Purpose of overloaded constructor in SynchronizedMap class

Question

In Collections class, class SynchronizedMap has two constructors. One takes only a map instance and another with a map and a mutex.

    SynchronizedMap(Map<K,V> m) {
        this.m = Objects.requireNonNull(m);
        mutex = this;
    }

    SynchronizedMap(Map<K,V> m, Object mutex) {
        this.m = m;
        this.mutex = mutex;
    }

However, SynchronizedMap class is a private static class and only way to access it using provided wrapper method:

public static <K,V> Map<K,V> synchronizedMap(Map<K,V> m) {
    return new SynchronizedMap<>(m);
}

As understood from this link, the idea of the second constructor to use user-supplied mutex other than this. Now, since the wrapper method is the only way to get an instance of SynchronizedMap (which takes only a map object) , what is the purpose of this second overloaded constructor?

Seems like it gives the map user to specify the object on which synchronization is done (which otherwise defaults to the map instance itself). — ernest_k, Jun 13 '18 at 05:22
Make up your mind. Either it is static or it is inner. `static inner` is a contradiction in terms. — user207421, Jun 13 '18 at 06:13

score 1 · Accepted Answer · answered Jun 13 '18 at 05:26

It is used e.g. in SynchronizedSortedMap which extends SynchronizedMap when creating submap view.

 public SortedMap<K,V> subMap(K fromKey, K toKey) {
            synchronized (mutex) {
                return new SynchronizedSortedMap<>(
                    sm.subMap(fromKey, toKey), mutex);
            }
        }

To share the same mutex.

Purpose of overloaded constructor in SynchronizedMap class

1 Answers1

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