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;

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);

    }

}