A class that implements java.io.Externalizable must have a public no-argument constructor. The constructor is used by the Java serialization framework when it creates the object during deserialization. If the class does not define such a constructor, the Java serialization framework throws an InvalidClassException.

The Java Development Kit API documentation for Externalizable states:

When an Externalizable object is reconstructed, an instance is created using the public no-arg constructor, then the readExternal method called.

Make sure that externalizable classes always have a no-argument constructor.

In the following example, WrongMemo does not declare a public no-argument constructor. When the Java serialization framework tries to deserialize the object, an InvalidClassException is thrown. However, Memo does declare a public no-argument constructor, so that the object is deserialized successfully.

Java API Documentation: Externalizable.