Reusability in computer programming is the measurement of the likelihood that a given unit of code can be successfully incorporated into another program. Computer programming languages offer varying degrees of reusability. In structured programming languages, programmers develop code libraries that contain frequently used routines, such as those related to opening a window and displaying it on the screen. To make use of a library routine, the programmer writes a statement that calls the routine.
Although this procedure sounds simple in practice, the programmer must know a great deal about the routine and must call the routine according to the routine’s precise (and often complex) rules. See library, library routine, object, object-oriented programming, structured programming.