What does it mean The serializable class does not declare a static final serialVersionUID field

Question

I have the warning message given in the title  I would like to understand and remove it  I found already some answers on this question but I do not understand these answers because of an overload with technical terms  Is it possible to explain this issue with simple words   P S  I know what OOP is  I know what is object  class  method  field and instantiation   P P S  If somebody needs my code it is here   import java awt    import javax swing      public class HelloWorldSwing extends JFrame            JTextArea m resultArea   new JTextArea 6  30                                                                     constructor         public HelloWorldSwing                       Set initial text  scrolling  and border              m resultArea setText  Enter more text to see scrollbars                JScrollPane scrollingArea   new JScrollPane m resultArea               scrollingArea setBorder BorderFactory createEmptyBorder 10 5 10 5                    Get the content pane  set layout  add to center             Container content   this getContentPane                content setLayout new BorderLayout                 content add scrollingArea  BorderLayout CENTER               this pack                       public static void createAndViewJFrame                 JFrame win   new HelloWorldSwing                win setTitle  TextAreaDemo                win setDefaultCloseOperation JFrame EXIT ON CLOSE               win setVisible true                                                                                      main         public static void main String   args                SwingUtilities invokeLater new Runnable                    public void run                        createAndViewJFrame

User · Answer

it must be changed whenever anything changes that affects the serialization (additional fields, removed fields, change of field order, ...)

That's not correct, and you will be unable to cite an authoriitative source for that claim. It should be changed whenever you make a change that is incompatible under the rules given in the Versioning of Serializable Objects section of the Object Serialization Specification, which specifically does not include additional fields or change of field order, and when you haven't provided readObject(), writeObject(), and/or readResolve() or /writeReplace() methods and/or a serializableFields declaration that could cope with the change.

User · Answer

The reasons for warning are documented here  and the simple fixes are to turn off the warning or put the following declaration in your code to supply the version UID   The actual value is not relevant  start with 999 if you like  but changing it when you make incompatible changes to the class is   public class HelloWorldSwing extends JFrame            JTextArea m resultArea   new JTextArea 6  30           private static final long serialVersionUID   1L

User · Answer

Any class that can be serialized  i e  implements Serializable  should declare that UID and it must be changed whenever anything changes that affects the serialization  additional fields  removed fields  change of field order        The field s value is checked during deserialization and if the value of the serialized object does not equal the value of the class in the current VM  an exception is thrown   Note that this value is special in that it is serialized with the object even though it is static  for the reasons described above

User · Answer

From the javadoc      The serialization runtime associates with each serializable class a version number  called a serialVersionUID  which is used during deserialization to verify that the sender and receiver of a serialized object have loaded classes for that object that are compatible with respect to serialization  If the receiver has loaded a class for the object that has a different serialVersionUID than that of the corresponding sender s class  then deserialization will result in an InvalidClassException  A serializable class can declare its own serialVersionUID explicitly by declaring a field named  serialVersionUID  that must be static  final  and of type long    You can configure your IDE to    ignore this  instead of giving a warning  autogenerate an id   As per your additional question  Can it be that the discussed warning message is a reason why my GUI application freeze     No  it can t be  It can cause a problem only if you are serializing objects and deserializing them in a different place  or time  where  when  the class has changed  and it will not result in freezing  but in InvalidClassException

User · Answer

The other answers so far have a lot of technical information  I will try to answer  as requested  in simple terms   Serialization is what you do to an instance of an object if you want to dump it to a raw buffer  save it to disk  transport it in a binary stream  e g   sending an object over a network socket   or otherwise create a serialized binary representation of an object   For more info on serialization see Java Serialization on Wikipedia     If you have no intention of serializing your class  you can add the annotation just above your class  SuppressWarnings  serial      If you are going to serialize  then you have a host of things to worry about all centered around the proper use of UUID  Basically  the UUID is a way to  version  an object you would serialize so that whatever process is de-serializing knows that it s de-serializing properly  I would look at Ensure proper version control for serialized objects for more information

[java] What does it mean: The serializable class does not declare a static final serialVersionUID field?

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