Add rollover to JTree handles

Question

I am trying to make a custom rollover effect on the Collapsed Icon for a JTree. However, I am unsure how to target an individual handle instead of all the handles.

If you run the code below, you will see that when you hover over any handle, node, or leaf of the JTree all the collapsed handles will change to the rollover. This is not desired. So how can I change just a single handle when I am hovering over that handle, and preferably not when hovering over the node next to it?

import java.awt.*;
import java.awt.event.*;

import javax.swing.*;
import javax.swing.tree.*;
import javax.swing.plaf.basic.*;

@SuppressWarnings("serial")
public class DirectoryExplorer extends JFrame {
    private DirectoryExplorer() {
        super("Directory Explorer");
        setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        setLayout(new GridLayout(1, 1));
        createPanel();
        setSize(800,600);
        setVisible(true);
    }

    private void createPanel() {
        JPanel panel = new JPanel(new GridLayout(1, 1));

        DefaultMutableTreeNode root = new DefaultMutableTreeNode("Hello");
        root.add(new DefaultMutableTreeNode("1"));
        root.add(new DefaultMutableTreeNode("2"));
        root.add(new DefaultMutableTreeNode("3"));

        JTree tree = new JTree();
        BasicTreeUI tUI = (BasicTreeUI) tree.getUI();
        tUI.setCollapsedIcon(new ImageIcon("resources/closed.png"));
        tUI.setExpandedIcon(new ImageIcon("resources/open.png"));
        tree.setShowsRootHandles(true);
        tree.addMouseMotionListener(new MouseHandler(tree));

        panel.add(new JScrollPane(tree));
        getContentPane().add(panel);
    }

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

    private class MouseHandler implements MouseMotionListener {
        JTree t = null;
        BasicTreeUI tUI = null;

        public MouseHandler(JTree tree) {
            t = tree;
            tUI = (BasicTreeUI) tree.getUI();
        }

        @Override
        public void mouseDragged(MouseEvent e) {

        }

        @Override
        public void mouseMoved(MouseEvent e) {
            TreePath selPath = t.getPathForLocation(e.getX(), e.getY());
            if(selPath != null) 
                tUI.setCollapsedIcon(new ImageIcon("resources/rollover.png"));
            else
                tUI.setCollapsedIcon(new ImageIcon("resources/closed.png"));
            t.repaint();
        }
    }
}

Answer 1

In order to achieve the desired result you need to override BasicTreeUI.paintExpandControl() and BasicTreeUI.MouseHandler.mouseMoved(). You will also need to create a few methods such as setRolloverIcon().

A working example of this might look like this

import java.awt.Component;
import java.awt.EventQueue;
import java.awt.Graphics;
import java.awt.GridLayout;
import java.awt.Insets;
import java.awt.Rectangle;
import java.awt.event.MouseEvent;
import java.net.MalformedURLException;
import java.net.URL;

import javax.swing.Icon;
import javax.swing.ImageIcon;
import javax.swing.JFrame;
import javax.swing.JPanel;
import javax.swing.JScrollPane;
import javax.swing.JTree;
import javax.swing.plaf.basic.BasicTreeUI;
import javax.swing.tree.DefaultMutableTreeNode;
import javax.swing.tree.TreePath;

@SuppressWarnings("serial")
public class DirectoryExplorer extends JFrame {
    private DirectoryExplorer() {
        super("Directory Explorer");
        setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        setLayout(new GridLayout(1, 1));
        createPanel();
        setSize(800,600);
        setVisible(true);
    }

    private void createPanel() {
        JPanel panel = new JPanel(new GridLayout(1, 1));

        DefaultMutableTreeNode root = new DefaultMutableTreeNode("Hello");
        root.add(new DefaultMutableTreeNode("1"));
        root.add(new DefaultMutableTreeNode("2"));
        root.add(new DefaultMutableTreeNode("3"));

        JTree tree = new JTree();

        //UI Stuff//
        TreeHandleUI tUI = new TreeHandleUI(tree);
        tree.setUI(tUI);
        try {
            tUI.setCollapsedIcon(new ImageIcon(new URL("https://i.sstatic.net/nKJFv.png")));
            tUI.setExpandedIcon(new ImageIcon(new URL("https://i.sstatic.net/NJvcp.png")));
            tUI.setRolloverIcon(new ImageIcon(new URL("https://i.sstatic.net/jN6uX.png")));
        } catch(MalformedURLException e) {
            System.out.println("Bad URL / URLs");
        }
        ////////////

        tree.setShowsRootHandles(true);

        panel.add(new JScrollPane(tree));
        getContentPane().add(panel);
    }

    public static void main(String[] args) {
        EventQueue.invokeLater(() -> new DirectoryExplorer());
    }

    private class TreeHandleUI extends BasicTreeUI {
        ///Variables
        private JTree t = null;
        private Icon rolloverIcon = null;
        private boolean rolloverEnabled = false;
        private UpdateHandler uH = null;

        private boolean isLeftToRight( Component c ) {
            return c.getComponentOrientation().isLeftToRight();
        }

        public TreeHandleUI(JTree tree) {
            t = tree;
            uH = new UpdateHandler(t);
            t.addMouseMotionListener(uH);
        }

        public void setRolloverIcon(Icon rolloverG) {
            Icon oldValue = rolloverIcon;
            rolloverIcon = rolloverG;
            setRolloverEnabled(true);
            if (rolloverIcon != oldValue) {
                t.repaint();
            }
        }

        private void setRolloverEnabled(boolean handleRolloverEnabled) {
            boolean oldValue = rolloverEnabled;
            rolloverEnabled = handleRolloverEnabled;
            if (handleRolloverEnabled != oldValue) {
                t.repaint();
            }
        }

        @Override
        protected void paintExpandControl(Graphics g,
                                          Rectangle clipBounds, Insets insets,
                                          Rectangle bounds, TreePath path,
                                          int row, boolean isExpanded,
                                          boolean hasBeenExpanded,
                                          boolean isLeaf) {
            Object value = path.getLastPathComponent();

            if (!isLeaf && (!hasBeenExpanded || treeModel.getChildCount(value) > 0)) {
                int middleXOfKnob;
                if (isLeftToRight(t)) {
                    middleXOfKnob = bounds.x - getRightChildIndent() + 1;
                } else {
                    middleXOfKnob = bounds.x + bounds.width + getRightChildIndent() - 1;
                }
                int middleYOfKnob = bounds.y + (bounds.height / 2);

                if (isExpanded) {
                    Icon expandedIcon = getExpandedIcon();
                    if(expandedIcon != null)
                      drawCentered(tree, g, expandedIcon, middleXOfKnob, middleYOfKnob );
                } else if(isLocationInExpandControl(path, uH.getXPos(), uH.getYPos()) && !isExpanded && rolloverEnabled) {
                    if(row == uH.getRow()) {
                        if(rolloverIcon != null)
                            drawCentered(tree, g, rolloverIcon, middleXOfKnob, middleYOfKnob);
                    } else {
                        Icon collapsedIcon = getCollapsedIcon();
                        if(collapsedIcon != null)
                          drawCentered(tree, g, collapsedIcon, middleXOfKnob, middleYOfKnob);
                    }
                } else {
                    Icon collapsedIcon = getCollapsedIcon();
                    if(collapsedIcon != null)
                      drawCentered(tree, g, collapsedIcon, middleXOfKnob, middleYOfKnob);
                }
            }
        }

        private class UpdateHandler extends BasicTreeUI.MouseHandler {
            private JTree t = null;
            private int xPos = 0;
            private int yPos = 0;

            private boolean leftToRight(Component c) {
                return c.getComponentOrientation().isLeftToRight();
            }

            public UpdateHandler(JTree tree) {
                t = tree;
            }

            @Override
            public void mouseMoved(MouseEvent e) {
                xPos = e.getX();
                yPos = e.getY();
                t.repaint();
            }

            public int getXPos() {
                return xPos;
            }

            public int getYPos() {
                return yPos;
            }

            public int getRow() {
                return getRowForPath(t, getClosestPathForLocation(t, xPos, yPos));
            }
        }
    }
}

Code will run without downloading images however they are available below

closed.png

open.png

rollover.png