Generate random number in java

Question

am using some simple code, which should get data from config

my config looks like:

name: test
locationA: -457.0,5.0,-186.0
locationB: -454.0,5.0,-186.0
prisonfile: ./plugins/JB/test.prison
prisons:
- -454.0,4.0,-176.0
- -460.0,4.0,-176.0
- -457.0,5.0,-186.0
police:
- -460.0,5.0,-186.0
- -454.0,5.0,-186.0
- -457.0,5.0,-176.0
open: true

my code looks like:

public void enter(Player player, String lines, String lines2) 
    {
        World world = player.getWorld();
        HashMap<String, Object> prison = plugin.prisons.getPrison(world.getName(), false);

        File configFile = new File(prison.get("config").toString());
        FileConfiguration config = YamlConfiguration.loadConfiguration(configFile);
        String listName = "police";
        List<String> list = config.getStringList(listName);
        Integer ListSize = list.size();
        Random r = new Random();
        int i1=r.nextInt(ListSize-1);
        String[] parts = list.get(i1).split(",");
        player.teleport(new Location(world, Float.parseFloat(parts[0]), Float.parseFloat(parts[1]), Float.parseFloat(parts[2])));

Code works corretly and teleporting me on random positions, but it always just port on first 2 positions and never port me on 3th one, i try to print out how many coordinations found ListSize in config and its 3 so i totaly dont understand.

p.s. i need to generate randoms between 0 and MaxNumber of positions

rgettman · Accepted Answer · 2013-12-10 17:46:09Z

4

The problem is the argument to the nextInt method in this line:

int i1=r.nextInt(ListSize-1);

The range of the random number returned is 0 (inclusive) through n - 1, n being the parameter. Quoting from the Javadocs for the nextInt method:

Returns a pseudorandom, uniformly distributed int value between 0 (inclusive) and the specified value (exclusive), drawn from this random number generator's sequence.

(emphasis mine)

There is no need to subtract 1 from the list size here. Try

int i1 = r.nextInt(ListSize);

answered Dec 10, 2013 at 17:46

rgettman

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Elliott Frisch · Accepted Answer · 2013-12-10 17:49:25Z

0

You need a good random and a good seed... so you could use one of

java.util.Random random = null; // Declare the random Instance.
random = new java.util.Random(
    System.currentTimeMillis());
// or
random = new java.util.Random(System.nanoTime());
// or
random = new java.util.Random(System.nanoTime()
    ^ System.currentTimeMillis());
// or
random = new java.security.SecureRandom(); // Because it makes "security" claims! :)

random.nextInt(MaxNumber + 1); // for the range 0-MaxNumber (inclusive).

answered Dec 10, 2013 at 17:49

Elliott Frisch

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