import java.awt.GridLayout;

import javax.swing.ImageIcon;
import javax.swing.JFrame;
import javax.swing.JTree;
import javax.swing.SwingUtilities;
import javax.swing.tree.DefaultMutableTreeNode;
import javax.swing.tree.DefaultTreeCellRenderer;
import javax.swing.tree.DefaultTreeModel;

public class CustomTreeIcons {

    public CustomTreeIcons(){
        JFrame frame = new JFrame("Custom Tree Icons");
        frame.setLayout(new GridLayout(1,1));
        frame.add(initTree());
        frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        frame.setSize(300, 300);
        frame.setLocationRelativeTo(null);
        frame.setVisible(true);
    }
    
    private JTree initTree(){
        DefaultMutableTreeNode root = new DefaultMutableTreeNode("Root");
        root.add(new DefaultMutableTreeNode("Blatt 1"));
        root.add(new DefaultMutableTreeNode("Blatt 2"));
        DefaultTreeModel model = new DefaultTreeModel(root);
        JTree tree = new JTree(model);
        tree.setCellRenderer(new MyCellRenderer());
        return tree;
    }
    
    public static void main(String[] args) {
        SwingUtilities.invokeLater(() -> new CustomTreeIcons());
    }
}

class MyCellRenderer extends DefaultTreeCellRenderer {
    
    public MyCellRenderer(){
        openIcon = createImageIcon("img/open.png");
        closedIcon = createImageIcon("img/closed.png");
        leafIcon = createImageIcon("img/leaf.png");
    }
    
    protected ImageIcon createImageIcon(String path) {
        java.net.URL imgURL = getClass().getClassLoader().getResource(path);
        if (imgURL != null) {
            return new ImageIcon(imgURL);
        }
        System.err.println(path + " nicht gefunden!");
        return null;
    }
}