《Java并发编程实践》中4.3.5章demo的一个疑问？

Question

Demo code结束后，有一句话没太理解：“PlublishingVehicleTracker是线程安全的，但是如果它在车辆位置的有效值上施加了任何约束，那么就不再是线程安全的”，SafePoint本身已经通过内置锁保证了对象内数据的线程安全，那么即使在set方法内加入了一些判断location位置的有效性代码也不会导致线程不安全吧。求指点！代码如下：@ThreadSafepublic 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);      }}@ThreadSafepublic 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;    }}