With 5 characters you would be safe for 900 days and then have to reset.
I wrote some code for another StackOverflow user a few weeks ago. It's a random generator, which only generates new numbers.
import java.util.BitSet;
import java.util.Random;
/**
* Random number generator without repeat in the given range at construction time.
*
* @author martijn
*/
public class NoRepeatRandom {
private Random random;
private BitSet used;
private int max;
/**
* Creates new instance of NoRepeatRandom, with the range <code>[0-max[</code>.
* @param max the maximum for the range
* @param seed the seed for the underlying {@link java.util.Random}
*/
public NoRepeatRandom(int max, long seed)
{
this.max = max;
this.used = new BitSet(max);
this.random = new Random(seed);
}
/**
* Creates new instance of NoRepeatRandom, with the range <code>[0-max[</code>.<br />
* <code>System.currentTimeMillis()</code> is used as seed for the underlying {@link java.util.Random}
* @param max the maximum for the range
*/
public NoRepeatRandom(int max)
{
this(max, System.currentTimeMillis());
}
/**
* Gives the next random number
* @return a new random number. When finished it returns -1.
*/
public int next()
{
if (isFinished())
{
return -1;
}
while (true)
{
int r = random.nextInt(max);
if (!used.get(r))
{
used.set(r);
return r;
}
}
}
/**
* Tells if the random generator has finished. Which means that all number in the range
* [0-max[ are used.
* @return true if all numbers are used, otherwise false.
*/
public boolean isFinished()
{
return max == used.cardinality();
}
/**
* Sets all the numbers in the range [0-max[ to unused. Which means that all the numbers
* can be reused.
*/
public void reset()
{
used.clear();
}
/**
*
* @return the maximum.
*/
public int getMax()
{
return max;
}
}
Then create an instance of it:
NoRepeatRandom nrr = new NoRepeatRandom(916132832);
And to generate a new code, use:
int codeInt = nrr.next();
if (codeInt == -1)
{
// All the codes are used, need to reset the random generator!
}
String code = toCode(codeInt);
The only remaining part is to design the toCode(int) method:
public static final String charset = "ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvw013456789";
public static String toCode(int i)
{
String code = "";
return code;
}
