tree 结构很常见，当persist 到数据库中。
有些操作，在db 中更好。
1。取得所有的叶子节点。
SELECT Name FROM Projects p
WHERE NOT EXISTS(
SELECT * FROM Projects
WHERE Parent=p.VertexId)
2。multilevel operation ，用数据库的辅助表， 用triger 。
CREATE TABLE ProjectPaths(
VertexId INTEGER,
Depth INTEGER,
Path VARCHAR(300) 。
)
3. 用 hibernate 时，如果 stack over flow,考虑用 stack 代替recursive algrithm
public void traverseDepthFirst( AST ast )
{
// Root AST node cannot be null or
// traversal of its subtree is impossible.
if ( ast == null )
{
throw new IllegalArgumentException(
"node to traverse cannot be null!" );
}
// Map to hold parents of each
// AST node. Unfortunately the AST
// interface does not provide a method
// for finding the parent of a node, so
// we use the Map to save them.
Map parentNodes = new HashMap();
// Start tree traversal with first child
// of the specified root AST node.
AST currentNode = ast.getFirstChild();
// Remember parent of first child.
parentNodes.put( currentNode , ast );
// Iterate through nodes, simulating
// recursive tree traversal, and add them
// to queue in proper order for later
// linear traversal. This "flattens" the
// into a linear list of nodes which can
// be visited non-recursively.
while ( currentNode != null )
{
// Visit the current node.
strategy.visit( currentNode );
// Move down to current node's first child
// if it exists.
AST childNode = currentNode.getFirstChild();
// If the child is not null, make it
// the current node.
if ( childNode != null )
{
// Remember parent of the child.
parentNodes.put( childNode , currentNode );
// Make child the current node.
currentNode = childNode;
continue;
}
while ( currentNode != null )
{
// Move to next sibling if any.
AST siblingNode = currentNode.getNextSibling();
if ( siblingNode != null )
{
// Get current node's parent.
// This is also the parent of the
// sibling node.
AST parentNode = (AST)parentNodes.get( currentNode );
// Remember parent of sibling.
parentNodes.put( siblingNode , parentNode );
// Make sibling the current node.
currentNode = siblingNode;
break;
}
// Move up to parent if no sibling.
// If parent is root node, we're done.
currentNode = (AST)parentNodes.get( currentNode );
if ( currentNode.equals( ast ) )
{
currentNode = null;
}
}
}
参考:
http://wordhoard.northwestern.edu/userman/hibernatechanges.html
《Tansact Sql cookbook.》
西津渡