如何在JAVA中使用PrintWriter从顶部写回？

Question

Using PrintWriter to write to a txt file. But now I have a problem: Can I write from the top line after I write some lines? For example, I have write:

PrintWriter out = new PrintWriter("result.txt");
out.print("first line");
out.print("second line");

Then, can I write back from the first line? So I can write the String top = "top line" before the first line like the following:

top line
first line
second line

The problem is, I can get the content of String top only after I write "top line" and "second line". How can I write to the beginning of the file?

Thank you!

Answer 1

A simple way to implement this, would simply be to read in the entire file as a string, append your pre-fix line onto the start of it, and overwrite the contents of the file with the String's content using a PrintWriter object. It's not exactly what you wanted, but it has the exact same effect. :)

Answer 2

Use RandomAccessFile class and its seek method, but write on top doesn't insert lines but overwrites them. It's not that you want. I agree with JB Nizet comment, the best is to compute first, then write.

If you known the size of the first line you have to write 'XXXXXXXXXXXXXXX' to fill it and after writing the others lines, back to the top and replace it by the good one with the same count of characters. Possible but not recommended.