将对象写入文件java

Question

I am creating an application that manages an agenda. Each contact must be written in the object file. How can I check if the file exists and is there a way to write the next object without overwriting??

My class:

import java.io.*;

public class WriteFile {
    public void setContact(Agenda contact){
        ObjectOutputStream out;

        try{
            File file = new File("Contacts.txt");
            out = new ObjectOutputStream(new FileOutputStream(file));
            out.writeObject(contact);


            out.flush();
            out.close();
            System.out.println("Object written to file");
        }
        catch (FileNotFoundException ex) {
            System.out.println("Error with specified file") ;
            ex.printStackTrace();
        }
        catch (IOException ex) {
            System.out.println("Error with I/O processes") ;
            ex.printStackTrace();
        }             
    }

}

Answer 1

Use file.exists() to check if a file exists.

If it does, read in the old data from the file then write the old data and the new data to a temporary file. Delete the old file and rename the temp file to 'Contacts.txt'.

Answer 2

Using your code, I believe the easiest thing to do would be to make use of the file.exists() method to check to see if the file exists.

boolean exists = file.exists();

Then you can use the following constructor for the the FileOutputStream:

public FileOutputStream(File file, boolean append) throws FileNotFoundException

to append to the end of the file. Obviously the construction of that object should be wrapped in an if-else clause, depending on the value of the file.exists().

Answer 3

first, object in which able to be written have to be an instance of a class which implements Serializable interface, that interface has no method to be implemented, it's just a flag which tell that it's instaces are can be serialized.

second to make writing countinously, or not overwrite the old data, use this in your code:

out = new ObjectOutputStream(new FileOutputStream(file), true);

there is a 'true' at the back seat, to make it appends,.