What is required of a class used as an entry or attribute?

Attributes must conform to some strict rules:

• Must implement net.jini.core.entry.Entry (which itself extends Serializable)
• Must have only public instance variables (transient, final, static, and non-public variables will not be serialized)
• All instance variables must be references to serializable objects, primitive types are not allowed
• Must have a public, no-argument constructor

An IllegalArgumentException will be thrown during serialization if the attribute has a non-object instance variable or lacks a no-argument constructor.