Question

I’m trying to write line into a file at line-position n.

Even if line n isn’t present. In that case the file has to grow with empty lines to reach n. Basically something like writer.writeLine(n, mycontent). mycontent is a binary representation from ObjectOutputStream. Each line of the file contains a serialized object. The line-number is the index.

How can I write to a specific line? – Without using FileUtils or any non-standard API components.

This answer pretty much sums up what I want – but with writing it seems to behave different.

edit: I clarified my question due to the comments

Answer 1

Is the notion of line is very important to you? Otherwise you could probably serialize a Map in a file, and use it to write or read your objects at a specific index (in that case, the index would be key of the map).

Here’s a little example.

    public static void main(String[] args) throws Exception {
    ObjectOutputStream tocStream = new ObjectOutputStream(new FileOutputStream("myfile.toc"));
    Map<Integer,Object> tableOfContent = new HashMap<Integer, Object>();
    String myString = "dataOne";
    Date myDate = new Date();
    tableOfContent.put(0,myDate);
    tableOfContent.put(1,myString);
    tocStream.writeObject(tableOfContent);
    tocStream.flush();
    tocStream.close();
    ObjectInputStream tocInputStream = new ObjectInputStream(new FileInputStream("myfile.toc"));
    Map<Integer,Object> restoredTableOfContent = (Map<Integer, Object>) tocInputStream.readObject();
    Object restoredMyString  =  restoredTableOfContent.get(1);
    System.out.println(restoredMyString);
    tocInputStream.close();
}