Without a doubt the idea of object-oriented programming has brought some motion into the field of programming methodology and enlarged the set of programming languages. Object-oriented programming is nothing new-it first arose in the sixties. The motivation came from the simulation of discrete event systems. The concept first manifested itself in the language Simula 67. It took nearly two decades for the method to gain impetus, and today object-oriented programming is an important concept and a powerful technique. Meanwhile, we can even speak of an over reaction, for the concept has become a buzzword. But buzzwords always appear where there is the hope of exploiting ill-informed clients because they see the new approach as the solution to all their problems. Thus object-oriented programming is often hailed as a panacea. And so the question is justified: What is really behind it? To let the cat out of the bag: There is more to object-oriented programming than merely putting data as objects in the fore ground, instead of algorithms to which the data are subject. It is more than purely an alternative view of programmed systems. To identify the essence of object-oriented programming, is the subject of this book. This is a textbook that shows in a didactically skillful way which concepts and constructs are new, where they can be employed reasonably, and what advantages they offer. For, not all programs are automatically improved by merely recasting them in an object-oriented style.