Software Engineering is the application of a systematic, disciplined, quantifiable approach to the development, operation, and maintenance of software, and the study of these approaches; that is, the application of engineering to software...
, the template method pattern is a design pattern.
It is a behavioral pattern
Behavioral pattern
In software engineering, behavioral design patterns are design patterns that identify common communication patterns between objects and realize these patterns...
Templates are a feature of the C++ programming language that allow functions and classes to operate with generic types. This allows a function or class to work on many different data types without being rewritten for each one....
.
Introduction
A template method defines the program skeleton of an algorithm
Algorithm
In mathematics and computer science, an algorithm is an effective method expressed as a finite list of well-defined instructions for calculating a function. Algorithms are used for calculation, data processing, and automated reasoning...
. One or more of the algorithm steps can be overridden by subclasses to allow differing behaviors while ensuring that the overarching algorithm is still followed.
In object-oriented programming, first a class is created that provides the basic steps of an algorithm design
Algorithm design
Algorithm design is a specific method to create a mathematical process in solving problems. Applied algorithm design is algorithm engineering....
. These steps are implemented using abstract methods. Later on, subclasses change the abstract methods to implement real actions. Thus the general algorithm is saved in one place but the concrete steps may be changed by the subclasses.
The template method thus manages the larger picture of task semantics
Semantics
Semantics is the study of meaning. It focuses on the relation between signifiers, such as words, phrases, signs and symbols, and what they stand for, their denotata....
, and more refined implementation details of selection and sequence of methods. This larger picture calls abstract and non-abstract methods for the task at hand. The non-abstract methods are completely controlled by the template method but the abstract methods, implemented in subclasses, provide the pattern's expressive power and degree of freedom. Some or all of the abstract methods can be specialized in a subclass, allowing the writer of the subclass to provide particular behavior with minimal modifications to the larger semantics. The template method (which is non-abstract) remains unchanged in this pattern, ensuring that the subordinate non-abstract methods and abstract methods are called in the originally-intended sequence.
The template method occurs frequently, at least in its simplest case, where a method calls only one abstract method, with object oriented languages. If a software writer uses a polymorphic
Polymorphism in object-oriented programming
Subtype polymorphism, almost universally called just polymorphism in the context of object-oriented programming, is the ability to create a variable, a function, or an object that has more than one form. The word derives from the Greek "πολυμορφισμός" meaning "having multiple forms"...
method at all, this design pattern may be a rather natural consequence. This is because a method calling an abstract or polymorphic function is simply the reason for being of the abstract or polymorphic method. The template method may be used to add immediate present value to the software or with a vision to enhancements in the future.
The Template method pattern is strongly related to the Non-Virtual Interface (NVI) pattern. The NVI pattern recognizes the benefits of a non-abstract method invoking the subordinate abstract methods. This level of indirection allows for pre and post operations relative to the abstract operations both immediately and with future unforeseen changes. The NVI pattern can be deployed with very little software production and runtime cost. Many commercial software framework
Software framework
In computer programming, a software framework is an abstraction in which software providing generic functionality can be selectively changed by user code, thus providing application specific software...
s employ the NVI pattern.
Template method implements the Protected variations GRASP principle, like the Adapter pattern
Adapter pattern
In computer programming, the adapter pattern is a design pattern that translates one interface for a class into a compatible interface...
does. The difference is that Adapter gives the same interface for several operations while Template Method does so only for one.
Method overriding, in object oriented programming, is a language feature that allows a subclass or child class to provide a specific implementation of a method that is already provided by one of its superclasses or parent classes...
) behavior that can vary
avoid duplication in the code: the general workflow structure is implemented once in the abstract class's algorithm
Algorithm
In mathematics and computer science, an algorithm is an effective method expressed as a finite list of well-defined instructions for calculating a function. Algorithms are used for calculation, data processing, and automated reasoning...
, and necessary variations are implemented in each of the subclasses.
In object-oriented programming , inheritance is a way to reuse code of existing objects, establish a subtype from an existing object, or both, depending upon programming language support...
is allowed. As opposed to a simple polymorphic override, where the base method would be entirely rewritten allowing radical change to the workflow, only the specific details of the workflow are allowed to change.
In software engineering, Inversion of Control is an abstract principle describing an aspect of some software architecture designs in which the flow of control of a system is inverted in comparison to procedural programming....
) that is the result of the application of a template pattern is often referred to as the Hollywood Principle
Hollywood Principle
In computer programming, the Hollywood principle is stated as "don't call us, we'll call you." It has applications in software engineering; see also implicit invocation for a related architectural principle.-Overview:...
: "Don't call us, we'll call you." Using this principle, the template method in a parent class controls the overall process by calling subclass methods as required. This is shown in the following Java example:
Example
/**
* An abstract class that is
* common to several games in
* which players play against
* the others, but only one is
* playing at a given time.
*/
/* A template method : */
public final void playOneGame(int playersCount) {
this.playersCount = playersCount;
initializeGame;
int j = 0;
while (!endOfGame) {
makePlay(j);
j = (j + 1) % playersCount;
}
printWinner;
}
}
//Now we can extend this class in order
//to implement actual games:
class Monopoly extends Game {
/* Implementation of necessary concrete methods */
void initializeGame {
// Initialize players
// Initialize money
}
void makePlay(int player) {
// Process one turn of player
}
boolean endOfGame {
// Return true if game is over
// according to Monopoly rules
}
void printWinner {
// Display who won
}
/* Specific declarations for the Monopoly game. */
// ...
}
class Chess extends Game {
/* Implementation of necessary concrete methods */
void initializeGame {
// Initialize players
// Put the pieces on the board
}
void makePlay(int player) {
// Process a turn for the player
}
boolean endOfGame {
// Return true if in Checkmate or
// Stalemate has been reached
}
void printWinner {
// Display the winning player
}
/* Specific declarations for the chess game. */
In object-oriented programming , inheritance is a way to reuse code of existing objects, establish a subtype from an existing object, or both, depending upon programming language support...
Method overriding, in object oriented programming, is a language feature that allows a subclass or child class to provide a specific implementation of a method that is already provided by one of its superclasses or parent classes...
