Dfyjkt

Student.txt

1,Giannis,Oreos,Man
2,Maria,Karra,Woman
3,Maria,Oaka,Woman

After run my code I take this:

Student.txt

1,Giannis,Oreos,Man
2,Maria,Karra,Woman
3,Maria,Oaka,Woman,2,3,1,3,4,6

But I want, if I search for ID=2 go to 2nd line and put the numbers, like that:

Student.txt

1,Giannis,Oreos,Man
2,Maria,Karra,Woman,2,3,1,3,4,6
3,Maria,Oaka,Woman

Code:

@FXML 
TextField ID1,glossa,math,fis,xim,prog,gym;

@FXML
public void UseAddLesson() throws IOException
 Scanner x = new Scanner("src/inware/students.txt");

 FileWriter fW = new FileWriter("src/inware/students.txt",true);
 BufferedWriter bW = new BufferedWriter(fW);

 boolean found= false;

 while(!found)
 String line = x.nextLine();
 if(line.contains(ID1.getText()))
 bW.write(","+glossa.getText()+",");
 bW.write(math.getText()+",");
 bW.write(fis.getText()+",");
 bW.write(xim.getText()+",");
 bW.write(prog.getText()+",");
 bW.write(gym.getText());
 System.out.println(line);
 found= true;
 
 
 bW.close();
 fW.close();
 x.close();

Do not attempt to read/write to the same file at the same time. You also cannot append/overwrite the structure of a text file requires all the text following the inserting point to be written at a different position.

I recommend creating a temporary file and replacing the old file with the new one you're done:

@FXML
public void UseAddLesson() throws IOException
 String searchText = ID1.getText();
 Path p = Paths.get("src", "inware", "students.txt");
 Path tempFile = Files.createTempFile(p.getParent(), "studentsTemp", ".txt");
 try (BufferedReader reader = Files.newBufferedReader(p);
 BufferedWriter writer = Files.newBufferedWriter(tempFile)) 
 String line;

 // copy everything until the id is found
 while ((line = reader.readLine()) != null) 
 writer.write(line);
 if (line.contains(searchText)) 
 writer.write(","+glossa.getText()+",");
 writer.write(math.getText()+",");
 writer.write(fis.getText()+",");
 writer.write(xim.getText()+",");
 writer.write(prog.getText()+",");
 writer.write(gym.getText());
 break;
 
 writer.newLine();
 

 // copy remaining lines
 if (line != null) 
 writer.newLine();
 while ((line = reader.readLine()) != null) 
 writer.write(line);
 writer.newLine();
 
 
 

 // copy new file & delete temporary file
 Files.copy(tempFile, p, StandardCopyOption.REPLACE_EXISTING);
 Files.delete(tempFile);

Note: If you distribute the app, probably the src directory will become unavailable.