A more database friendly TreeModel implementation

LazyTreeModel.java

public class LazyTreeModel<T> implements TreeModel<T> {
    private final ValueEncoder<T> encoder;
    private final LazyTreeModelSource<T> source;
    
    public LazyTreeModel<T>(ValueEncoder<T> encoder, LazyTreeModelSource<T> source) {
        this.encoder = encoder;
        this.source = source;
    }
    
    @Override
    public List<TreeNode<T>> getRootNodes() {
        List<T> roots = source.getRoots();
        return LazyTreeModel.createTreeNodes(encoder, source, roots);
    }
    
    @Override
    public TreeNode<T> getById(String id) {
        return find(encoder.toValue(id));
    }
    
    @Override
    public TreeNode<T> find(T value) {
        return new LazyTreeNode<T>(encoder, source, value);
    }
    
    public static List<TreeNode> createTreeNodes(ValueEncoder<T> encoder, LazyTreeModelSource<T> source, List<T> siblings) {
        if (siblings == null || siblings.isEmpty()) {
            return Collections.emptyList();
        }
        List<TreeNode> treeNodes = new ArrayList<TreeNode>(siblings.size());
        for (T value : siblings) {
            treeNodes.add(new LazyTreeNode<T>(encoder, source, value));
        }
        return treeNodes;
    }
}

LazyTreeModelSource.java

public interface LazyTreeModelSource<T> {
    /**
     * Get the label from the value. Note: for efficiency this should be sourced 
     * directly from the value and should not require an extra database lookup
     **/
    String getLabel(T value);

    /**
     * Get the isLeaf flag from the value. Note: for efficiency this should be sourced 
     * directly from the value and should not require an extra database lookup
     **/
    boolean isLeaf(T value);

    /**
     * Get the hasChildren flag from the value. Note: for efficiency this should be sourced 
     * directly from the value and should not require an extra database lookup
     **/
    boolean hasChildren(T value);

    /**
     * Get the children of the value
     **/
    List<T> getChildren(T value);
    
    /**
     * Get the roots of the TreeModel
     **/
    List<T> getRoots();
}

LazyTreeNode.java

public class LazyTreeNode<T> implements TreeNode<T> {
    private final ValueEncoder<T> encoder;
    private final LazyTreeModelSource<T> source;
    private final T value;
    
    public LazyTreeNode<T>(ValueEncoder<T> encoder, LazyTreeModelSource<T> source, T value) {
        this.encoder = encoder;
        this.source = source;
        this.value = value;
    }
    
    @Override
    public T getValue() {
        return value;
    }
    
    @Override
    public String getId() {
        return encoder.toClient(value);
    }
    
    @Override
    public boolean isLeaf() {
        return source.isLeaf(value);
    }
    
    @Override
    public boolean getHasChildren() {
        return source.hasChildren(value);
    }
    
    @Override
    public String getLabel() {
        return source.getLabel(value);
    }
    
    @Override
    public List<TreeNode<T>> getChildren() {
        List<T> children = source.getChildren(value);
        return LazyTreeModel.createTreeNodes(encoder, source, children);
    }
}

in the class LazyTreeModel, a declaration for roots is missing?

The roots are intentionally not part of LazyTreeModel. The roots are looked up via LazyTreeNodeSource on demand. The roots are ONLY required when rendering the root of the tree and are NOT required when expanding a node. I prefer this approach to DefaultTreeModel which ALWAYS requires the roots.

This has been implemented in tapestry-stitch demo here