Encyclopedia of Computer Science and Technology

Oobject-oriented programming (OOP)During the last two decades the way in which programmersview the data structures and functions that make up programshas significantly changed. In simplified form the earliestapproach to programming was roughly the following:• Determine what results (or output) the user needs.• Choose or devise an algorithm (procedure) for gettingthat result.• Declare the variables needed to hold the input data.• Get the data from the file or user input.• Assign the data to the variables.• Execute the algorithm using those variables.• Output the result.While this type of approach often works well for small“quick and dirty” programs, it becomes problematic as thecomplexity of the program increases. In real-world applicationsdata structures (such as for a customer record orinventory file) are accessed and updated by many differentroutines, such as billing, inventory, auditing, summaryreport generation, and so on. It is easy for a programmerworking on one part of the program to make a change in adata field specification (such as changing its size or underlyingdata type) without other programmers finding out.Suddenly, other parts of the program that relied on theoriginal definitions start to “break,” giving errors, or worse,silently produce incorrect results.During the 1970s, computer scientists advocated a varietyof reforms in programming practices (see structuredprogramming) in an attempt to make code both more readableand safer from unwanted side effects. For example, the“goto” or arbitrary jump from one part of the program toanother was discouraged in favor of strictly controlled iterativestructures (see loop). Also encouraged was the declarationof local variables that could not be changed fromoutside the procedure in which they were defined.Development of Object-Oriented LanguagesHowever, a more radical programming paradigm was alsoin the making. In existing languages, there is no inherentconnection between data and the procedures that operateupon that data. For example, the employee record may bedeclared somewhere near the beginning of the program,while procedures to update fields in the record, copy therecord, print the record, and so on may well be found manypages deeper into the program.A new approach, object-oriented programming is basedon the fact that in daily life we interact with thousandsof objects. An object, such as a ball, has properties (suchas size and color) and capabilities (such as bouncing). Ininteracting with an object, we use its capabilities. It is muchmore natural to think of an object as a whole than to haveits properties and capabilities jumbled together with thoseof other objects.Simula 67, developed in the late 1960s, was the firstobject-oriented language (see Simula). It was followed inthe 1970s by Smalltalk, a language developed at the Xerox339