Deleting duplicate lines in a file using Java

Question

As part of a project I'm working on, I'd like to clean up a file I generate of duplicate line entries. These duplicates often won't occur near each other, however. I came up with a method of doing so in Java (which basically made a copy of the file, then used a nested while-statement to compare each line in one file with the rest of the other). The problem, is that my generated file is pretty big and text heavy (about 225k lines of text, and around 40 megs). I estimate my current process to take 63 hours! This is definitely not acceptable.

I need an integrated solution for this, however. Preferably in Java. Any ideas? Thanks!

Answer 1

Something like this, perhaps:

BufferedReader in = ...;
Set<String> lines = new LinkedHashSet();
for (String line; (line = in.readLine()) != null;)
    lines.add(line); // does nothing if duplicate is already added
PrintWriter out = ...;
for (String line : lines)
    out.println(line);

LinkedHashSet keeps the insertion order, as opposed to HashSet which (while being slightly faster for lookup/insert) will reorder all lines.

Answer 2

Does it matter in which order the lines come, and how many duplicates are you counting on seeing?

If not, and if you're counting on a lot of dupes (i.e. a lot more reading than writing) I'd also think about parallelizing the hashset solution, with the hashset as a shared resource.