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| Licensed to the Apache Software Foundation (ASF) under one |
| or more contributor license agreements. See the NOTICE file |
| distributed with this work for additional information |
| regarding copyright ownership. The ASF licenses this file |
| to you under the Apache License, Version 2.0 (the |
| "License"); you may not use this file except in compliance |
| with the License. You may obtain a copy of the License at |
| |
| http://www.apache.org/licenses/LICENSE-2.0 |
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| Unless required by applicable law or agreed to in writing, |
| software distributed under the License is distributed on an |
| "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY |
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| ////////////////////////////////////////// |
| |
| = Strategy Pattern |
| |
| The http://en.wikipedia.org/wiki/Strategy_pattern[Strategy Pattern] allows you to abstract away particular algorithms from their usage. This allows you to easily swap the algorithm being used without having to change the calling code. The general form of the pattern is: |
| |
| [plantuml, StrategyClasses, png] |
| .... |
| !pragma layout smetana |
| hide fields |
| hide <<Hidden>> circle |
| skinparam ClassBorderColor<<Hidden>> Transparent |
| skinparam ClassBackgroundColor<<Hidden>> Transparent |
| skinparam ClassStereotypeFontColor<<Hidden>> Transparent |
| skinparam ClassFontSize<<Hidden>> 24 |
| skinparam ClassFontStyle<<Hidden>> bold |
| skinparam shadowing<<Hidden>> false |
| class Context { |
| +Strategy getStrategy() |
| } |
| class Strategy { |
| +algorithmMethod() |
| } |
| class ConcreteStrategy1 { |
| +algorithmMethod() |
| } |
| class ConcreteStrategy2 { |
| +algorithmMethod() |
| } |
| class "..." as ConcreteHidden |
| class ConcreteHidden <<Hidden>> { |
| } |
| class ConcreteStrategyN { |
| +algorithmMethod() |
| } |
| Context o---r--- Strategy |
| Strategy <|-- ConcreteStrategy1 |
| Strategy <|-- ConcreteStrategy2 |
| Strategy <|-[hidden]- ConcreteHidden |
| Strategy <|-- ConcreteStrategyN |
| .... |
| |
| In Groovy, because of its ability to treat code as a first class object using anonymous methods (which we loosely call __Closures__), the need for the strategy pattern is greatly reduced. You can simply place algorithms inside Closures. |
| |
| == Example using traditional class hierarchy |
| |
| First let's look at the traditional way of encapsulating the Strategy Pattern. |
| |
| [source,groovy] |
| ---- |
| include::../test/DesignPatternsTest.groovy[tags=strategy_traditional,indent=0] |
| ---- |
| |
| Here we have defined an interface `Calc` which our concrete strategy classes will implement (we could also have used an abstract class). |
| We then defined two algorithms for doing simple multiplication: `CalcByMult` the normal way, and ++CalcByManyAdds++ using only addition (don't try this one using negative numbers - yes we could fix this but it would just make the example longer). |
| We then use normal http://en.wikipedia.org/wiki/Polymorphism_in_object-oriented_programming[polymorphism] to invoke the algorithms. |
| |
| == Example using closures |
| |
| Here is the Groovier way to achieve the same thing using Closures: |
| |
| [source,groovy] |
| ---- |
| include::../test/DesignPatternsTest.groovy[tags=strategy_groovy_way,indent=0] |
| ---- |
| |
| == Example using lambdas |
| |
| For Groovy 3+, we can leverage lambda syntax: |
| |
| [source,groovy] |
| ---- |
| include::../test/DesignPatternsTest.groovy[tags=strategy_lambdas_with_explicit_interface,indent=0] |
| ---- |
| |
| Or we can use the built-in JDK `BiFunction` class: |
| |
| [source,groovy] |
| ---- |
| include::../test/DesignPatternsTest.groovy[tags=strategy_lambdas_using_bifunction,indent=0] |
| ---- |