Develop Reference

Saving data to a file doesn't behave as expected

I have a simple project where I created a Store with customers, products and employees. Each is represented by a Class of course and I also have a CSV file for each one of them to be able to load data from and save data to it.
I'm facing issues where the file reading/writing is working, but not really. For example, I have the ability to save each file individually so if for instance I want to create a new customer, I'd save it to the list and then to the file. Issue is, once I do it for another Class (i.e if I create a new employee) and then save it again, the customer file object I saw in the CSV earlier is deleted. BUT, once I add a new object again, that same object reappears again. Hope you can somehow understand, but here is a more detailed view:
customer.csv is empty:
Me creating a new customer:
Created and saved to CSV:
Now, if I go to the other menu, and click on "Save all data" that jon snow customer object will be gone. Then if I create a new customer, then it will be added to the CSV file, along with the jon snow I added earlier. So why is it gone in the first place?
So here is the whole file reader/writer code I'm using:
import java.io.BufferedWriter;
import java.io.File;
import java.io.FileNotFoundException;
import java.io.FileWriter;
import java.util.ArrayList;
import java.io.IOException;
import java.util.List;
import java.util.Scanner;
class CSV {
static void CreateFile(String filename) { //Create new file
try {
File fileToCreate = new File(filename);
if (fileToCreate.createNewFile()) {
System.out.println("File created sucessfully: " + fileToCreate.getName());
}
} catch (IOException e) {
System.out.println("Cannot create file!");
}
}
static void ReadFile(String path_and_filename){
try {
File fileToRead = new File(path_and_filename);
Scanner myReader = new Scanner(fileToRead);
System.out.println("Reading file "+path_and_filename+" :");
while (myReader.hasNextLine()) {
String data = myReader.nextLine();
System.out.println(data);
}
myReader.close();
System.out.println();
} catch (FileNotFoundException e) {
System.out.println("There is no such file "+"\"path_and_filename\""+".\n");
}
}
// The StringBuilder in Java represents a mutable sequence of characters.
// Java's built in String class is not mutable.
static void saveArrayListToFile(List<Output> listToSave, String fileName, String sep) throws Exception {
StringBuilder ans = new StringBuilder();
for (Output record : listToSave) {
ans.append(record.createOutput());
ans.append(sep);
}
saveStringToFile(ans.toString(), fileName);
System.out.println("\nData saved to "+ fileName);
}
static void saveArrayListToFile1(ArrayList<String> listToSave, String fileName, String sep){
StringBuilder ans = new StringBuilder();
for (Object record : listToSave) {
ans.append(record.toString());
ans.append(sep);
}
saveStringToFile(ans.toString(), fileName);
System.out.println("\nList was saved to file "+fileName+"\n");
}
static void saveStringToFile(String data, String fileName){
BufferedWriter bufferedWriter=null;
try {
bufferedWriter = new BufferedWriter(
new FileWriter(fileName,false));
bufferedWriter.write(data);
} catch (IOException e) {
System.out.println("Cannot write to file");
} finally {
try {
bufferedWriter.close();
} catch (IOException e) {
System.out.println("Cannot write to file");
}
}
}
}
When I'm creating a new customer, I call it from a menu and it looks like this:
switch (selection) {
case 1:
try {
System.out.println("You're registering as a new customer");
String custID = ObjectIDs.generateID();
System.out.println("Enter first name:");
String firstName = sc.next();
System.out.println("Enter last name:");
String lastName = sc.next();
st.newCustomer(custID, firstName, lastName);
st.saveCustomersList();
} catch (Exception e) {
e.printStackTrace();
}
break;
the saveCustomerList() function is this:
#SuppressWarnings("unchecked")
void saveCustomersList() throws Exception {
CSV.saveArrayListToFile((List<Output>)(List<?>) customers, CUSTOMERS_FILE_PATH,"\n");
}
And then the functions calls saveArrayListToFile() to save it.
The behavior is the same with Product and Employee projects, so I randomly chose to show how it acts when creating a new Product.
I hope I added enough information. If needed, I can paste more code in but I already feel it's very cluttered. Hopefully it's ok.
Thank you very much :)

At the moment it's hard to say, as one can only hypothesise as to what happens when you click on "Save all data". There are some weird things (what is saveArrayListToFile and saveArrayListToFile11? Why does one declare an exception? When are these called?).
Having said that, look at the actual file writing method saveStringToFile, it says:
bufferedWriter = new BufferedWriter(new FileWriter(fileName,false));
This false there means 'do not append to file, rewrite it from scratch'. So each time you call it, file contents are discarded and replaced from what you provide to the method call. So my somewhat educated guess would be:
You save customer one to file (gets cleared, customer 1 written) and
append the customer to a list of customers (that's my guess)
You
save customer two to file (file gets cleared, so only customer 2 is
saved), you add to list to customers (do you?)
Then you choose 'save all' which gets list of customers, and save them in one go, a single call to the method. The file is cleared, all customers are saved.
But it's all guessing. Try creating a minimal, reproducible example

In addition to pafau k. I would like to add some things at least I would do differently...
First of all:
Things that can cause errors or unexpected behaviour:
Everything below is in saveStringToFile
Like already pointed out the Initialisation of the BufferedWriter: It should be initialized like this:
bufferedWriter = new BufferedWriter(new FileWriter(filename, true));
This puts the File into appending mode (if you want to append to a file you can also get rid of the boolean (second argument) entirely because appending is standard: new FileWriter(filename))
If for some case the Creation of the BufferedWriter failed you will still have a null-pointing object as bufferedWriter. This however means that you will be surprised with a NullPointerException in your finally block. To prevent this first of all do a check in your finally block:
if (bufferedWriter != null) {
// Close your bufferedWriter in here
}
Also, if you run into an error you will likely be presented with the same error message twice.
Now cosmetics:
Things that I would write differently for aesthetic reasons:
Java methods (and static "methods") are always starting with a small letter :)
This means it should be public static void createFile() for example or static void readFile()
variables and parameters of methods do not contain seperators like _ but instead if you want to make it more readable you start with a small letter and for each seperation you use a capital letter for that: e.g. String thisIsAVeryLongVariableWithALotOfSeperations = "Foo";
The generic types in saveArrayListToFile1() work like a placeholder. So you declare ArrayList<String> listToSave so you don't need a cast in the following for-loop: You can simply write:
for (String record : listToSave) {
ans.append(record);
ans.append(sep);
}
I hope this fixes all errors or complications. :)

reading variables buried in java exception handling

I am writing a function to take a text file and count how many lines it has while outputting the lines to an array of strings. Doing this I have several exceptions I need to look out for. The class function has several variables that should have a scope throughout the function but when I write a value to the function inside of an exception, the return statement cannot find it. I've moved the declaration around and nothing helps
The value returned "h5Files" "Might not have been initialized" Since I don't know how long the array will be I cannot initialize it to a certain length. I do this within the code and I need a way to tell the return statement that I now have a values
Here is the code
public String[] ReadScanlist(String fileIn){
int i;
String directory ="c:\\data\\"; // "\" is an illegal character
System.out.println(directory);
int linereader = 0;
String h5Files[];
File fileToRead = new File(directory + fileIn);
System.out.println(fileToRead);
try {
FileInputStream fin = new FileInputStream(fileToRead); // open this file
}
catch(FileNotFoundException exc) {
System.out.println("File Not Found");
}
try{
//read bytes until EOF is detected
do {
FileReader fr = new FileReader(fileToRead);// Need to convert to reader
LineNumberReader lineToRead = new LineNumberReader(fr); // Use line number reader class
//
while (lineToRead.readLine() != null){
linereader++;
}
linereader = 0;
lineToRead.setLineNumber(0); //reset line number
h5Files = new String[linereader];
while (lineToRead.readLine() != null){
h5Files[linereader] = lineToRead.readLine(); // deposit string into array
linereader++;
}
return h5Files;
}
while(i !=-1); // When i = -1 the end of the file has been reached
}
catch(IOException exc) {
System.out.println("Error reading file.");
}
try{
FileInputStream fin = new FileInputStream(fileToRead);
fin.close(); // close the file
}
catch(IOException exc) {
System.out.println("Error Closing File");
}
return h5Files;
}

Your code is very very odd. For example these two blocks make no sense:
try {
FileInputStream fin = new FileInputStream(fileToRead); // open this file
}
catch(FileNotFoundException exc) {
System.out.println("File Not Found");
}
try{
FileInputStream fin = new FileInputStream(fileToRead);
fin.close(); // close the file
}
catch(IOException exc) {
System.out.println("Error Closing File");
}
I don't know what you think they do, but besides the first one leaking memory, they do nothing at all. The comments are more worrying, they suggest that you need to do more reading on IO in Java.
Deleting those blocks and tidying the code a (moving declarations, formatting) gives this:
public String[] ReadScanlist(String fileIn) {
String directory = "c:\\data\\";
String h5Files[];
File fileToRead = new File(directory + fileIn);
try {
int i = 0;
do {
FileReader fr = new FileReader(fileToRead);
LineNumberReader lineToRead = new LineNumberReader(fr);
int linereader = 0;
while (lineToRead.readLine() != null) {
linereader++;
}
linereader = 0;
lineToRead.setLineNumber(0);
h5Files = new String[linereader];
while (lineToRead.readLine() != null) {
h5Files[linereader] = lineToRead.readLine();
linereader++;
}
return h5Files;
} while (i != -1);
} catch (IOException exc) {
System.out.println("Error reading file.");
}
return h5Files;
}
My first bone of contention is the File related code. First, File abstracts from the underlying OS, so using / is absolutely fine. Second, there is a reason File has a File, String constructor, this code should read:
File directory = new File("c:/data");
File fileToRead = new File(directory, fileIn);
But it should really be using the new Path API anyway (see below).
So, you declare h5Files[]. You then proceed to read the whole file to count the lines. You then assign h5Files[] to an array of the correct size. Finally you fill the array.
If you have an error anywhere before you assign h5Files[] you have not initialised it and therefore cannot return it. This is what the compiler is telling you.
I don't know what i does in this code, it is assigned to 0 at the top and then never reassigned. This is an infinite loop.
So, you need to rethink your logic. I would recommend throwing an IOException if you cannot read the file. Never return null - this is an anti-pattern and leads to all those thousands of null checks in your code. If you never return null you will never have to check for it.
May I suggest the following alternative code:
If you are on Java 7:
public String[] ReadScanlist(String fileIn) throws IOException {
final Path root = Paths.get("C:/data");
final List<String> lines = Files.readAllLines(root.resolve(fileIn), StandardCharsets.UTF_8);
return lines.toArray(new String[lines.size()]);
}
Or, if you have Java 8:
public String[] ReadScanlist(String fileIn) throws IOException {
final Path root = Paths.get("C:/data");
try (final Stream<String> lines = Files.lines(root.resolve(fileIn), StandardCharsets.UTF_8)) {
return lines.toArray(String[]::new);
}
}

Since I don't know how long the array will be I cannot initialize it
to a certain length.
I don't think an array is the correct solution for you then - not to say it can't be done, but you would be re-inventing the wheel.
I would suggest you use a LinkedList instead, something like:
LinkedList<String> h5Files = new LinkedList<>();
h5Files.add(lineToRead.readLine());
Alternatively you could re-invent the wheel by setting the array to an arbritary value, say 10, and then re-size it whenever it gets full, something like this:
h5Files = new String[10];
if (linereader = h5Files.size())
{
String[] temp = h5Files;
h5Files = new String[2 * linereader];
for (int i = 0; i < linereader; i++)
{
h5Files[i] = temp[i];
}
}
Either one of these solutions would allow you to initialize the array (or array alternative) in a safe constructor, prior to your try block, such that you can access it if any exceptions are thrown

Here is your problem. Please take a look on digested version of your code with my comments.
String h5Files[]; // here you define the variable. It still is not initialized.
try{
..................
do {
h5Files = new String[linereader]; // here you initialize the variable
} while(i !=-1); // When i = -1 the end of the file has been reached
..................
catch(IOException exc) {
// if you are here the variable is still not initialized
System.out.println("Error reading file.");
}
// you continue reading file even if exception was thrown while opening the file
I think that now the problem is clearer. You try to open the file and count lines. If you succeed you create array. If not (i.e. when exception is thrown) you catch the exception but still continue reading the file. But in this case you array is not initialized.
Now how to fix this?
Actually if you failed to read the file first time you cannot continue. This may happen for example if file does not exist. So, you should either return when first exception is thrown or just do not catch it at all. Indeed there is nothing to do with the file if exception was thrown at any phase. Exception is not return code. This is the reason that exceptions exist.
So, just do not catch exceptions at all. Declare your method as throws IOException and remove all try/catch blocks.

What is the best way to handle an exception for an invalid file?

I have a java class where a user provides a file path and if the path doesn't exist I ask them to try again. My professor says we should use an exception to handle this.
Here is a snippet of how I'm currently doing it:
public class SalesUtil {
public static void processSales() {
Scanner keyboard = new Scanner(System.in);
System.out.println("Enter sales file name: ");
String salesFile = keyboard.nextLine();
try {
Scanner scanFile = new Scanner(new File(salesFile));
//do stuff
}
} catch(FileNotFoundException fnfe) {
System.out.println("Invalid file name supplied, please try again.");
processSales();
}
}
}
Well in the do stuff section, I'm calculating values and printing data to the console. If I enter the correct file name correctly on the first try all the data is correct. If it is incorrect one or more times the data is not correct.
I imagine this is because of adding function calls on top of my initial stack and never 'getting out' of the initial stack while supplying subsequent stack calls until the correct file is supplied?
I'm still new to java and would appreciate some tips in understanding how to solve this using an exception.

The FileNotFoundException is the correct one to catch, however I gather that you're worried about the stacks building up? I tested reading back the file after multiple failed attempts and it was fine. The recursive call is at the end of the method so it is the last line of code and therefore the stacks shouldn't have any effect.
However, if you want, you could use a while loop instead of recursion to avoid stack buildup:
public static void processSales() {
Scanner scanFile = null;
Scanner keyboard = new Scanner(System.in);
while (scanFile == null) {
System.out.println("Enter sales file name: ");
String salesFile = keyboard.nextLine();
try {
scanFile = new Scanner(new File(salesFile));
while (scanFile.hasNextLine()) {
System.out.println(scanFile.nextLine());
}
} catch(FileNotFoundException fnfe) {
System.out.println("Invalid file name supplied, please try again.");
}
}
}

use the file.exist() method to check, if that what you want to do is to make sure it exist then this is the codes:
File sfile = new File(salesFile);
if (sfile.exists()) {
// ok, file exist do something.
...
}
On the other hand, when you say "invalid file" could be anything, if it is bad filename, then it is another animal (well, different exeception)...
To use try/catch for a readonly file then:
try {
FileInputStream sfile = new FileInputStream(salesFile);
...
} catch (FileNotFoundException e) {
System.out.println(e.getMessage());
}

In Java, why am I getting a NullPointerException when trying to display a FileNotFoundException?

I think it's easier to just show the code and the output I'm getting than trying to explain it :)
This is from my main method:
//prompt user for filename
System.out.println("Please enter the text file name. (Example: file.txt):");
String filename = ""; //will be used to hold filename
//loop until user enters valid file name
valid = false;
while(!valid)
{
filename = in.next();
try
{
reader.checkIfValid(filename);
valid = true; //file exists and contains text
}
catch (Exception e)
{
System.out.println(e + "\nPlease try again.");
}
}
And this is the reader.checkIfValid method:
public void checkIfValid(String filename) throws InvalidFileException, FileNotFoundException
{
try
{
in = new Scanner(new File(filename));
if (!in.hasNextLine()) // can't read first line
throw new InvalidFileException("File contains no readable text.");
}
finally
{
in.close();
}
}
This is the output I get when a nonexistent file is entered:
Please enter the text file name. (Example: file.txt):
doesNotExist.txt
java.lang.NullPointerException
Please try again.
Why is the System.out.println(e) getting a NullPointerException? When I enter an empty file or a file with text, it works just fine. The empty file prints the InvalidFileException (a custom exception) message.
When I put a try-catch statement around the "in = new Scanner(new File(filename));", and have the catch block display the exception, I do get the FileNotFoundException printed out, followed by the NullPointerException (I'm not entirely sure why the catch block in the main method would be activated if the exception was already caught in the checkIfValid method...).
I've spent a while on this and I'm completely clueless as to what's wrong. Any help would be appreciated. Thanks!

edited: I think the null pointer comes from the call to reader, it is poor practise to catch all exceptions as you no longer know where they came from!
Maybe the checkIfValid method should just check if the filename is valid?
public boolean checkIfValid(String filename) {
try {
File file = new File(filename);
return file.exists();
} catch (FileNotFoundException) {
System.out.println("Invalid filename ["+filename+"] "+e);
}
}
Then the code calling it could look like;
filename = in.next();
valid = reader.checkIfValid(filename);
if (valid)
List<String> fileContents = readFromFile(filename);
Then contain all the file reading logic in it's own method like this;
public List<String> readFromFile(filename) {
List<String> fileContents = new ArrayList<String>();
try {
in = new Scanner(new File(filename));
while (in.hasNextLine()) {
fileContents.add(in.nextLine);
}
} catch (IOException e){
//do something with the exception
} finally {
in.close();
}
return fileContents;
}

My mistake was something only I could've seen. I was catching all the exceptions so I wasn't able to see where it was coming from. Thank you for helping!

How do I call method if exception thrown?

I have a method called readinFile and if the user enters a wrong file instead of exiting I wanted to call the method readinFile again inside the readinFile method I ask the user for new filename. The problem I am running into is the first time it goes through it and gives the exception file not found than it goes through the catch(). I want it to call the method and not run the last inputStream.
try
{
inputStream = new Scanner(new FileInputStream(fileName));
}
catch(FileNotFoundException E)
{
readinfile(table, numberOfColumns, header,
original, sntypes,displaySize,
writeOut,inputStream,fileName );
System.out.print("It got here after doing the method call");
}

You should generally not use exceptions for branching. Just check for the existance of the file using File.exists, like so:
new File(fileName).exists()

You probably want to do something like this:
String fileName;
do {
System.out.println("Please enter filename");
fileName = getFileNameFromInput();
File file = new File(fileName);
} while (!file.exists());
readFile(file);
EDIT:
As Bruno Reis has pointed out, this will only check if the file exists when the user specified the file name. If the file was to be moved/deleted between specifying the file name and reading it then a FileNotFoundException would still be thrown.
To reduce the risk of this you can lock the file as discussed in this question.

bool invalidFilename = true;
string fileName;
while(invalidFilename)
{
readinfile(...);
invalidFilename = !new File(fileName).exists();
}
inputStream = new Scanner(new FileInputStream(fileName));

You can check if the filename the user input does exists or not, and don't need to catch the exception. (which is not a good design code, decrease the readability of the code)....
as inflagranti said,
you can do this pseudocode
if (!new File(filename).exists()){
//read your other file from user
readinfile(....)
}

To get what you are after, without the chance of the file being deleted after you check for it existing but before you open it do something like:
boolean done = false;
String fileName = fileNameParameter;
while(!done)
{
try
{
inputStream = new Scanner(new FileInputStream(fileName));
done = true;
}
catch(FileNotFoundException E)
{
fileName = /* ask the user for the file name */
}
}