Question

I’m writing a Java Tree in which tree nodes could have children that take a long time to compute (in this case, it’s a file system, where there may be network timeouts that prevent getting a list of files from an attached drive).

The problem I’m finding is this:

1. getChildCount() is called before the user specifically requests opening a particular branch of the tree. I believe this is done so the JTree knows whether to show a + icon next to the node.

2. An accurate count of children from getChildCount() would need to perform the potentially expensive operation

3. If I fake the value of getChildCount(), the tree only allocates space for that many child nodes before asking for an enumeration of the children. (If I return ‘1’, I’ll only see 1 child listed, despite that there are more)

The enumeration of the children can be expensive and time-consuming, I’m okay with that. But I’m not okay with getChildCount() needing to know the exact number of children.

Any way I can work around this?

Added: The other problem is that if one of the nodes represents a floppy drive (how archaic!), the drive will be polled before the user asks for its files; if there’s no disk in the drive, this results in a system error.

Update: Unfortunately, implementing the TreeWillExpand listener isn’t the solution. That can allow you to veto an expansion, but the number of nodes shown is still restricted by the value returned by TreeNode.getChildCount().

Answer 1

There are a few parts to the solution:

• Like Lorenzo Boccaccia said, use the TreeWillExpandListener

• Also, need to call nodesWereInserted on the tree, so the proper number of nodes will be displayed. See this code

• I have determined that if you don’t know the child count, TreeNode.getChildCount() needs to return at least 1 (it can’t return 0)