Tracker task

Tracker.java

import java.util.concurrent.ConcurrentHashMap;
import java.util.concurrent.ConcurrentSkipListSet;
import java.util.concurrent.atomic.AtomicInteger;

import static java.lang.Integer.parseInt;
import static java.util.Objects.requireNonNull;

/**
 * Host Tracker: Server names consist of an alphabetic host type (e.g. "apibox") concatenated with the server number, with server numbers allocated sequentially (so, "apibox:1", "apibox:2", etc. are valid hostnames). Write a  tracking class with two operations, "allocate(hostType)" and "deallocate(hostName)". The former should reserve and return the next available hostname, while the latter should release that hostname back into the pool.
 * For example:
 *
 * tracker = new Tracker();
 *
 * tracker.allocate("apibox");
 * >> "apibox:1"
 *
 * tracker.allocate("apibox");
 * >> "apibox:2"
 *
 * tracker.deallocate("apibox:1");
 * tracker.allocate("apibox");
 * >> "apibox:1"
 *
 * tracker.allocate("apibox");
 * >> "apibox:3"
 *
 * tracker.deallocate("apibox:2");
 * tracker.allocate("apibox");
 * >> "apibox:2"
 *
 * tracker.allocate("sitebox");
 */
public class Tracker {

    private final ConcurrentHashMap<String, AtomicInteger> byTypeCounters = new ConcurrentHashMap<>();
    private final ConcurrentHashMap<String, ConcurrentSkipListSet<Integer>> byTypeDeAllocations = new ConcurrentHashMap<>();

    public String allocate(String hostType) {
        requireNonNull(hostType);
        if (hostType.isBlank()) throw new IllegalArgumentException("hostType should not be blank");

        Integer nextVersion = null;

        var deallocations = byTypeDeAllocations.get(hostType);
        if (deallocations != null) {
            nextVersion = deallocations.pollFirst();
        }

        if (nextVersion == null) {
            var byTypeCounter = byTypeCounters.getOrDefault(hostType, new AtomicInteger(0));
            nextVersion = byTypeCounter.incrementAndGet();
            byTypeCounters.put(hostType, byTypeCounter);
        }

        return hostType + ":" + nextVersion;
    }

    public void deallocate(String hostname) {
        requireNonNull(hostname);
        if (hostname.isBlank()) throw new IllegalArgumentException("hostname should not be blank");

        var parts = hostname.split(":");
        if (parts.length != 2) throw new IllegalArgumentException("invalid hostname definition");

        var hostType = parts[0];
        var version = parseInt(parts[1]);

        var deallocations = byTypeDeAllocations.getOrDefault(hostType, new ConcurrentSkipListSet<>());
        deallocations.add(version);
        byTypeDeAllocations.put(hostType, deallocations);

    }

}