Blue is a system for teaching object-oriented programming, developed at the University of Sydney, Australia. It is an integrated development environment (IDE) and a programming language. Blue has been used for teaching since 1997. The development was stopped in 1999 when one of its principals, Michael Kölling, began applying the IDE design to the Java programming language, resulting in BlueJ. Many of the language features it has inherited from Eiffel make it suitable for teaching, such as design by contract (pre-conditions, post-conditions, class invariants) and automatic garbage collection.