Name: gm110360 Date: 10/01/2002


FULL PRODUCT VERSION :
Occurs in both 1.3.0 and 1.3.1:

java version "1.3.0_02"
Java(TM) 2 Runtime Environment, Standard Edition (build 1.3.0_02)
Java HotSpot(TM) Client VM (build 1.3.0_02, mixed mode)

java version "1.3.1"
Java(TM) 2 Runtime Environment, Standard Edition (build 1.3.1-b24)
Java HotSpot(TM) Client VM (build 1.3.1-b24, mixed mode)

FULL OPERATING SYSTEM VERSION :
Microsoft Windows 2000 [Version 5.00.2195]
(Service Pack 2)

A DESCRIPTION OF THE PROBLEM :
The collections returned by calls to
java.util.HashMap#entrySet, #keySet and #values are not
Serializable which, although consistent with the
documentation, is confusing and counter-intuitive because
HashMap itself is Serializable.


STEPS TO FOLLOW TO REPRODUCE THE PROBLEM :
1. Create a HashMap.
2. Attempt to serialize the result of calling keySet(),
entrySet() or values() on that HashMap.


EXPECTED VERSUS ACTUAL BEHAVIOR :
I would (naively, optimistically :-) assume these
collections to be Serializable, since the object from which
they come is itself Serializable.

ERROR MESSAGES/STACK TRACES THAT OCCUR :
keySet():
Trying to serialize [two, one, three]...failed:
java.io.NotSerializableException: java.util.HashMap$1
        at java.io.ObjectOutputStream.outputObject(ObjectOutputStream.java:1148)
        at java.io.ObjectOutputStream.writeObject(ObjectOutputStream.java:366)
        at
test.MapCollectionsSerializeMain.trySerialize(MapCollectionsSerializeMain.java:15)
        at
test.MapCollectionsSerializeMain.main(MapCollectionsSerializeMain.java:36)
entrySet():
Trying to serialize [two=two, one=one, three=three]...failed:
java.io.NotSerializableException: java.util.HashMap$3
        at java.io.ObjectOutputStream.outputObject(ObjectOutputStream.java:1148)
        at java.io.ObjectOutputStream.writeObject(ObjectOutputStream.java:366)
        at
test.MapCollectionsSerializeMain.trySerialize(MapCollectionsSerializeMain.java:15)
        at
test.MapCollectionsSerializeMain.main(MapCollectionsSerializeMain.java:38)
values():
Trying to serialize [two, one, three]...failed:
java.io.NotSerializableException: java.util.HashMap$2
        at java.io.ObjectOutputStream.outputObject(ObjectOutputStream.java:1148)
        at java.io.ObjectOutputStream.writeObject(ObjectOutputStream.java:366)
        at
test.MapCollectionsSerializeMain.trySerialize(MapCollectionsSerializeMain.java:15)
        at
test.MapCollectionsSerializeMain.main(MapCollectionsSerializeMain.java:40)

REPRODUCIBILITY :
This bug can be reproduced rarely.

---------- BEGIN SOURCE ----------
package test;

import java.io.*;
import java.util.*;

public class MapCollectionsSerializeMain
{

    private static void trySerialize (Object o) {
        try {
            ObjectOutputStream stream
                = new ObjectOutputStream(new ByteArrayOutputStream());
            System.out.print("Trying to serialize " + o + "...");
            stream.writeObject(o);
            stream.flush();
            System.out.println("done");
        }
        catch (Exception e) {
            System.out.print("failed: ");
            e.printStackTrace();
        }
    }

    // ----------------------------------------------------------------------
    // Entry point

    public static void main (String[] args) {

        Map map = new HashMap();
        map.put("one", "one");
        map.put("two", "two");
        map.put("three", "three");

        System.out.println("keySet():");
        trySerialize(map.keySet());
        System.out.println("entrySet():");
        trySerialize(map.entrySet());
        System.out.println("values():");
        trySerialize(map.values());

    }
}

---------- END SOURCE ----------

CUSTOMER WORKAROUND :
For the keySet() and valueSet() cases, one could create a
new HashSet from those sets, but that's inefficient and it
doesn't cover the values() case.

The right answer in the first place is presumably that any
Serializable object which contains a Set (or other abstract
type not documented as Serializable) must implement its own
readObject and writeObject methods which (de-)serialize the
set manually. Either that, or define the object to contain
a HashSet (or other type documented as Serializable).
(Review ID: 165184)
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