In object-oriented programming , a class library is a collection of prewritten class es or coded templates, any of which can be specified and used by a programmer when developing an application program. The programmer specifies which classes are being used and furnishes data that instantiate s each class as an object that can be called when the program is executed. Access to and use of a class library greatly simplifies the job of the programmer since standard, pretested code is available that the programmer doesn't have to write.