How to update a binary file - object

Question

I'm creating a GUI, and I use a method "getStudentInfo()" of the Student object return data type to retrieve information from the JTextFields and storing them into the "student" object.
public Student getStudentInfo() {
Student student = new Student();
String name = jtfName.getText();
student.setName(name);
String idNumber = jtfIDNumber.getText();
student.setIdNumber(idNumber);
String address = jtfAddress.getText();
student.setAddress(address);
String phoneNumber = jtfPhoneNumber.getText();
student.setPhoneNumber(phoneNumber);
String major = jtfMajor.getText();
student.setMajor(major);
return student;
}
Then, in a different class, I create an "Add" button that, when clicked, is supposed to add the "student" object into an ArrayList, and then write the ArrayList into a binary file.
private class AddButtonListener implements ActionListener {
public void actionPerformed(ActionEvent e) {
File studentFile = new File(FILENAME);
ArrayList<Student> studentList = new ArrayList<Student>();
studentList.add(text.getStudentInfo());
try {
FileOutputStream fos = new FileOutputStream(studentFile);
ObjectOutputStream oos = new ObjectOutputStream(fos);
oos.writeObject(studentList);
}
catch (FileNotFoundException fnf) {
fnf.printStackTrace();
}
catch (IOException ioe) {
ioe.printStackTrace();
}
}
}
But when I run the program and I write a student's info and add it to the binary file, then I go to add another student, it overwrites the previous student's info completely. Any help would be greatly appreciated.

Answer 1

In the actionPerformed method of your class, AddButtonListener, you have the following line of code:
FileOutputStream fos = new FileOutputStream(studentFile);
This constructor will open the file so bytes are written to the beginning of your file. Since you reopen this file each time the button is clicked, you are replacing the file contents with new data. Instead, use the constructor with the boolean parameter for appending bytes rather than overwriting...
FileOutputStream fos = new FileOutputStream(studentFile, true);
You can check out this constructor's details in the java documentation...
FileOutputStream constructor documentation