UniqueRandomInts.java

import java.util.HashMap;
import java.util.Iterator;
import java.util.Map;
import java.util.Random;

class UniqueInts implements Iterable<Integer> {
    private int imin;
    private int emax;
    private Random rand;
    private Map<Integer, Integer> trans;
    private int dictsize;
    private int num;

    public UniqueInts(int imin, int emax, int num, Random rand) {
        dictsize = num;
        int half = (emax - imin + 1) / 2;
        if (half < dictsize)
            dictsize = (int)half;
        trans = new HashMap<>(dictsize);
        this.imin = imin;
        this.emax = emax;
        this.rand = rand;
        this.num = num;
    }
    public UniqueInts(int imin, int emax, int num) {
        this(imin, emax, num, new Random());
    }

    @Override
    public Iterator<Integer> iterator() {
        return new Iterator<Integer>(){
            private int i = 0;
            @Override
            public boolean hasNext() {
                return i < num;
            }
            @Override
            public Integer next() {
                int current = imin + i;
                int r = rand.nextInt(emax - current) + current;
                int right;
                if (!trans.containsKey(r))
                    right = r;
                else
                    right = trans.get(r);
                int left;
                if (trans.containsKey(current)) {
                    left = trans.get(current);
                    trans.remove(current);
                }
                else
                    left = current;
                if (r > current)
                    trans.put(r, left);
                ++i;
                return right;
            }
            @Override
            public void remove() {
                throw new UnsupportedOperationException();
            }
        };
    }
}

public class UniqueRandomInts {
    public static void main(String[] args) {
        UniqueInts iter = new UniqueInts(0, 10, 9);
        for (int v : iter) {
            System.out.print(v + " ");
        }
    }
}