Can we replace a line from file with other line (i.e. new string) having different size?

Question

Example-

aa
bbb
ccc
jjj

Output-

zzzzzz      <- replaced aa by zzzzzz
bbb
ccc
jjj

Answer 1

No, there is no "insert-mode" in which you can squeeze in new bytes in the middle of the file.

You'll have to read through and "regenerate" the entire file with the characters inserted along the way:

File input = new File("data.txt");
File tmp = File.createTempFile("tmp", null);

String search = "aa";
String replacement = "zzzzzz";

PrintWriter pw = new PrintWriter(tmp);
BufferedReader br = new BufferedReader(new FileReader(input));
String line;
while ((line = br.readLine()) != null) {
    if (line.equals(search))
        pw.println(replacement);
    else
        pw.println(line);
}
br.close();
pw.close();

input.delete();
tmp.renameTo(input);