Java.io.NotSerializableException: java.util.HashMap $ Node

Question

I wrote a class that works in conjunction with serializing objects. There seems to be an exception at the serialization stage. I could not find anything about this exception. I also failed to find the inner class with the alias Node in the HashMap.

[04/01/15 2:33 PM]: java.io.NotSerializableException: java.util.HashMap$Node
[04/01/15 2:33 PM]:     at java.io.ObjectOutputStream.writeObject0(ObjectOutputStream.java:1184)
[04/01/15 2:33 PM]:     at java.io.ObjectOutputStream.writeObject(ObjectOutputStream.java:348)
[04/01/15 2:33 PM]:     at java.util.ArrayList.writeObject(ArrayList.java:762)
[04/01/15 2:33 PM]:     at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
[04/01/15 2:33 PM]:     at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62)
[04/01/15 2:33 PM]:     at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
[04/01/15 2:33 PM]:     at java.lang.reflect.Method.invoke(Method.java:483)
[04/01/15 2:33 PM]:     at java.io.ObjectStreamClass.invokeWriteObject(ObjectStreamClass.java:988)
[04/01/15 2:33 PM]:     at java.io.ObjectOutputStream.writeSerialData(ObjectOutputStream.java:1496)
[04/01/15 2:33 PM]:     at java.io.ObjectOutputStream.writeOrdinaryObject(ObjectOutputStream.java:1432)
[04/01/15 2:33 PM]:     at java.io.ObjectOutputStream.writeObject0(ObjectOutputStream.java:1178)
[04/01/15 2:33 PM]:     at java.io.ObjectOutputStream.defaultWriteFields(ObjectOutputStream.java:1548)
[04/01/15 2:33 PM]:     at java.io.ObjectOutputStream.writeSerialData(ObjectOutputStream.java:1509)
[04/01/15 2:33 PM]:     at java.io.ObjectOutputStream.writeOrdinaryObject(ObjectOutputStream.java:1432)
[04/01/15 2:33 PM]:     at java.io.ObjectOutputStream.writeObject0(ObjectOutputStream.java:1178)
[04/01/15 2:33 PM]:     at java.io.ObjectOutputStream.writeObject(ObjectOutputStream.java:348)
[04/01/15 2:33 PM]:     at ab.server.data.ServerData.saveField(ServerData.java:157)
[04/01/15 2:33 PM]:     at ab.server.data.ServerData.processQueue(ServerData.java:195)
[04/01/15 2:33 PM]:     at ab.Server.lambda$0(Server.java:260)
[04/01/15 2:33 PM]:     at ab.Server$$Lambda$15/2011997442.run(Unknown Source)
[04/01/15 2:33 PM]:     at java.util.concurrent.Executors$RunnableAdapter.call(Executors.java:511)
[04/01/15 2:33 PM]:     at java.util.concurrent.FutureTask.runAndReset(FutureTask.java:308)
[04/01/15 2:33 PM]:     at java.util.concurrent.ScheduledThreadPoolExecutor$ScheduledFutureTask.access$301(ScheduledThreadPoolExecutor.java:180)
[04/01/15 2:33 PM]:     at java.util.concurrent.ScheduledThreadPoolExecutor$ScheduledFutureTask.run(ScheduledThreadPoolExecutor.java:294)
[04/01/15 2:33 PM]:     at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1142)
[04/01/15 2:33 PM]:     at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:617)
[04/01/15 2:33 PM]:     at java.lang.Thread.run(Thread.java:745)

This is a class that is serializable.

public class WellOfGoodWill implements Serializable {

    private static final long serialVersionUID = -3335621107999346623L;

    /**
     * The total number of winners possible for each draw
     */
    private static final int MAXIMUM_WINNERS = 3;

    /**
     * Compares the value of an entry
     */
    private static final Comparator<Entry<String, Integer>> HIGHEST_VALUE = Entry.comparingByValue();

    /**
     * Contains all of the entries for the week
     */
    private HashMap<String, Integer> entries = new HashMap<>();

    /**
     * Winners of the previous week
     */
    private Collection<Entry<String, Integer>> winners;

    /**
     * The date the week is over. The initial date is set.
     */
    private Date date = Misc.getFutureDate(2014, Calendar.DECEMBER, 28, 18, 0, 0);

    /**
     * Requests that an update be made on the entries variable.
     * @param player    the key being updated
     * @param amount    the amount being added 
     */
    public void update(String player, Integer amount) {
        if (!entries.containsKey(player)) {
            entries.put(player, amount);
        } else {
            int oldValue = entries.get(player);
            entries.replace(player, oldValue, oldValue + amount);
        }
        Server.getServerData().setWellOfGoodWill(this);
    }

    /**
     * Clears all entries in the map
     */
    public void clear() {
        entries.clear();
    }

    /**
     * Determines the weekly winner based on their contributions
     * @return  the winner
     */
    public Collection<Entry<String, Integer>> getSortedResults() {
        List<Entry<String, Integer>> list = new ArrayList<>(entries.entrySet());
        list.sort(HIGHEST_VALUE);
        Collections.reverse(list);
        return new ArrayList<>(list.subList(0, list.size() < MAXIMUM_WINNERS ? list.size() : MAXIMUM_WINNERS));
    }

    /**
     * The map containing all of the entries for the week
     * @return  the entries
     */
    public Map<String, Integer> getEntries() {
        return entries;
    }

    /**
     * The winner of the previous week
     * @return  the winner
     */
    public Collection<Entry<String, Integer>> getWinners() {
        return winners;
    }

    /**
     * Sets the new winner of the week
     * @param winner    the winner
     */
    public void setWinners(Collection<Entry<String, Integer>> winners) {
        this.winners = winners;
    }

    /**
     * The date the week started
     * @return  the date
     */
    public Date getDate() {
        return date;
    }

    /**
     * Sets the date of the week the event starts on
     * @param date  the date
     */
    public void setDate(Date date) {
        this.date = date;
    }

}

Answer 1

It is not enough that the class you are trying to serialize Serializable, all the fields of these classes should also be Serializable. In particular, most interfaces, including Collectionand Map.Entry, do not implement Serializable.

, Serializable. NotSerializableException .

, (, ArrayList, List Collection), .

, writeObject() Serializable transient, , .

: https://docs.oracle.com/javase/8/docs/technotes/guides/serialization/index.html