Save HashSet data after restart

I created a HashSet which saves Blocks like so:

Code:

    HashSet<Block> set = new HashSet<Block>();
//Later on in the program
    set.add(event.getBlockPlaced());
//Even later
    if (set.contains(event.getClickedBlock()) {
//Do something
}

The HashSet works fine, but when my server restarts, all of the data in the HashSet is lost. How do I save the data so that when the server restarts, the block will behave the same as it would if the restart never took place?

 

Snowshovel
Several ways to do it - you can use an Object Output Stream
http://docs.oracle.com/javase/1.4.2/docs/api/java/io/ObjectOutputStream.html
Buffered writer is capable of it - but it requires a little more effort then the ObjectOutputStream.
I'm personally a big fan of BufferedWriter because I can just write text and if anything goes wrong it is easy to fix and debug because it's a text file. Certainly not efficient compared to other options though.
And there is other options too.

 

Sessional

What's the simplest way to save it over a restart?

 

Snowshovel
Personal preference - I like text files personally using BufferedWriter and then a BufferedReader to read them in.

 

Sessional

Alright, how do I do that?

 

are buffered writers (textfiles) a plausible alternative to databases (mysql?) for custom server plugins?

 

I don't think you can just save and restore blocks into and from a file. You schouldl have mentioned that in your other thread too. In this case, because you only need the location of the block, I'd recommend to save String instead of Block in the HashSet. Do this by serializing them.

Code:

private String serializeBlock(Block b) {
    StringBuilder s = new StringBuilder();
    s.append(b.getWorld().getName());
    s.append(":");
    s.append(b.getX());
    s.append(":");
    s.append(b.getY());
    s.append(":");
    s.append(b.getZ());
    return s.toString();
}
 
    HashSet<String> set = new HashSet<String>();
//Later on in the program
    set.add(serializeBlock(event.getBlockPlaced()));
//Even later
    if (set.contains(serializeBlock(event.getClickedBlock())) {
//Do something
}

 

https://github.com/Sessional/Waypoints/tree/master/src/waypoints
Check out this code, it's hard to explain, but it can get you started, and will work for flat files.

As for the mysql comment - any database is more efficient, but server owners rarely want to deal with setting one up if they can just use flat files.