This week's book giveaway is in the OO, Patterns, UML and Refactoring forum. We're giving away four copies of Refactoring for Software Design Smells: Managing Technical Debt and have Girish Suryanarayana, Ganesh Samarthyam & Tushar Sharma on-line! See this thread for details.
A marker interface is one that has no methods defined, such as java.io.Serializable.
These types of interfaces usually imply some type of a commitment on the part of the developer. Loosely stated, with Serializable, a commitment is made by the developer to use only the basic, standard, primitive data-types, or objects made up of them, and when using something that falls beyond that boundary, to mark it as being transient.