FileWriter not appending to existing file

Question

I am writing a method that takes in a List of Twitter Status objects as a parameter, opens a log file containing String represenatations of Tweets, checks if any of the String representations of the Status objects are already written to the file - if so, it removes them from the list, if not it appends the Status to the file.

Everything is working up until I attempt to write to the file. Nothing is being written at all. I am led to believe that it is due to the method having the file open in two different places: new File("tweets.txt") and new FileWriter("tweets.txt, true).

Here is my method:

    private List<Status> removeDuplicates(List<Status> mentions) {
        File mentionsFile = new File("tweets.txt");
        try {
            mentionsFile.createNewFile();
        } catch (IOException e1) {
            // Print error + stacktrace
        }

        List<String> fileLines = new ArrayList<>(); 
        try {
            Scanner scanner = new Scanner(mentionsFile);
            while (scanner.hasNextLine()) {
                fileLines.add(scanner.nextLine());
            }
            scanner.close();
        } catch (FileNotFoundException e) {
            // Print error + stacktrace
        }

        List<Status> duplicates = new ArrayList<>();    
        for (Status mention : mentions) {
            String mentionString = "@" + mention.getUser().getScreenName() + " \"" + mention.getText() + "\" (" + mention.getCreatedAt() + "\")";
            if (fileLines.contains(mentionString)) {
                duplicates.add(mention);
            } else {
                try {
                    Writer writer = new BufferedWriter(new FileWriter("tweets.txt", true));
                    writer.write(mentionString);
                } catch (IOException e) {
                    // Print error + stacktrace
                }

            }
        }

        mentions.removeAll(duplicates);
        return mentions;
    }

Answer 1

I wrote here few thoughts looking your code.

Remember to always close the object Reader and Writer.

Have a look at try-with-resources statement :

try (Writer writer = new BufferedWriter(new FileWriter("tweets.txt", true))) {
   writer.write(mentionString);
} catch (IOException e) {
   // Print error + stacktrace
}

To read an entire file in a List<String>:

List<String> lines = Files.readAllLines(Paths.get("tweets.txt"), StandardCharsets.UTF_8);

And again, I think it's a bad practice write in the same file you're reading of.

I would suggest to write in a different file if you don't have a particular constraint.

But if you really want have this behavior there are few alternative.

1. Create a temporary file as output and, when you process is successfully completed, than move it to the old one using Files.move(from, to).