Template method pattern

From Wikipedia, the free encyclopedia

In software engineering, the template method pattern is a design pattern. It is a so-called behavioral pattern, and is unrelated to C++ templates.

1 Introduction
2 Usage
3 Example (in Java)
4 External link

[edit] Introduction

A template method defines the program skeleton of an algorithm. The algorithm itself is made abstract, and the subclasses of the method override the abstract methods to provide concrete behavior.

First a class is created that provides the basic steps of an algorithm. 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, 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. The expressive power and degrees of freedom occur in abstract methods that may be implemented in subclasses. Some or all of the abstract methods can be specialized in the subclass, the abstract method is the smallest unit of granularity, allowing the writer of the subclass to provide particular behavior with minimal modifications to the larger semantics. In contrast the template method need not be changed and is not an abstract operation and thus may guarantee required steps before and after the abstract operations. Thus the template method is invoked and as a consequence the subordinate non-abstract methods and abstract methods are called in the correct 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 method at all, this design pattern may be a rather natual 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 is strongly related to the NVI Non-Virtual Interface 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 unforseen changes. The NVI pattern can be deployed with very little software production and runtime cost. Many commercial frameworks employ the NVI pattern.

[edit] Usage

The template method is used to:

let subclasses implement behaviour that can vary
avoid duplication in the code: you look for the general code in the algorithm, and implement the variants in the subclasses
to control at what point(s) subclassing is allowed.

The control structure (inversion of control) that is the result of the application of a template pattern is often referred to as the Hollywood Principle: "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 example:

[edit] Example (in Java)

/**
 * 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.
 */

abstract class Game {

    private int playersCount;

    abstract void initializeGame();

    abstract void makePlay(int player);

    abstract boolean endOfGame();

    abstract void printWinner();

    /* A template method : */
    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() {
        // ...
    }

    void makePlay(int player) {
        // ...
    }

    boolean endOfGame() {
        // ...
    }

    void printWinner() {
        // ...
    }
 
    /* Specific declarations for the Monopoly game. */

    // ...

}

class Chess extends Game {

    /* Implementation of necessary concrete methods */

    void initializeGame() {
        // ...
    }

    void makePlay(int player) {
        // ...
    }

    boolean endOfGame() {
        // ...
    }

    void printWinner() {
        // ...
    }
 
    /* Specific declarations for the chess game. */

    // ...

}