[] args) {
new MCVE();
}

public MCVE() {
initialize();
}

public void initialize () {
this.setTitle("Halp");
this.setDefaultCloseOperation(EXIT_ON_CLOSE);
this.setBounds(50, 50, 500, 300);
this.setResizable(false);

JPanel mainPanel = new JPanel(new GridBagLayout());

/** The JPanel everything is put into **/
scrollPanel = new JPanel();
scrollPanel.setLayout(new BoxLayout(scrollPanel, BoxLayout.Y_AXIS));

/** The JScrollPane we're using **/
pane = new JScrollPane(scrollPanel);
pane.setVerticalScrollBarPolicy(ScrollPaneConstants.VERTICAL_SCROLLBAR_ALWAYS);
pane.getVerticalScrollBar().setUnitIncrement(10);

/** The button which keeps getting cut off.... **/
addRow = new JButton("...");
addRow.setBackground(Color.WHITE);
addRow.setMnemonic('R');
addRow.setFocusable(false);
addRow.addActionListener(new ActionListener() {
public void actionPerformed(ActionEvent e) {
addNewRow();
}
});

/** I wrap the button into this panel so I can affix it to the left **/
panel = new JPanel();
panel.setMinimumSize(new Dimension(0, 20));
panel.setLayout(null);
addRow.setBounds(0, 0, 35, 15);
panel.add(addRow);

/** Faking some data to get the table to populate **/
ArrayList<List<String>> allData = new ArrayList<List<String>>();
ArrayList<String> fakeData = new ArrayList<String>();
fakeData.addAll(Arrays.asList(
new String[]{"this", "is", "just", "sample", "data"}));
for(int i = 0; i < 5; i++)
allData.add(fakeData);

List<String> columnNames = Arrays.asList(new String[] {"", "", "", "", ""});
tableModel = new MCVEModel(columnNames, allData);

table = new JTable();
table.setModel(tableModel);

/** Adding it all together **/
GridBagConstraints c = new GridBagConstraints();
c.fill = GridBagConstraints.BOTH;
c.weightx = 1;
c.weighty = 1;
scrollPanel.add(table);
scrollPanel.add(panel);
mainPanel.add(pane, c);
this.add(mainPanel);

this.setVisible(true);
}

public void addNewRow () {
tableModel.addRow(tableModel.getRowCount(),
new String[]{"true", "", "", "false", "false"});
tableModel.fireTableRowsInserted(
tableModel.getRowCount(), tableModel.getRowCount());
}
}
/**
* Just here to keep things compilable. Seriously cut back for the MCVE,
* but still replicates the problem without any errors.
* Nothing below here should be relevant to the issue.
*/
class MCVEModel extends DefaultTableModel {

private static final long serialVersionUID = -6598574844380686148L;
private List<String> columnNames;
private List<List<String>> values;

public MCVEModel (List<String> columnNames, List<List<String>> strings) {
this.columnNames = columnNames;
this.values = strings;
}

public int getColumnCount() {
return columnNames.size();
}

public int getRowCount() {
return values == null || values.size() == 0 ? 0 : values.get(0).size();
}

public String getColumnName(int col) {
return columnNames.get(col);
}

public Object getValueAt(int row, int col) {
return values.get(col).get(row);
}

@SuppressWarnings({ "unchecked", "rawtypes" })
public Class getColumnClass(int c) {
return String.class;
}

public boolean isCellEditable(int row, int col) {
return true;
}

public void setValueAt(Object value, int row, int col) {
values.get(col).set(row, (String) value);
fireTableCellUpdated(row, col);
}

public void removeRow(int row) {
for(int i = 0; i < values.size(); i++)
values.get(i).remove(row);
this.fireTableRowsDeleted(row, row);
}

public void addRow(int row, String[] strings) {
for(int i = 0; i < values.size(); i++)
values.get(i).add(row, strings[i]);
fireTableRowsInserted(row, row);
}
}


The problem is with this line:

panel.setMinimumSize(new Dimension(0, 20));


More precisely, it's with the word "Minimum". By changing this to:

panel.setPreferredSize(new Dimension(0, 20));


I got exactly the functionality I needed. Now when the table gets too large for the
JScrollPanel
, we're still able to scroll down and see the
JButton
; it's no longer cut off.

I'm assuming this means the
JPanel
's parent didn't honor its minimum dimensions, but that it did honor its preferred dimensions. Why is this? I had thought
setPreferredSize()
,
setMinimumSize()
, and
setMaximumSize()
interacted like, "I'd prefer to be this big, but no matter what I can't be smaller than my minimum or larger than my maximum," but it seems this isn't the case. I know none of these methods should be used too frequently, but when should I use
setMinimumSize()
over
setPreferredSize()
or vice versa?

Answer

The magic is this line:

scrollPanel.add(panel);

So, scrollPanel will contain this panel. Then, JScrollPane honors the preferredSize-s. Which makes sense, since its purpose is, by using the scroll bars, to make enough room for the contained components. In other words, JScrollPane -s impementation ignores the minimumSize-s.

Update:

From an other angle, JScrollPane -s source code checks the preferredSize of its children, but not the minimumSize. There's no deep philosophy here, JScrollPane is implemented this way.

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