how to delete the content of text file without deleting itself

Question

I want to copy the content of file \'A\' to file \'B\'. after the copying is done I want to clear the content of file \'A\' and want to write on it from its beginning. I can

Answer 1

Simple, write nothing!

FileOutputStream writer = new FileOutputStream("file.txt");
writer.write(("").getBytes());
writer.close();