Template Method Pattern High Level Algorithm Monis Iqbal Snr. Software Engineer Creative Chaos (pvt.) Limited

Definition Define the skeleton of an algorithm in an operation, deferring some steps to subclasses. Template Method lets subclasses redefine certain steps of an algorithm without changing the algorithm's structure. <GOF>

Define the skeleton of an algorithm in an operation, deferring some steps to subclasses. Template Method lets subclasses redefine certain steps of an algorithm without changing the algorithm's structure. <GOF>

Different implementations of algorithms leading to different algorithm altogether. (Similar code maintained differently) Every class requires full knowledge of the algorithm. Code duplication (Yuc!!)

Different implementations of algorithms leading to different algorithm altogether. (Similar code maintained differently)

Every class requires full knowledge of the algorithm.

Code duplication (Yuc!!)

Algorithm encapsulated in single class Single control/entry point. Subclasses only need to provide specialized algorithm details.

Algorithm encapsulated in single class

Single control/entry point.

Subclasses only need to provide specialized algorithm details.

Primitive Methods Abstract class declares primitive methods leaving the subclasses to provide functionality. Primitive methods used in Template Method . Nicely documented and meaningfully named.

Abstract class declares primitive methods leaving the subclasses to provide functionality.

Primitive methods used in Template Method .

Nicely documented and meaningfully named.

Concrete methods Used in Template method Defined in abstract class Declared final

Used in Template method

Defined in abstract class

Declared final

Hooks Hook placed in the Template method. Abstract class provides implementation. Subclasses can “hook into” the algorithm if they want.

Hook placed in the Template method.

Abstract class provides implementation.

Subclasses can “hook into” the algorithm if they want.

abstract class AbstractClass { final void templateMethod() { primitiveOperation1(); primitiveOperation2(); concreteOperation(); if (hookIsTrue()) hook(); } abstract void primitiveOperation1(); abstract void primitiveOperation2(); void final concreteOperation() { … .. } boolean hookIsTrue() { return false; } void hook() {} }

abstract class AbstractClass {

final void templateMethod() {

primitiveOperation1();

primitiveOperation2();

concreteOperation();

if (hookIsTrue())

hook();

}

abstract void primitiveOperation1();

abstract void primitiveOperation2();

void final concreteOperation() {

… ..

}

boolean hookIsTrue() { return false; }

void hook() {}

}

Cache revisited Example by Code First in First Out (FIFO) Cache Least Recently Used (LRU) Cache

Example by Code

First in First Out (FIFO) Cache

Least Recently Used (LRU) Cache

Intuition Encapsulation of Algorithm Don’t call us, we’ll call you. (Hollywood Principle).

Encapsulation of Algorithm

Don’t call us, we’ll call you. (Hollywood Principle).

Questions ???