public class IntIDGenerator {

    private static int sequence = 1;

    public synchronized static int generate(){
        sequence++;
        return sequence;
    }

}

public final class SDGIntIDGenerator {

    private static final AtomicInteger sequence = new AtomicInteger(1);

    private SDGIntIDGenerator() {}

    public static int generate(){
        return sequence.getAndIncrement();
    }

}

public final class SDGIntIDGenerator {

    private static final ConcurrentHashMap<Class,AtomicInteger> mapper = new ConcurrentHashMap<Class,AtomicInteger>();

    private SDGIntIDGenerator () {}

    public static int generateId (Class _class) {
        mapper.putIfAbsent(_class, new AtomicInteger(1));
        return mapper.get(_class).getAndIncrement();
    }
}