Guess my object: an 'objects first' game on objects' behavior and implementation with bluej

Introductory programming education following the Objects First approach introduces the concepts of object-oriented programming early on. Objects with state (fields) and behavior (methods) that offer services to their clients (via their public interface) and hide the way these services are implemented (in their implementation) are the building blocks of any larger object system. These basic properties of objects are so crucial for understanding object-oriented programming (and later on object-oriented design) that diverse approaches to teaching them should be offered. In this paper we introduce Guess My Object (GMO) as a new approach to getting in contact with objects early that can complement existing teaching approaches. In essence, GMO is a way of using BlueJ for an interactive round-based game, each consisting of two stages, behavior exploration and behavior implementation.