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Modifying txt file in Java without creating temp file

I have a .txt file that will be accessed by many users, possibly at the same time (or close to that) and because of that I need a way modify that txt file without creating a temporary file and I haven't found answer or solution to this. So far, I only found this approach ->
Take existing file -> modify something -> write it to a new file (temp file) -> delete the old file.
But his approach is not good to me, I need something like: Take existing file -> modify it -> save it.
Is this possible? I'm really sorry if this question already exists, I tried searching Stack-overflow and I read thru Oracle Docs but I haven't found solution that suits my needs.
EDIT:
After modification, file would stay the same size as before. For example imagine list of students, each student can have value 1 or 0 (passed or failed the exam)
So in this case I would just need to update one character per row in a file (that is per, student). Example:
Lee Jackson 0 -> Lee Jackson 0
Bob White 0 -> would become -> Bob White 1
Jessica Woo 1 -> Jessica Woo 1
In the example above we have a file with 3 records one below other and I need to update 2nd record while 1st and 3rd would became the same and all that without creating a new file.

Here's a potential approach using RandomAccessFile. The idea would be to use readline to read it in strings but to remember the position in the file so you can go back there and write a new line. It's still risky in case anything in the text encoding would change byte lenght, because that could overwrite the line break for example.
void modifyFile(String file) throws IOException {
try (RandomAccessFile raf = new RandomAccessFile(file, "rw")) {
long beforeLine = raf.getFilePointer();
String line;
while ((line = raf.readLine()) != null) {
// edit the line while keeping its length identical
if (line.endsWith("0")) {
line = line.substring(0, line.length() - 1) + "1";
}
// go back to the beginning of the line
raf.seek(beforeLine);
// overwrite the bytes of that line
raf.write(line.getBytes());
// advance past the line break
String ignored = raf.readLine();
// and remember that position again
beforeLine = raf.getFilePointer();
}
}
}
Handling correct String encoding is tricky in this case. If the file isn't in the encoding used by readline() and getBytes(), you could workaround that by doing
// file is in "iso-1234" encoding which is made up.
// reinterpret the byte as the correct encoding first
line = new String(line.getBytes("ISO-8859-1"), "iso-1234");
... modify line
// when writing use the expected encoding
raf.write(line.getBytes("iso-1234"));
See How to read UTF8 encoded file using RandomAccessFile?

Try storing the changes you want to make to a file in the RAM (string or linked list of strings). If you read in the file to a linked list of strings (per line of the file) and write a function to merge the string you want to insert into that linked list of lines from the file and then rewrite the file entirely by putting down every line from the linked list it should give you what you want. Heres what I mean in psudocode the order is important here.
By reading in the file and setting after input we minimize interference with other users.
String lineYouWantToWrite = yourInput
LinkedList<String> list = new LinkedList<String>()
while (file has another line)
list.add(file's next line)
add your string to whatever index of list you want
write list to file line by line, file's first line = list[1]...

Java String contains/indexof fails due to wrong encoding from local file

EDIT:
I have a semi-working solution at the bottom.
Or, the original text:
I have a local CSV file. The file is encoded in utf16le. I want to read the file into memory in java, modify it, then write it out. I have been having incredibly strange problems for hours.
The source of the file is Facebook leads generation. It is a CSV. Each line of the file contains the text "2022-08-08". However when I read in the line with a buffered reader, all String methods fail. contains("2022-08-08") returns false. I print out the line directly after checking, and it indeed contains the text "2022-08-08". So the String methods are totally failing.
I think it's possibly due to encoding but I'm not sure. I tried pasting the code into this website for help, but any part of the code that includes copy pasted strings from the CSV file refuses to paste into my browser.
int i = s.indexOf("2022");
if (i < 0) {
System.out.println(s.contains("2022") + ", "+s);
continue;
}
Prints: false, 2022-08-08T19:57:51+07:00
There are tons of invisible characters in the CSV file and in my IDE everywhere I have copy pasted from the file. I know the characters are there because when I backspace them it deletes the invisible character instead of the actual character I would expect it to delete.
Please help me.
EDIT:
This code appears to fix the problem. I think partially the problem is Facebook's encoding of the file, and partially because the file is from user generated inputs and there are a few very strange inputs. If anyone has more to add or a better solution I will award it. Not sure exactly why it works. Combined from different sources that had sparse explanation.
Is there a way to determine the encoding automatically? Windows Notepad is able to do it.
BufferedReader fr = new BufferedReader(new InputStreamReader(new FileInputStream(new File("C:\\New folder\\form.csv")), "UTF-16LE"));
BufferedWriter fw = Files.newBufferedWriter(Paths.get("C:\\New folder", "form3.txt"));
String s;
while ((s = fr.readLine()) != null) {
s = s.replaceAll("\\p{C}", "?").replaceAll("[^A-Za-z0-9],", "").replaceAll("[^\\x00-\\x7F]", "");
//doo stuff with s normally
}

You can verify what you're getting from the stream by
byte[] b = s.getBytes(StandardCharsets.UTF_16BE);
System.out.println(Arrays.toString(b));

I think the searching condition for indexOf could be wrong:
int i = s.indexOf("2022");
if (i < 0) {
System.out.println(s.contains("2022") + ", "+s);
continue;
}
Maybe the condition should be (i != -1), if I'm not wrong too much.
It's a little tricky, because for (i < 0) the string should not contain "2022".

How to keep zero begin string when export data using opencsv library

I using opencsv library in java and export csv. But i have problem. When i used string begin zero look like : 0123456 , when i export it remove 0 and my csv look like : 123456. Zero is missing. I using way :
"\"\t"+"0123456"+ "\""; but when csv export it look like : "0123456" . I don't want it. I want 0123456. I don't want edit from excel because some end user don't know how to edit. How to export csv using open csv and keep 0 begin string. Please help

I think it is not really the problem when generating CSV but the way excel treats the data when opened via explorer.
Tried this code, and viewed the CSV in a text editor ( not excel ), notice that it shows up correctly, though when opened in excel, leading 0s are lost !
CSVWriter writer = new CSVWriter(new FileWriter("yourfile.csv"));
// feed in your array (or convert your data to an array)
String[] entries = "0123131#21212#021213".split("#");
List<String[]> a = new ArrayList<>();
a.add(entries);
//don't apply quotes
writer.writeAll(a,false);
writer.close();
If you are really sure that you want to see the leading 0s for numeric values when excel is opened by user, then each cell entry be in format ="dataHere" format; see code below:
CSVWriter writer = new CSVWriter(new FileWriter("yourfile.csv"));
// feed in your array (or convert your data to an array)
String[] entries = "=\"0123131\"#=\"21212\"#=\"021213\"".split("#");
List<String[]> a = new ArrayList<>();
a.add(entries);
writer.writeAll(a);
writer.close();
This is how now excel shows when opening excel from windows explorer ( double clicking ):
But now, if we see the CSV in a text editor, with the modified data to "suit" excel viewing, it shows as :
Also see link :
format-number-as-text-in-csv-when-open-in-both-excel-and-notepad

have you tried to use String like this "'"+"0123456". ' char will mark number as text when parse into excel

For me OpenCsv works correctly ( vers. 5.6 ).
for example my csv file has a row as the following extract:
"999739059";;;"abcdefgh";"001024";
and opencsv reads the field "1024" as 001024 corretly. Of course I have mapped the field in a string, not in a Double.
But, if you still have problems, you can grab a simple yet powerful parser that fully adheres with RFC 4180 standard:
mykong.com
Mykong shows you some examples using opencsv directly and, in the end, he writes a simple parser to use if you don't want to import OpenCSV , and the parser works very well , and you can use it if you still have any problems.
So you have an easy-to-understand source code of a simple parser that you can modify as you want if you still have any problem or if you want to customize it for your needs.

.contains not working when reading from a text file?

Recently started Java and have been trying to make a database sorts of program which reads from a preset text file, the user can either search for a definition using the term or keywords/terms within the definition itself. The searching by term works fine but the key term always outputs not found.
FileReader fr = new FileReader("text.txt");
BufferedReader br = new BufferedReader(fr);
boolean found = false;
String line = br.readLine(); // first line so the term itself
String lineTwo = br.readLine(); // second line which is the definition
do {
if (lineTwo.toLowerCase().contains(keyterm.toLowerCase())) {
found = true;
System.out.println("Found "+keyterm);
System.out.println(line);
System.out.println(lineTwo);
}
} while ((br.readLine()!=null)&(!found));
if (!found){System.out.println("Not Found");} br.close(); fr.close();
This is my method used to check for the key term which works partially, it seems to be able to find the first two lines. Which causes it to output the definition of the first term if the key term is there however it doesn't work for any of the other terms.
edit
The text file it reads from looks something like this:
term
definition
term
definition
Each have their own line.
Edit 2
Thanks to #Matthew Kerian it now checks through the whole file, changing the end of the do while loop to
while (((lineTwo = br.readLine())!=null)&(!found));
It now finds the actual definition but is now outputting the wrong term with it.
Edit 3 The key term is defined by the users input
Edit 4 If it wasn't clear the output in the end I am looking for is either the definition of the term/key term if it is in the txt file or just not found if its not found.
Edit 5 Tried to look at what it was outputting and noticed it was outputting array (the first term in the text file) after every "lineTwo" it seems as though line is not updating.
Final Edit Managed to crudely solve the problem by making another text file with it flipped in the way it goes term definition it now goes definition term, lets me call upon the next line once the definition is found so it reads properly.

lineTwo is not begin refreshed with new data. Something like this would work better:
do {
if (lineTwo.toLowerCase().contains(keyterm.toLowerCase())) {
found = true;
System.out.println("Found "+keyterm);
System.out.println(line);
System.out.println(lineTwo);
}
} while (((lineTwo = br.readLine())!=null)&(!found));
We're still checking for EOF by checking nullness, but by setting it equal to line two we're constantly refreshing our buffer.

Reading specific parts of a text file using Scanner class in Java

I have gone through most of the questions related to reading a Text file using the Scanner class here, but I still don't understand how I parse the following text into different variables and then replace that text with the changed variable values.
Here is the text :
## UserID1.Credentials :
[Username] = {default}
[Password] = {passwordforadmin}
[AccType] = {Admin.FirstDefault}
[AccessLevel] = {1}
##
Now, I need the above text parsed into the following variables, where :
int UserID = 1;
String Username = "default";
String Password = "passwordforadmin";
String AccType = "Admin";
String AccUnderType = "FirstDefault";
int AccessLevel = 1;
The method I am using to read the text file is :
String content = new Scanner(new File("D:/EncryptedCredentials.txt")).useDelimiter("\\Z").next();
Where content is the String which reads the text file (D:/EncryptedCredentials.txt) and the delimiter '\Z' is used. (I need to load the whole text file into a String, because it would be continuously modified by the program, and further details should be added automatically, like if the User logged in at a specific time, say 11:00 AM, and logged out at 3:00 PM, and used A,B and C features, then the above text should be modified to :
## UserID1.Credentials :
[Username] = {default}
[Password] = {passwordforadmin}
[AccType] = {Admin.FirstDefault}
[AccessLevel] = {1}
[LastLogInTime] = {1100}
[LastLogOutTime] = {1500}
[TotalTimeSpent] = {0400}
[FeaturesUsed] = {A,B,C}
##
How do I go about this reading and writing to a text file ?
Another question is, since anyone can evidently see I am doing a login system, I need to know how encryption can be implemented into the text file so that people can access the contents of the text file only through the program. The text can be changed, of course. All I need is for the program to read it and store specific information in the appropriate variables, and then to write it back to the encrypted text file.
So, in all, what I need to know is :
How to read only specific parts of a text file loaded onto a String ?
How to modify those parts (including adding text that wasn't present before) and write them again to the text file ?
How to do the above steps while implementing encryption into the text file ?
Please explain them to me, because I don't have much knowledge about this. Thanks in advance.
Note : I'm still a beginner in Java, and I have no idea of anything more than Scanner classes, Arrays, loops, etc. and some random stuff. Please help me out.

I suggest simpler approach by using properties file.
Here is a link to an example:
http://www.mkyong.com/java/java-properties-file-examples/