Defines the skeleton of an algorithm deferring some steps to subclasses. Template methods lets subclasses redefine certain steps of algorithm without changing the algorithm structure.
Pattern type - Behavioral
Static elements of an algorithm are defined in base abstract class while what varies is implemented in subclasses.
In such case we rather speak about abstract class rather than an interface.
UML diagram shows that templateMethod implements some known algorithm
which elements are modified by specific implementations of method1 and method2
in concrete class (this methods may also be called operations in other examples).
Template Method and Strategy UML diagrams look alike, however, pattern purposes are different.
While Strategy is about implementing unified interface for family of algorithms, Template intends
to modify specific algorithm. It can be used when general approach (algorithm) is known (!!) and only some parts of it are expected to be modified.
Strategy focus on delivering composition approach while Template focus on inheritance.
Both of them help to meet open/close principal.
Template, can be tricky to manage in a multilevel inheritance environment.
In a given example data extraction class is implemented. DataExtractionABC provides
analysing data from files. Two classes are implemented:
TextDataExtractionCSVDataExtraction
which provides mechanisms dealing with text/csv files.
Method processData calls getFileContent and parseFileContent.
parseFileContent is a virtual method in DataExtractionABC class and is implemented
separately for TextDataExtraction and CSVDataExtraction.