List accDetails = (List)tm.get(accNo)

TreeMap tm = new TreeMap(); 
 
 
		try {
			FileInputStream fiStream = new FileInputStream("C:\\Work\\Tracker\\TEST.txt");
			DataInputStream diStream = new DataInputStream(fiStream);
			BufferedReader bReader = new BufferedReader(new InputStreamReader(diStream));
			String inLine;
 
			while ((inLine = bReader.readLine()) != null) {
				// Row gets inserted into the Treemap as the key, with the account number as the value 
				String [] record = inLine.split("\\|", -1); // Splits the record, keeps empty fields.
 
				String accNo = record[0];
				String restOfLine = "";
 
				for (int i = 1; i < record.length; i++) {
					restOfLine = restOfLine + "," + record[i];
				}
 
				if (!record[0].equals("ACCOUNT NUMBER")) {
				tm.put(accNo, restOfLine);
				}
				//type mismatch: cannot convert from Object to List
				List accDetails = (List)tm.get(accNo);
				if (accDetails == null) {
					accDetails = new ArrayList();
				}
				accDetails.add(restOfLine);
 
				tm.put(accNo, accDetails);
			}
			bReader.close();