input and output streams

[Iamthecheese]

I have a program that runs a binary search tree and lists a menu of options. Two of the options are the only ones I can't seem to get to work which are saveToFile and buildFromFile. These methods include asking for a name to save as an output file (save) and asking for a name of the input file (open). Say I wanted to save a whole binary tree, nodes and all, and then be able to load that file and pick up right where I left off. Sounds easy, right? Apparently it's too hard for me hahaha. I get a null pointer exception at the saveTree() call, but I can see the file in WinSCP.

Error for saving a tree with multiple nodes:
java.io.NotSerializableException: treePkg.TreeNode
at java.io.ObjectOutputStream.writeObject0(ObjectOutputStream.java:1180)
at java.io.ObjectOutputStream.writeObject(ObjectOutputStream.java:346)
at treePkg.Tree.saveTree(Tree.java:244)
at treePkg.Tree.saveToFile(Tree.java:217)
at TreeTest.main(TreeTest.java:98)

Error for loading an existing file:
java.io.WriteAbortedException: writing aborted; java.io.NotSerializableException : treePkg.TreeNode
at java.io.ObjectInputStream.readObject0(ObjectInputStream.java:1350)
at java.io.ObjectInputStream.readObject(ObjectInputStream.java:368)
at treePkg.Tree.buildFromFile(Tree.java:270)
at TreeTest.main(TreeTest.java:102)
Caused by: java.io.NotSerializableException: treePkg.TreeNode
at java.io.ObjectOutputStream.writeObject0(ObjectOutputStream.java:1180)
at java.io.ObjectOutputStream.writeObject(ObjectOutputStream.java:346)
at treePkg.Tree.saveTree(Tree.java:243)
at treePkg.Tree.saveToFile(Tree.java:217)
at TreeTest.main(TreeTest.java:98)

Here's my code thus far.
Any help is greatly appreciated :)

//SAVE
	public boolean saveToFile(){
		String name;
		boolean save = true;
		System.out.print("Name of the output file: ");
		name = input.nextLine();
		try{	
			ObjectOutputStream output = new ObjectOutputStream(new FileOutputStream(name));	
			saveTree(output,root);
			System.out.print("Saved.");
			save = true;		
			output.close();
		}
		catch(FileNotFoundException fnfe){
			System.out.println("File not found.");
			return false;
		}
		catch(SecurityException se){
			System.out.println("No write access to this file.");
			return false;
		}
		catch(Exception e){
			e.printStackTrace();
			return false;
		}
		return save;
	}
		
	private boolean saveTree(ObjectOutputStream output, TreeNode<T> tree){
		//boolean save = saveTree(output,root);
		try{
			output.writeObject(tree.data);
			//saveTree(output,tree.leftNode);
			//saveTree(output,tree.rightNode);
			output.writeObject(tree.leftNode);
			output.writeObject(tree.rightNode);
		}
		catch(java.io.IOException exp){
			exp.printStackTrace();
		}
		
		
		return true;
	}
//END SAVE
//BUILD
	public boolean buildFromFile(){
		String name;
		boolean end = false;

		System.out.print("Name of the input file: ");
		name = input.nextLine();
		
		try{
			//ObjectInputStream ois = new ObjectInputStream(new FileInputStream(name));
			FileInputStream fis = new FileInputStream(name);
			ObjectInputStream ois = new ObjectInputStream(fis);
			if(input==null)
				System.out.println("\nInput file did not open.\n");		
			while(end==false)
			{
				T obj = (T)ois.readObject();
				insertItem(obj);
				System.out.print("Opened.");
			}
			input.close();
		}
		catch(FileNotFoundException fnfe){
			System.out.println("File not found.");
			return false;
		}
		catch(NoSuchElementException nsee){
			System.out.println("Invalid input.  Please try again");
			return false;
		}
		catch(IllegalStateException ise){
			System.out.println("Illegal state.");
			return false;
		}
		catch(EOFException eofe){
			return true;
		}
		catch(Exception e){
			System.out.println("Error opening file.");
			return false;
		}
		return end;
	}
//END BUILD