Little ID Generator, that makes ids reusable after freeing

ReusableIDGenerator.java

public class ReusableIDGenerator {
    private static Map<Integer, Boolean> map = Collections.synchronizedMap(new HashMap<Integer, Boolean>());

    private ReusableIDGenerator() {}

    public static int get() {
        for (int i = 0; i < Integer.MAX_VALUE; i++) {
            if (map.containsKey(i)) {
                if (!map.get(i)) {
                    map.put(i, true);
                    return i;
                }
            } else {
                map.put(i, true);
                return i;
            }
        }
        return -1;
    }

    public static void free(int i) {
        map.put(i, false);
    }
}