private Command parse(String userInput) {
		if (userInput == null) {
			return null;
		}

		StringTokenizer tokenizer = new StringTokenizer(userInput);
		if (tokenizer.countTokens() == 0) {
			return null;
		}

		CommandName commandName = null;
		String userName = null;
		String sellerName = null;
		float amount = 0;
		int userInputTokenNo = 1;

		while (tokenizer.hasMoreTokens()) {
			switch (userInputTokenNo) {
			case 1:
				try {
					String commandNameString = tokenizer.nextToken();
					commandName = CommandName.valueOf(CommandName.class,
							commandNameString);
				} catch (IllegalArgumentException commandDoesNotExist) {
					System.out.println("Illegal command");
					return null;
				}
				break;
			case 2:
				userName = tokenizer.nextToken();
				break;
			case 3:
				try {
					amount = Float.parseFloat(tokenizer.nextToken());
				} catch (NumberFormatException e) {
					System.out.println("Illegal amount");
					return null;
				}
				break;
			case 4:
				sellerName = tokenizer.nextToken();
				break;
			default:
				System.out.println("Illegal command");
				return null;
			}
			userInputTokenNo++;
		}
		return new Command(commandName, userName, amount, sellerName);
	}