java.lang.ClassCastException: java.util.HashSet cannot be cast to Custom Class

Question

FATAL EXCEPTION: AsyncTask #2
Process: my.app, PID: 15441
java.lang.ClassCastException: java.util.HashSet cannot be cast to my.app.ChatNotificationHashSet
at my.app.UserHandler.getChatNotificationMessage(UserHandler.java:127)
at my.app.NotificationUtil.getChatTitle(NotificationUtil.java:100)
at my.app.NotificationUtil.showChat(NotificationUtil.java:71)
at my.app.service.GCMListenerService.onMessageReceived(GCMListenerService.java:47)
at com.google.android.gms.gcm.GcmListenerService.zzq(Unknown Source)
at com.google.android.gms.gcm.GcmListenerService.zzp(Unknown Source)
at com.google.android.gms.gcm.GcmListenerService.zzo(Unknown Source)
at com.google.android.gms.gcm.GcmListenerService.zza(Unknown Source)
at com.google.android.gms.gcm.GcmListenerService$1.run(Unknown Source)
at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1112)
at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:587)
at java.lang.Thread.run(Thread.java:818)

ChatNotificationHashSet

public class ChatNotificationHashSet<E> extends LinkedHashSet<String> {

    @Override
    public String toString() {
        // ...
    }
}

UserHandler (where the exception occurs)

public ChatNotificationHashSet<String> getChatNotificationMessage() {
        return (ChatNotificationHashSet<String>) pref.getStringSet(CHAT_NOTIFICATION_MESSAGE, null); // <- Exception occurs here
    }

    public void setChatNotificationMessage(ChatNotificationHashSet<String> messages) {
        SharedPreferences.Editor editor = pref.edit();
        editor.putStringSet(CHAT_NOTIFICATION_MESSAGE, messages);
        editor.commit();
    }

How can this happen? There shouldn't be any problems when casting null to ChatNotificationHashSet<String>, right? I couldn't think of any other problem.

Can you post the implementation of getStringSet() ? It seems that it doesn't return ChatNotificationHashSet. — Grisha Weintraub
– Grisha Weintraub, Commented Jun 20, 2016 at 7:42
It's androids SharedPreferences class (grepcode.com/file_/repository.grepcode.com/java/ext/…) — Chris
– Chris, Commented Jun 20, 2016 at 7:43

Zbynek Vyskovsky - kvr000 · Accepted Answer · 2016-06-20 07:41:21Z

3

The Set that is returned by getStringSet() does not need to be the same as you pass to SharedPreferences.Editor.putStringSet() method. It's generally up to internal implementation. I assume it serializes the data and puts it into some storage and unserializes when retrieving back.

If you need your specific structure, it's probably better to implement some wrapper which would either read all settings in the constructor or retrieve the settings on demand from underlying Set.

answered Jun 20, 2016 at 7:41

Zbynek Vyskovsky - kvr000

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Comments

Community · Accepted Answer · 2017-05-23 12:32:19Z

1

Values are simply copied into an existing HashSet. Your collection is only a source for the copy.

Refer the comments on this answer: https://stackoverflow.com/a/13446387/1364747

Interface used for modifying values in a SharedPreferences object. All changes you make in an editor are batched, and not copied back to the original SharedPreferences until you call commit() or apply() Source: SharedPreferences.Editor

Only after commit changes are copied to the original SharedPreferences.

edited May 23, 2017 at 12:32

CommunityBot

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answered Jun 20, 2016 at 7:47

Teddy

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Comments

Henning Luther · Accepted Answer · 2016-06-20 11:39:22Z

1

pref.getStringSet() returns a HashSet and not your class, even when it can handle your class as HashSet while saving.

edited Jun 20, 2016 at 11:39

user5547025

answered Jun 20, 2016 at 7:41

Henning Luther

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Comments

Vikrant Kashyap · Accepted Answer · 2016-06-20 07:45:22Z

0

Exception came in your program with this line

 return (ChatNotificationHashSet<String>) pref.getStringSet(CHAT_NOTIFICATION_MESSAGE, null); // <- Exception occurs here

because your ChatNotificationHashSet class is a Strict-Type Argument (simply say generics Supported). So Type-Safety is enabled for this class.

Here getStringSet method return the Object of Type java.util.HashSet<String> which is not a type of ChatNotificationHashSet<String>.

So, Converting a HashSet<String> type Object to ChatNotificationHashSet<String> will prone to ClassCastException as You see in your Code

answered Jun 20, 2016 at 7:45

Vikrant Kashyap

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1 Comment

Teddy Over a year ago

Generics has nothing to do with the error. The object being returned is not a ChatNotificationHashSet. Its a basic java util HashSet.

Chris · Accepted Answer · 2016-06-20 08:45:07Z

0

Thank you guys. I've chosen to save the list as an simple JSON String, which works fine.

answered Jun 20, 2016 at 8:45

Chris

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1 Comment

Teddy Over a year ago

You could probably accept one of the answers, because some of them did answer why the ClassCastException happened.

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