Question

Is there a way to get a list of components within a JPanel according to the order that they display in the JPanel (Top left to bottom right), and not the order that they were added to the JPanel?

This seems to get the components in the order that they were added to the panel

Component[] comps = myJPanel.getComponents();

Answer 1

Take a look at Container#getFocusTraversalPolicy, which returns a FocusTraversalPolicy which has methods to determine the focus movement through a container.

This (should) provide you with the natural order of the components (from the perspective of the layout manager and focus manager)

I’d start with FocusTraversalPolicy#getFirstComponent(Container) and FocusTraversalPolicy#getComponentAfter(Container, Component)

If that doesn’t work, you may need to write your self a custom Comparator and sort the array of components accordingly

UPDATED – Focus Traversal Example

public class ComponentOrder {

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

    public ComponentOrder() {
        EventQueue.invokeLater(new Runnable() {
            @Override
            public void run() {
                try {
                    UIManager.setLookAndFeel(UIManager.getSystemLookAndFeelClassName());
                } catch (ClassNotFoundException | InstantiationException | IllegalAccessException | UnsupportedLookAndFeelException ex) {
                }

                BodyPane body = new BodyPane();

                JFrame frame = new JFrame("Testing");
                frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
                frame.setLayout(new BorderLayout());
                frame.add(body);
                frame.pack();
                frame.setLocationRelativeTo(null);
                frame.setVisible(true);

                Container focusRoot = body.getFocusCycleRootAncestor();
                FocusTraversalPolicy ftp = focusRoot.getFocusTraversalPolicy();
                Component comp = ftp.getFirstComponent(body);
                Component first = comp;
                while (comp != null) {
                    System.out.println(" - " + comp);
                    comp = ftp.getComponentAfter(focusRoot, comp);
                    if (comp.equals(first)) {
                        break;
                    }
                }
            }
        });
    }

    public class BodyPane extends JPanel {

        private JTextField fldFirstName;
        private JTextField fldMiddleName;
        private JTextField fldLastName;
        private JTextField fldDateOfBirth;
        private JTextField fldEMail;
        private JButton okButton;
        private JButton cancelButton;

        public BodyPane() {

            setLayout(new BorderLayout());
            add(createFieldsPane());
            add(createButtonsPane(), BorderLayout.SOUTH);

        }

        public JPanel createButtonsPane() {

            JPanel panel = new JPanel(new FlowLayout());
            panel.add((okButton = createButton("Ok")));
            panel.add((cancelButton = createButton("Cancel")));

            return panel;

        }

        protected JButton createButton(String text) {

            return new JButton(text);

        }

        public JPanel createFieldsPane() {

            JPanel panel = new JPanel(new GridBagLayout());
            GridBagConstraints gbc = new GridBagConstraints();
            gbc.insets = new Insets(2, 2, 2, 2);
            gbc.gridx = 0;
            gbc.gridy = 0;
            gbc.anchor = GridBagConstraints.WEST;

            panel.add(createLabel("First Name:"), gbc);
            gbc.gridy++;
            panel.add(createLabel("Middle Name:"), gbc);
            gbc.gridy++;
            panel.add(createLabel("Last Name:"), gbc);
            gbc.gridy++;
            panel.add(createLabel("Date of Birth:"), gbc);
            gbc.gridy++;
            panel.add(createLabel("EMail:"), gbc);

            gbc.gridy = 0;
            gbc.gridx++;
            gbc.weightx = 1;
            panel.add((fldFirstName = createField()), gbc);
            gbc.gridy++;
            panel.add((fldLastName = createField()), gbc);
            gbc.gridy++;
            panel.add((fldMiddleName = createField()), gbc);
            gbc.gridy++;
            panel.add((fldDateOfBirth = createField()), gbc);
            gbc.gridy++;
            panel.add((fldEMail = createField()), gbc);

            JPanel filler = new JPanel();
            filler.setOpaque(false);

            gbc.gridy++;
            gbc.weightx = 1;
            gbc.weighty = 1;
            panel.add(filler, gbc);

            return panel;

        }

        protected JLabel createLabel(String text) {

            return new JLabel(text);

        }

        protected JTextField createField() {

            JTextField field = new JTextField(12);
            return field;

        }
    }
}

Comparator example

The following example uses a comparator to determine the flow of the components. This version translates all the components from a parent container into that containers coordinate space, so it should be possible to use this with compound containers.

nb I stole the comparator from javax.swing.LayoutComparator which is package protected (which was nice of the SwingTeam) and modified it to convert the component coordinates back into the parent coordinate space

public class ComponentOrder {

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

    public ComponentOrder() {
        EventQueue.invokeLater(new Runnable() {
            @Override
            public void run() {
                try {
                    UIManager.setLookAndFeel(UIManager.getSystemLookAndFeelClassName());
                } catch (ClassNotFoundException | InstantiationException | IllegalAccessException | UnsupportedLookAndFeelException ex) {
                }

                BodyPane body = new BodyPane();

                JFrame frame = new JFrame("Testing");
                frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
                frame.setLayout(new BorderLayout());
                frame.add(body);
                frame.pack();
                frame.setLocationRelativeTo(null);
                frame.setVisible(true);

                List<Component> components = new ArrayList<Component>(25);
                getContents(body, components);
                LayoutComparator lc = new LayoutComparator(body);
                lc.setComponentOrientation(body.getComponentOrientation());
                Collections.sort(components, lc);
                for (Component comp : components) {
                    System.out.println(comp);
                }
            }
        });
    }

    protected void getContents(Container container, List<Component> components) {
        for (Component comp : container.getComponents()) {
            components.add(comp);
            if (comp instanceof Container) {
                getContents((Container) comp, components);
            }
        }
    }

    public class BodyPane extends JPanel {

        private JTextField fldFirstName;
        private JTextField fldMiddleName;
        private JTextField fldLastName;
        private JTextField fldDateOfBirth;
        private JTextField fldEMail;
        private JButton okButton;
        private JButton cancelButton;

        public BodyPane() {
            setLayout(new BorderLayout());
            add(createFieldsPane());
            add(createButtonsPane(), BorderLayout.SOUTH);
        }

        public JPanel createButtonsPane() {
            JPanel panel = new JPanel(new FlowLayout());
            panel.add((okButton = createButton("Ok")));
            panel.add((cancelButton = createButton("Cancel")));
            return panel;
        }

        protected JButton createButton(String text) {
            return new JButton(text);
        }

        public JPanel createFieldsPane() {
            JPanel panel = new JPanel(new GridBagLayout());
            GridBagConstraints gbc = new GridBagConstraints();
            gbc.insets = new Insets(2, 2, 2, 2);
            gbc.gridx = 0;
            gbc.gridy = 0;
            gbc.anchor = GridBagConstraints.WEST;

            panel.add(createLabel("First Name:"), gbc);
            gbc.gridy++;
            panel.add(createLabel("Middle Name:"), gbc);
            gbc.gridy++;
            panel.add(createLabel("Last Name:"), gbc);
            gbc.gridy++;
            panel.add(createLabel("Date of Birth:"), gbc);
            gbc.gridy++;
            panel.add(createLabel("EMail:"), gbc);

            gbc.gridy = 0;
            gbc.gridx++;
            gbc.weightx = 1;
            panel.add((fldFirstName = createField("FirstName")), gbc);
            gbc.gridy++;
            panel.add((fldLastName = createField("LastName")), gbc);
            gbc.gridy++;
            panel.add((fldMiddleName = createField("MiddleName")), gbc);
            gbc.gridy++;
            panel.add((fldDateOfBirth = createField("DateOfBirth")), gbc);
            gbc.gridy++;
            panel.add((fldEMail = createField("EMail")), gbc);

            JPanel filler = new JPanel();
            filler.setOpaque(false);

            gbc.gridy++;
            gbc.weightx = 1;
            gbc.weighty = 1;
            panel.add(filler, gbc);

            return panel;
        }

        protected JLabel createLabel(String text) {
            JLabel label = new JLabel(text);
            label.setName(text);
            return label;
        }

        protected JTextField createField(String name) {
            JTextField field = new JTextField(12);
            field.setName("Field-" + name);
            return field;
        }
    }

    public class LayoutComparator implements Comparator<Component>, java.io.Serializable {

        private static final int ROW_TOLERANCE = 10;
        private boolean horizontal = true;
        private boolean leftToRight = true;

        private Component parent;

        public LayoutComparator(Component parent) {
            this.parent = parent;
        }

        void setComponentOrientation(ComponentOrientation orientation) {
            horizontal = orientation.isHorizontal();
            leftToRight = orientation.isLeftToRight();
        }

        public int compare(Component a, Component b) {
            if (a == b) {
                return 0;
            }

            // Row/Column algorithm only applies to siblings. If 'a' and 'b'
            // aren't siblings, then we need to find their most inferior
            // ancestors which share a parent. Compute the ancestory lists for
            // each Component and then search from the Window down until the
            // hierarchy branches.
            if (a.getParent() != b.getParent()) {
                LinkedList<Component> aAncestory = new LinkedList<Component>();

                for (; a != null; a = a.getParent()) {
                    aAncestory.add(a);
                    if (a instanceof Window) {
                        break;
                    }
                }
                if (a == null) {
                    // 'a' is not part of a Window hierarchy. Can't cope.
                    throw new ClassCastException();
                }

                LinkedList<Component> bAncestory = new LinkedList<Component>();

                for (; b != null; b = b.getParent()) {
                    bAncestory.add(b);
                    if (b instanceof Window) {
                        break;
                    }
                }
                if (b == null) {
                    // 'b' is not part of a Window hierarchy. Can't cope.
                    throw new ClassCastException();
                }

                for (ListIterator<Component> aIter = aAncestory.listIterator(aAncestory.size()),
                                bIter = bAncestory.listIterator(bAncestory.size());;) {
                    if (aIter.hasPrevious()) {
                        a = aIter.previous();
                    } else {
                        // a is an ancestor of b
                        return -1;
                    }

                    if (bIter.hasPrevious()) {
                        b = bIter.previous();
                    } else {
                        // b is an ancestor of a
                        return 1;
                    }

                    if (a != b) {
                        break;
                    }
                }
            }

            Point pa = SwingUtilities.convertPoint(a, a.getLocation(), parent);
            Point pb = SwingUtilities.convertPoint(b, b.getLocation(), parent);

            int ax = pa.x, ay = pa.y, bx = pb.x, by = pb.y;

            int zOrder = a.getParent().getComponentZOrder(a) - b.getParent().getComponentZOrder(b);
            if (horizontal) {
                if (leftToRight) {

                    // LT - Western Europe (optional for Japanese, Chinese, Korean)

                    if (Math.abs(ay - by) < ROW_TOLERANCE) {
                        return (ax < bx) ? -1 : ((ax > bx) ? 1 : zOrder);
                    } else {
                        return (ay < by) ? -1 : 1;
                    }
                } else { // !leftToRight

                    // RT - Middle East (Arabic, Hebrew)

                    if (Math.abs(ay - by) < ROW_TOLERANCE) {
                        return (ax > bx) ? -1 : ((ax < bx) ? 1 : zOrder);
                    } else {
                        return (ay < by) ? -1 : 1;
                    }
                }
            } else { // !horizontal
                if (leftToRight) {

                    // TL - Mongolian

                    if (Math.abs(ax - bx) < ROW_TOLERANCE) {
                        return (ay < by) ? -1 : ((ay > by) ? 1 : zOrder);
                    } else {
                        return (ax < bx) ? -1 : 1;
                    }
                } else { // !leftToRight

                    // TR - Japanese, Chinese, Korean

                    if (Math.abs(ax - bx) < ROW_TOLERANCE) {
                        return (ay < by) ? -1 : ((ay > by) ? 1 : zOrder);
                    } else {
                        return (ax > bx) ? -1 : 1;
                    }
                }
            }
        }
    }
}