Hi, I am working through an example in Java Concurrency in Practice and am not understanding why a concurrent-safe container is necessary in the following code.

I'm not seeing how the container "locations" 's state could be modified after construction; so since it is published through an 'unmodifiableMap' wrapper, it appears to me that an ordinary HashMap would suffice.

EG, it is accessed concurrently, but the state of the map is only accessed by readers, no writers. The value fields in the map are syncronized via delegation to the 'SafePoint' class, so while the points are mutable, the keys for the hash, and their associated values (references to SafePoint instances) in the map never change.

I think my confusion is based on what precisely the state of the collection is in the problem.

Thanks!! -Mike

Listing 4.12, Java Concurrency in Practice, (this listing available as .java here, and also in chapter form via google)

/////////////begin code

@ThreadSafe
public class PublishingVehicleTracker {

private final Map<String, SafePoint> locations;
private final Map<String, SafePoint> unmodifiableMap;

public PublishingVehicleTracker(
                        Map<String, SafePoint> locations) {
    this.locations
        = new ConcurrentHashMap<String, SafePoint>(locations);
    this.unmodifiableMap
        = Collections.unmodifiableMap(this.locations);
}

public Map<String, SafePoint> getLocations() {
    return unmodifiableMap;
}

public SafePoint getLocation(String id) {
    return locations.get(id);
}

public void setLocation(String id, int x, int y) {
    if (!locations.containsKey(id))
        throw new IllegalArgumentException(
            "invalid vehicle name: " + id);
    locations.get(id).set(x, y);
  }
}

// monitor protected helper-class

@ThreadSafe
public class SafePoint {

@GuardedBy("this") private int x, y;

private SafePoint(int[] a) { this(a[0], a[1]); }

public SafePoint(SafePoint p) { this(p.get()); }

public SafePoint(int x, int y) {
    this.x = x;
    this.y = y;
}

public synchronized int[] get() {
    return new int[] { x, y };
}

public synchronized void set(int x, int y) {
    this.x = x;
    this.y = y;
}

}

///////////end code