[sql] What is the most efficient/elegant way to parse a flat table into a tree?

This was written quickly, and is neither pretty nor efficient (plus it autoboxes alot, converting between int and Integer is annoying!), but it works.

It probably breaks the rules since I'm creating my own objects but hey I'm doing this as a diversion from real work :)

This also assumes that the resultSet/table is completely read into some sort of structure before you start building Nodes, which wouldn't be the best solution if you have hundreds of thousands of rows.

public class Node {

    private Node parent = null;

    private List<Node> children;

    private String name;

    private int id = -1;

    public Node(Node parent, int id, String name) {
        this.parent = parent;
        this.children = new ArrayList<Node>();
        this.name = name;
        this.id = id;
    }

    public int getId() {
        return this.id;
    }

    public String getName() {
        return this.name;
    }

    public void addChild(Node child) {
        children.add(child);
    }

    public List<Node> getChildren() {
        return children;
    }

    public boolean isRoot() {
        return (this.parent == null);
    }

    @Override
    public String toString() {
        return "id=" + id + ", name=" + name + ", parent=" + parent;
    }
}

public class NodeBuilder {

    public static Node build(List<Map<String, String>> input) {

        // maps id of a node to it's Node object
        Map<Integer, Node> nodeMap = new HashMap<Integer, Node>();

        // maps id of a node to the id of it's parent
        Map<Integer, Integer> childParentMap = new HashMap<Integer, Integer>();

        // create special 'root' Node with id=0
        Node root = new Node(null, 0, "root");
        nodeMap.put(root.getId(), root);

        // iterate thru the input
        for (Map<String, String> map : input) {

            // expect each Map to have keys for "id", "name", "parent" ... a
            // real implementation would read from a SQL object or resultset
            int id = Integer.parseInt(map.get("id"));
            String name = map.get("name");
            int parent = Integer.parseInt(map.get("parent"));

            Node node = new Node(null, id, name);
            nodeMap.put(id, node);

            childParentMap.put(id, parent);
        }

        // now that each Node is created, setup the child-parent relationships
        for (Map.Entry<Integer, Integer> entry : childParentMap.entrySet()) {
            int nodeId = entry.getKey();
            int parentId = entry.getValue();

            Node child = nodeMap.get(nodeId);
            Node parent = nodeMap.get(parentId);
            parent.addChild(child);
        }

        return root;
    }
}

public class NodePrinter {

    static void printRootNode(Node root) {
        printNodes(root, 0);
    }

    static void printNodes(Node node, int indentLevel) {

        printNode(node, indentLevel);
        // recurse
        for (Node child : node.getChildren()) {
            printNodes(child, indentLevel + 1);
        }
    }

    static void printNode(Node node, int indentLevel) {
        StringBuilder sb = new StringBuilder();
        for (int i = 0; i < indentLevel; i++) {
            sb.append("\t");
        }
        sb.append(node);

        System.out.println(sb.toString());
    }

    public static void main(String[] args) {

        // setup dummy data
        List<Map<String, String>> resultSet = new ArrayList<Map<String, String>>();
        resultSet.add(newMap("1", "Node 1", "0"));
        resultSet.add(newMap("2", "Node 1.1", "1"));
        resultSet.add(newMap("3", "Node 2", "0"));
        resultSet.add(newMap("4", "Node 1.1.1", "2"));
        resultSet.add(newMap("5", "Node 2.1", "3"));
        resultSet.add(newMap("6", "Node 1.2", "1"));

        Node root = NodeBuilder.build(resultSet);
        printRootNode(root);

    }

    //convenience method for creating our dummy data
    private static Map<String, String> newMap(String id, String name, String parentId) {
        Map<String, String> row = new HashMap<String, String>();
        row.put("id", id);
        row.put("name", name);
        row.put("parent", parentId);
        return row;
    }
}