How does DelegatingVehicleTracker (p. 65 Goetz) return a “live” view?

Question

On page 65 and 66 of Java Concurrency in Practice Brian Goetz lists the following code:

@ThreadSafe
public class DelegatingVehicleTracker {
private final Con         


        

Answer 1

getLocations() will return a read-only map which will reflect updates after getLocations() is called.

getLocationsAsStatic() on the other hand, will return a read-only-snapshot (aka deep copy) of the Location map at the time getLocationsAsStatic() is called.

To illustrate:

Map locs     = // a map with point A(1,1)  in it
DelegatingVehicleTracker tracker = DelegatingVehicleTracker(locs);

Map snapshot = getLocationsAsStatic();
Map live     = getLocations();

Point newB = // a point A(2,2)
tracker.setLocation(newB);

snapshot.get("A"); // will read A(1,1)
live.get("A");      // will read A(2,2)