What is serialVersionUID and why do I use it?

[java.io.Serializable] If you look at the Java document, the answer you want is well explained.

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:

ANY-ACCESS-MODIFIER static final long serialVersionUID = 42L;

If a serializable class does not explicitly declare a serialVersionUID, then the serialization runtime will calculate a default serialVersionUID value for that class based on various aspects of the class, as described in the Java(TM) Object Serialization Specification. However, it is strongly recommended that all serializable classes explicitly declare serialVersionUID values, since the default serialVersionUID computation is highly sensitive to class details that may vary depending on compiler implementations, and can thus result in unexpected InvalidClassExceptions during deserialization. Therefore, to guarantee a consistent serialVersionUID value across different java compiler implementations, a serializable class must declare an explicit serialVersionUID value. It is also strongly advised that explicit serialVersionUID declarations use the private modifier where possible, since such declarations apply only to the immediately declaring class--serialVersionUID fields are not useful as inherited members.

Converting objects into streams is called serialization of objects. Conversely, converting a stream to an object is called de-serialization of the object. Java Virtual Machines (JVMs) give version numbers for classes at the point of serialization and deserialization. Assign a new version number if the class definition has changed at that point. Therefore, if the version number when serializing is different from the version number when serializing, deserialization may not be possible. To solve this problem, developers can decide the version number themselves. That's the long-type static final field serialVersionUID. Setting the value of the field allows serialization/reverse serialization even if the class definition is changed.