文件编写Java; 如何改写

Question

i'm making an Highscore implementation for my game. Here there's what i want to do: I have object Score which contains String Name and Integer score. Now :

• if Name isn't already in the file, add it
• if Name is on the file, after a space take the String and convert into integer, so i got the score.

Now, if score is better than the actual, i have to OVERWRITE it on the file... and here's my problem..i can i do that? how can i write exactly a string over another in a certain point of the file?

Answer 1

Generally it's considered too fiddly to replace text in text files for this kind of requirement, so the usual way to do it is just to read in the whole file, make the replacement and write a new version of the whole file. If you have large amounts of data you would use a database or a NoSQL solution instead.

PS consider using serialization, it can make things easier.

Answer 2

I have object Score which contains String Name and Integer score.

Use a Properties file for this instead. It provides an easy interface to load & save the file, get the keys (Name) and set or retrieve values (Score). String values are stored, but they can be converted to/from integer easily.

Answer 3

I concur that this is best done by fully re-serializing the entire database. On modern computers, you can push 30MB/s per disk for linear writes (more if there's sufficient cache). And if you're dealing with more than 30MB of data, you REALLY need a DB (HSQLDB, DerbyDB, BerkleyDB) are trivial DBs. Or go all the way to postgres/mysql.

However, the way to overwrite a FIXED sized section of an existing file (or rather, the way to emulate doing so), is to use:

RandomAccessFile raf = new RandomAccessFile(fileName, "rw");
try {
  raf.seek(position);
  raf.writeInt(newScore);
  raf.close();
} finally { raf.close(); }

Note using the writeInt instead of raf.write(Integer.toHexString(newScore).getBytes()), because you really really need that to be fixed in size.

Now if the text file is intrinsicly ascii (eg humans will read the file), and thus the value can't be binary.. Perhaps you could keep it HexString (because that will be fixed in size), or you could a zero-padded decimal string:

But what you absoluately positively can not do is grow the string by 1 byte. So:

bob 15
joe 7
nina 981

Can't have joe's score replaced with 10, UNLESS you've padded a bunch of spaces.

If this is your data-file, then you will absolutely have to rewrite the whole file (even if you write the extra code to only rewrite from the point of change on - statistically that'll be 50% of the file and thus not worth bothering.

One other thing - if you do rewrite, you have the risk of shortening the file.. For that you need to call raf.setLength(0); before writing the first byte.. Otherwise, you'll see phantom text beyond the end of your new file.

Answer 4

final Map<String,Long> symbol2Position = new ConcurrentHashMap<>();
final Map<String,Integer> symbol2Score    = new ConcurrentHashMap<>();
final String fileName;
final RandomAccessFile raf;
// ... skipped code
void storeFull() {
     raf.position(0);
     raf.setLength(0);
     for (Map.Entry<String,Long> e : symbol2Position.entrySet()) {

        raf.writeUTF8(e.getKey());
        raf.write(',');
        symbol2Score.put(e.getKey(), raf.position());
        raf.writeUTF8(String.format("%06d",e.getValue()));
        raf.write('\n');
     }
}
void updateScore(String key, int newScore) {
    if (symbol2Score.containsKey(key)) {
       symbol2Score.put(key, newScore);
       raf.position(symbol2Position.get(key));
       raf.writeString(String.format("%06d", newScore));
    } else {
       symbol2Score.put(key, newScore); 
       long fileLen = raf.length();
       symbol2Position.put(key, fileLen);
       raf.position(fileLen);
       raf.writeString(String.format("%06d", newScore));
    }
}

I'd probably rather use a DB or binary map file.. meaning file with 8B per field, 4B pointing to user-name position, and 4B representing score. But this allows for a human readable data-file, while making updates faster than just rewriting the property file.

Check out LevelDB - fastest damn DB on the planet for embedded systems. :) Main thing it has over the above, is thousands / millions of updates per second without the rand-seek-rewrite cost of updating 6 bytes randomly across a multi-GB file.

Answer 5

Just a thought, any specific reason for the file storage of names and scores ?

Seems like a Map<String, Integer> would serve you much better...