Why is my "users" list still empty after all I've done?

Level 41 , Bayreuth, Germany

6 November, 11:58

I tried to answer your question already yesterday but you deleted it before I was able to hit send. Luckily I still had the code otherwise you'd have had to wait for Lupe ;) One issue you have solved I see and you added some debug, that's excellent. I'll try to explain the problem with the mentioned code (and how to create a log file that you can easily open).

// how to create temp files:
// createFile(String prefix, String suffix) -> that's what your code invokes, you just changed the prefix
// you need:
// createFile(String prefix, String suffix, File path)
File yourFile = File.createTempFile("task2002", ".txt", new File("c:\\Temp"));

// that's what you have:
OutputStream outputStream = new FileOutputStream(yourFile);
InputStream inputStream = new FileInputStream(yourFile);

// inside the save method you get outputStream and create a PrintWriter object
PrintWriter out = new PrintWriter(outputStream);
out.println("Testing write");

outputStream.flush();
// now content is saved and flushed (flushing is a trick... totally redundant)


// because: inputStream is still occupied by PrintWriter
BufferedReader in = new BufferedReader(new InputStreamReader(inputStream));
System.out.println("Trying to read line: " + in.readLine());
// result: null, your in object can not read the content from the temp file

// try again but closing PrintWriter first, writing its buffer:
out.close();

System.out.println("Trying to read line with out closed: " + in.readLine());
// wohooo, now we can access the temp file

// so don't forget to close streams/ resources you open in methods and don't need later again.

Level 32 , Budapest, Hungary

6 November, 13:26

Thank you very much, Thomas! Let me ask you some more questions about these two things. 1. Creating a temp file. If I write

File yourFile = File.createTempFile("task2002", ".txt", new File("C:\\Users\\passe\\Documents"));

will the programme create a temporary txt file called task2022-something in the Documents directory? Is that the correct syntax? 2. Closing streams. This is a snippet from my code:

codeGym.save(outputStream);
outputStream.flush();
outputStream.close();

InputStream inputStream = new FileInputStream(yourFile);
CodeGym loadedObject = new CodeGym();
loadedObject.load(inputStream);

Doesn't it mean that I first closed the outputStream and only then opened the inputStream to read the data? If yes, why doesn't it work? If not, how come?

Level 41 , Bayreuth, Germany

6 November, 15:07

1. yes 2. Yesterday you didn't have that close there ;) and the answer was easier. Internally if you pass a File or String to the Input- OutputStream constructor a FileDescriptor object gets created. That object represents 'an opaque handle to the underlying machine-specific structure representing an open file'. Here I assume that FileDescriptor returns the same handle for the same file --> if you open an Input- and OutputStream to the same file at the same time both use the same handle and if you close one of them, then effectively both get closed. If you just close the wrapper object all is working smooth. And I alwas recommend try-with-resources. Use that in your methods to close PrintWriter and BufferedReader. Unfortunately one can not look that up easily as FileDescriptor uses a lot of native methods.

Level 37 , Tampa, United States

6 November, 19:42

"Luckily I still had the code otherwise you'd have had to wait for Lupe ;)" This made me smile 🤣

Level 37 , Tampa, United States

6 November, 19:43

@Evgeniaa I mentioned last time that I have been impressed at how well you are learning this course, I do have a recommendation for you continuing forward: Focus on simplifying your code as much as possible. As an example, when you write a Date to file and then subsequently read it, there is no need to format it. The purpose of formatting is to make pretty looking output, or to read in unusual input. Take this example:

public static void main(String[] args) throws Exception {
    // use a date format for the unusual input
    SimpleDateFormat sdf = new SimpleDateFormat("mm-dd-yyyy");

    // create a date object with a date matching the format
    Date test1 = sdf.parse("12-25-2020");

    // this string is what would be written to file
    String dateData = test1.toString();

    // output first date to see what value it stores
    System.out.println("first date: " + test1);

    // create new date object with the String from above
    Date test2 = new Date(dateData);

    // output second date to see what value it stores
    System.out.println("Second date" + test2);
}

If you can imagine writing that dateString to the file in the save() method, then reading the String in the load() method and just using that to initialize a new Date which can be passed to the setBirthDate() method. You can simplify the save/load logic of the Country enum too. Take this bit of code for example:

public static void main(String[] args) throws Exception {
    Country test = Country.UNITED_KINGDOM;
    // This is what would be written to file
    System.out.println(test.toString());

    // you can load an enum with it's value
    Country test2 = Country.valueOf("UNITED_STATES");

    System.out.println(test2);
}

So you could write the value of the enum to file and then, in the load method, pass that String to the valueOf() method to get the correct enum.

Level 37 , Tampa, United States

6 November, 19:51

You can also do the same with the Boolean:

public static void main(String[] args) throws Exception {
        boolean save = false;
        String x = String.valueOf(save);
        System.out.println(x);

        boolean load = Boolean.parseBoolean(x);

        System.out.println(load);
}

In this example the top boolean save would be written to file, I simulate this by storing the value in a String x and then outputting the value of x to show it holds the boolean value. Then, in the load method, you can parse the String you read from the file, which i simulate by creating the boolean load and outputting the result just to show it worked.

Level 37 , Tampa, United States

6 November, 20:13

Finally I also would recommend not writing the int (line 73):

printWriter.println(users.size());

When you save it will write each user exactly as instructed, in your code each item on a new line like this: user.getFirstName() user.getLastName() user.getBirthDate() user.isMale() user.getCountry() Then, the next User data would start on the line directly after the end of the last one with the same exact items. So your saved file would look like this: 5 lines of a user 5 lines of a user 5 lines of a user 5 lines of a user **for however many users you have** So your load method should look more like this:

while there are still lines to be read {
    read 5 lines while initializing a new User.
}

Using my advice from the 2 posts above you should be able to shorten the amount of code in the read() and load() methods by quite a bit.

Level 37 , Tampa, United States

6 November, 20:26

Now, with all that being said, I think you can salvage your code with one simple line: Go to line 90 of the Solution.CodeGym class and outside the if block do this:

}  // line 90
    printWriter.flush(); // add this line
} // was line 91, now it will be 92.

Level 41 , Bayreuth, Germany

6 November, 21:06

@Lupe If the date must not necessarily be human readable it is (imho) better to save it as long

Date date = new Date();

// convert to long and save as Sting in a file
String millis = String.valueOf(date.getTime());

// saved data is similar to: 1730926769006

// now reading again into a Date object
Date readDate = new Date(Long.parseLong(millis));
System.out.println(readDate);

Level 37 , Tampa, United States

6 November, 21:27

That would definitely save characters in the file which, if the file becomes HUGE, would benefit from that saved space.

Level 32 , Budapest, Hungary

8 November, 11:17

Guadalupe, thank you very much! I simplified my code a lot:). The reminder to use printWriter.flush(); helped considerably. Let me ask why it's enough to use flush(). Why shouldn't I use close() here?

Level 32 , Budapest, Hungary

8 November, 11:18

Thomas, thank you very much, that was really helpful!

Level 41 , Bayreuth, Germany

8 November, 13:13

Imho you should use close here. Always close resources. When you leave the save method then the data hasn't necessarily been written to disk. It may still be buffered inside the PrintWriter objects cache. The buffer gets written using flush() or when you close the stream.

Level 37 , Tampa, United States

8 November, 19:42

Thomas is correct. The close() method is the more proper to use in this case because you are done using the resource (the PrintWriter). The flush() method is used to ensure that the data in the buffer is promptly written to the file when you are planning on continued usage of the resource.

Level 32 , Budapest, Hungary

12 November, 09:30

Thank you very much, I hope I got it)