Reading HashMap Values From a File

Question

Hi,
I haven't done file I/O in quite a while, so I'm a little rusty.
I have a HashMap<TreePath, Entity> that I need to store persistently. Thus far I have been creating a new HashMap every time a new session starts and populating it with Entity objects that I create from JDBC ResultSets. The TreePath is generated by adding each Entity object to its parent in the JTree. That all works well.
Now I'm trying to write the HashMap to file using ObjectOutputStream. Basically I want to be able to load the JTree (which does not need to be persistent at this stage), and as I load it I get the TreePath of each TreeNode. If that TreePath is a key in the persistent HashMap, then I want to display the details stored in the corresponding Entity. However, when I try to read the HashMap using an ObjectInputStream, I get a NullPointer.
I'm not sure whether the problem is in the way I'm trying to write or read the HashMap, or both. I've followed the examples in the API but they don't seem to be doing the job.
Any suggestions?

Answer 1

I have isolated the problem.
HashMap<TreePath, Entity> map = memory.getMap();
TreePath nodePath = new TreePath(node.getPath()); //where node is a DefaultMutableTreeNode
System.out.println("1. Desired key is: " + nodePath);
System.out.println("2. Map contains key: " + map.containsKey(nodePath));
System.out.println("3. Keys in map: " + map.keySet().toString());When I use a volatile Java object to store the map, an example of the output produced by the following code is:
1. Desired key is: [DW, Acct]
2. Map contains key: true
3. Keys in map: [[DW], [DW, Acct]]
However, when I use file I/O to make the Java object permanent, an example of the output produced by the following code is:
1. Desired key is: [DW, Acct]
2. Map contains key: false
3. Keys in map: [[DW], [DW, Acct]]
As far as I can see, line 2 of the I/O output should be true, not false, since line 3 shows that the key is present in the map.
Am I missing something here, or is the containsKey() method returning the incorrect result? It looks like it is getting its reference to map all wrong.