[Java OOP] Template method pattern (abstract class)

Overview

Template method pattern is a behavioral design pattern that defines the program skeleton of an algorithm in a method, called template method, which defers some steps to subclasses.

It lets one redefine certain steps of an algorithm without changing the algorithm’s structure.

Usage

The template method is used in frameworks, where each implements the invariant parts of a domain’s architecture.

Example in Java

Refer to code from WIKI:

/**
 * 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 {
 /* Hook methods. Concrete implementation may differ in each subclass*/
    protected int playersCount;
    abstract void initializeGame();
    abstract void makePlay(int player);
    abstract boolean endOfGame();
    abstract void printWinner();

    /* 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. */

    // ...
}