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COMPUTING SCIENCE

# Nouns and Verbs

The true history of software development is not a straight line but a meandering river with dozens of branches. Some of the tributaries—functional programming, declarative programming, methods based on formal proofs of correctness—are no less interesting than the mainstream, but here I have room to explore only one channel: object- oriented programming.

Consider a program for manipulating simple geometric figures. In a non-OOP environment, you might begin by writing a series of procedures with names such as rotate, scale, reflect, calculate-area, calculate-perimeter. Each of these verblike procedures could be applied to triangles, squares, circles and many other shapes; the figures themselves are nounlike entities embodied in data structures separate from the procedures. For example, a triangle might by represented by an array of three vertices, where each vertex is a pair of x and y coordinates. Applying the rotate procedure to this data structure would alter the coordinates and thereby turn the triangle.

What's the matter with this scheme? One likely source of trouble is that the procedures and the data structures are separate but interdependent. If you change your mind about the implementation of triangles—perhaps using a linked list of points instead of an array—you must remember to change all the procedures that might ever be applied to a triangle. Also, choosing different representations for some of the figures becomes awkward. If you describe a circle in terms of a center and a radius rather than a set of vertices, all the procedures have to treat circles as a special case. Yet another pitfall is that the data structures are public property, and the procedures that share them may not always play nicely together. A figure altered by one procedure might no longer be valid input for another.

Object-oriented programming addresses these issues by packing both data and procedures—both nouns and verbs—into a single object. An object named triangle would have inside it some data structure representing a three-sided shape, but it would also include the procedures (called methods in this context) for acting on the data. To rotate a triangle, you send a message to the triangle object, telling it to rotate itself. Sending and receiving messages is the only way objects communicate with one another; outsiders are not allowed direct access to the data. Because only the object's own methods know about the internal data structures, it's easier to keep them in sync.

This scheme would not have much appeal if every time you wanted to create a triangle, you had to write out all the necessary data structures and methods—but that's not how it works. You define the class triangle just once; individual triangles are created as instances of the class. A mechanism called inheritance takes this idea a step further. You might define a more-general class polygon, which would have triangle as a subclass, along with other subclasses such as quadrilateral, pentagon and hexagon. Some methods would be common to all polygons; one example is the calculation of perimeter, which can be done by adding the lengths of the sides, no matter how many sides there are. If you define the method calculate-perimeter in the class polygon, all the subclasses inherit this code.

Object-oriented programming traces its heritage back to simula, a programming language devised in the 1960s by Ole-Johan Dahl and Kristen Nygaard. Some object-oriented ideas were also anticipated by David L. Parnas. And the Sketchpad system of Ivan Sutherland was yet another source of inspiration. The various threads came together when Alan Kay and his colleagues created the Smalltalk language at the Xerox Palo Alto Research Center in the 1970s. Within a decade several more object-oriented languages were in use, most notably Bjarne Stroustrup's C++, and later Java. Object-oriented features have also been retrofitted onto older languages, such as Lisp.

As OOP has transformed the way programs are written, there has also been a major shift in the nature of the programs themselves. In the software-engineering literature of the 1960s and '70s, example programs tend to have a sausage-grinder structure: Inputs enter at one end, and outputs emerge at the other. An example is a compiler, which transforms source code into machine code. Programs written in this style have not disappeared, but they are no longer the center of attention. The emphasis now is on interactive software with a graphical user interface. Programming manuals for object-oriented languages are all about windows and menus and mouse clicks. In other words, OOP is not just a different solution; it also solves a different problem.

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