Whenever a class implements the Serializable interface, most IDE’s like Eclipse, IntelliJ Idea generates the serialVersionUID field, which is static, final, and type long. But what is serialVersionUID and why should we use it inside the Serializable class in Java?

A Serializable is a unique identifier for a serializable class in Java. When a class is serializable, the serialization runtime assigns it a version number called a serialVersionUID. This number helps to check if the sender and receiver of a serialized object have compatible classes for that object. If the classes have different serialVersionUIDs, then deserialization will fail and throw an InvalidClassException. A serializable class can declare its own serialVersionUID by declaring a static, final, and long field with that name. For instance:

 
The access modifier can be any valid access modifier in the serialVersionUID declaration. However, it should be private, if possible, because it only applies to the class that declares it and not to its subclasses. Array classes do not have an explicit serialVersionUID, so they use the default value computed by the serialization runtime. However, array classes do not need to match their serialVersionUID values with other classes.

 
If a serializable class does not declare a serialVersionUID, then the serialization runtime will generate a default serialVersionUID for that class. The default value depends on various aspects of the class as described in the Java™ Object Serialization Specification. However, this is not a good practice because different compilers may produce different default values, which can lead to InvalidClassException when deserializing an object. Therefore, it is strongly recommended that all serializable classes declare their own serialVersionUID values to ensure that it is consistent across different Java compiler implementations and help avoid InvalidClassException during deserialization.

To sum up, the serialVersionUID is a unique identifier for a serializable class that helps the serialization runtime to verify the compatibility of the sender and receiver of a serialized object. If the serialVersionUID of the class changes, then the serialization runtime will throw an InvalidClassException when deserializing an object of that class. Therefore, we should use serialVersionUID inside the Serializable class in Java to ensure that the class can be serialized and deserialized without any errors. The serialVersionUID can be declared explicitly by the programmer or generated automatically by most IDEs. However, it is recommended to declare it explicitly to avoid any inconsistencies across different Java compilers.