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I am trying to clear the contents of a file I made in java. The file is created by a PrintWriter call. I read here that one can use RandomAccessFile to do so, and read somewhere else that this is in fact better to use than calling a new PrintWriter and immediately closing it to overwrite the file with a blank one.

However, using the RandomAccessFile is not working, and I don't understand why. Here is the basic outline of my code.

PrintWriter writer = new PrintWriter("temp","UTF-8");

while (condition) {
writer.println("Example text");

if (clearCondition) {
new RandomAccessFile("temp","rw").setLength(0);
      //  Although the solution in the link above did not include ',"rw"'
      //  My compiler would not accept without a second parameter
writer.println("Text to be written onto the first line of temp file");
}
}
writer.close();

Running the equivalent of the above code is giving my temp file the contents:
(Lets imagine that the program looped twice before clearCondition was met)

Example Text
Example Text
Text to be written onto the first line of temp file



NOTE: writer needs to be able to write "Example Text" to the file again after the file is cleared. The clearCondition does not mean that the while loop gets broken.

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ErikAGriffin
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2 Answers2

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You want to either flush the PrintWriter to make sure the changes in its buffer are written out first, before you set the RandomAccessFile's length to 0, or close it and re-open a new PrintWriter to write the last line (Text to be written...). Preferably the former:

if (clearCondition) {
writer.flush();
new RandomAccessFile("temp","rw").setLength(0);
Martin Dinov
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  • So adding `writer.flush()` caused `^@` _(which I believe is the UNIX null character)_ to be written to the file a number of times equal to (numberOfcharactersPreviouslywritten + 1) * timesLooped. The previous text was deleted but these ^@^@ characters appear before new text. – ErikAGriffin Jan 31 '14 at 23:07
  • Preferably the latter actually. The former may not work on all platforms, Windows for example. – user207421 Feb 01 '14 at 03:18
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You'll be lucky if opening the file twice at the same time works. It isn't specified to work by Java.

What you should do is close the PrintWriter and open a new one without the 'append' parameter, or with 'append' set to 'false'.

user207421
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  • I don't believe I can do what you are suggesting, because after the file is cleared the original 'writer' needs to still be able to add text to the file. – ErikAGriffin Jan 31 '14 at 23:20
  • Why? Why can't you just assign the new PrintWriter to that 'writer' variable? I don't believe you've even tried it. – user207421 Feb 01 '14 at 03:15