serialVersionUID from java 5
I started using eclipse with java 5. I got this a compilation warning in one of the realizable class, which said something to the effect that it needs a 'serialVersionUID' field.
The 'serialVersionUID' field helps with making sure that the version of the class that has been serialized, is the same as the version of the class that is de-serialized and if it is not the a invalidClassException is thrown.
The field looks like 'private static final long serialVersionUID = 42L'.
Since it needs to be a static field, i guess each class and it's subclasses will need have one field per class declared if there is a heirarchy of classes that need to be serialized.