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#590 Add explanation to Decorator pattern

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Ilkka Seppälä 2017-09-02 23:10:39 +03:00
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91
decorator/README.md
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@ -19,7 +19,96 @@ Attach additional responsibilities to an object dynamically.
Decorators provide a flexible alternative to subclassing for extending
functionality.
![alt text](./etc/decorator.png "Decorator")
## Explanation
Real world example
> There is an angry troll living in the nearby hills. Usually it goes bare handed but sometimes it has a weapon. To arm the troll it's not necessary to create a new troll but to decorate it dynamically with a suitable weapon.
In plain words
> Decorator pattern lets you dynamically change the behavior of an object at run time by wrapping them in an object of a decorator class.
Wikipedia says
> In object-oriented programming, the decorator pattern is a design pattern that allows behavior to be added to an individual object, either statically or dynamically, without affecting the behavior of other objects from the same class. The decorator pattern is often useful for adhering to the Single Responsibility Principle, as it allows functionality to be divided between classes with unique areas of concern.
**Programmatic Example**
Lets take the troll example. First of all we have a simple troll implementing the troll interface
```
public interface Troll {
void attack();
int getAttackPower();
void fleeBattle();
}
public class SimpleTroll implements Troll {
private static final Logger LOGGER = LoggerFactory.getLogger(SimpleTroll.class);
@Override
public void attack() {
LOGGER.info("The troll tries to grab you!");
}
@Override
public int getAttackPower() {
return 10;
}
@Override
public void fleeBattle() {
LOGGER.info("The troll shrieks in horror and runs away!");
}
}
```
Next we want to add club for the troll. We can do it dynamically by using a decorator
```
public class ClubbedTroll implements Troll {
private static final Logger LOGGER = LoggerFactory.getLogger(ClubbedTroll.class);
private Troll decorated;
public ClubbedTroll(Troll decorated) {
this.decorated = decorated;
}
@Override
public void attack() {
decorated.attack();
LOGGER.info("The troll swings at you with a club!");
}
@Override
public int getAttackPower() {
return decorated.getAttackPower() + 10;
}
@Override
public void fleeBattle() {
decorated.fleeBattle();
}
}
```
Here's the troll in action
```
// simple troll
Troll troll = new SimpleTroll();
troll.attack(); // The troll tries to grab you!
troll.fleeBattle(); // The troll shrieks in horror and runs away!
// change the behavior of the simple troll by adding a decorator
Troll clubbed = new ClubbedTroll(troll);
clubbed.attack(); // The troll tries to grab you! The troll swings at you with a club!
clubbed.fleeBattle(); // The troll shrieks in horror and runs away!
```
## Applicability
Use Decorator

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76
decorator/etc/decorator.ucls
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@ -1,76 +0,0 @@
<?xml version="1.0" encoding="UTF-8"?>
<class-diagram version="1.1.11" icons="true" automaticImage="PNG" always-add-relationships="false"
generalizations="true" realizations="true" associations="true" dependencies="false" nesting-relationships="true"
router="FAN">
<class id="1" language="java" name="com.iluwatar.decorator.App" project="decorator"
file="/decorator/src/main/java/com/iluwatar/decorator/App.java" binary="false" corner="BOTTOM_RIGHT">
<position height="113" width="114" x="177" y="237"/>
<display autosize="true" stereotype="true" package="true" initial-value="false" signature="true"
sort-features="false" accessors="true" visibility="true">
<attributes public="true" package="true" protected="true" private="true" static="true"/>
<operations public="true" package="true" protected="true" private="true" static="true"/>
</display>
</class>
<class id="2" language="java" name="com.iluwatar.decorator.SimpleTroll" project="decorator"
file="/decorator/src/main/java/com/iluwatar/decorator/SimpleTroll.java" binary="false" corner="BOTTOM_RIGHT">
<position height="149" width="125" x="496" y="237"/>
<display autosize="true" stereotype="true" package="true" initial-value="false" signature="true"
sort-features="false" accessors="true" visibility="true">
<attributes public="true" package="true" protected="true" private="true" static="true"/>
<operations public="true" package="true" protected="true" private="true" static="true"/>
</display>
</class>
<class id="3" language="java" name="com.iluwatar.decorator.ClubbedTroll" project="decorator"
file="/decorator/src/main/java/com/iluwatar/decorator/ClubbedTroll.java" binary="false" corner="BOTTOM_RIGHT">
<position height="149" width="125" x="249" y="426"/>
<display autosize="true" stereotype="true" package="true" initial-value="false" signature="true"
sort-features="false" accessors="true" visibility="true">
<attributes public="true" package="true" protected="true" private="true" static="true"/>
<operations public="true" package="true" protected="true" private="true" static="true"/>
</display>
</class>
<class id="4" language="java" name="com.iluwatar.decorator.TrollDecorator" project="decorator"
file="/decorator/src/main/java/com/iluwatar/decorator/TrollDecorator.java" binary="false" corner="BOTTOM_RIGHT">
<position height="149" width="125" x="331" y="237"/>
<display autosize="true" stereotype="true" package="true" initial-value="false" signature="true"
sort-features="false" accessors="true" visibility="true">
<attributes public="true" package="true" protected="true" private="true" static="true"/>
<operations public="true" package="true" protected="true" private="true" static="true"/>
</display>
</class>
<interface id="5" language="java" name="com.iluwatar.decorator.Troll" project="decorator"
file="/decorator/src/main/java/com/iluwatar/decorator/Troll.java" binary="false" corner="BOTTOM_RIGHT">
<position height="113" width="125" x="414" y="426"/>
<display autosize="true" stereotype="true" package="true" initial-value="false" signature="true"
sort-features="false" accessors="true" visibility="true">
<attributes public="true" package="true" protected="true" private="true" static="true"/>
<operations public="true" package="true" protected="true" private="true" static="true"/>
</display>
</interface>
<realization id="6">
<end type="SOURCE" refId="4"/>
<end type="TARGET" refId="5"/>
</realization>
<generalization id="7">
<end type="SOURCE" refId="3"/>
<end type="TARGET" refId="4"/>
</generalization>
<realization id="8">
<end type="SOURCE" refId="2"/>
<end type="TARGET" refId="5"/>
</realization>
<association id="9">
<end type="SOURCE" refId="4" navigable="false">
<attribute id="10" name="decorated"/>
<multiplicity id="11" minimum="0" maximum="1"/>
</end>
<end type="TARGET" refId="5" navigable="true"/>
<display labels="true" multiplicity="true"/>
</association>
<classifier-display autosize="true" stereotype="true" package="true" initial-value="false" signature="true"
sort-features="false" accessors="true" visibility="true">
<attributes public="true" package="true" protected="true" private="true" static="true"/>
<operations public="true" package="true" protected="true" private="true" static="true"/>
</classifier-display>
<association-display labels="true" multiplicity="true"/>
</class-diagram>

38
decorator/etc/decorator.urm.puml
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@ -1,38 +0,0 @@
@startuml
package com.iluwatar.decorator {
class App {
- LOGGER : Logger {static}
+ App()
+ main(args : String[]) {static}
}
class ClubbedTroll {
- LOGGER : Logger {static}
+ ClubbedTroll(decorated : Troll)
+ attack()
+ getAttackPower() : int
}
class SimpleTroll {
- LOGGER : Logger {static}
+ SimpleTroll()
+ attack()
+ fleeBattle()
+ getAttackPower() : int
}
interface Troll {
+ attack() {abstract}
+ fleeBattle() {abstract}
+ getAttackPower() : int {abstract}
}
class TrollDecorator {
- decorated : Troll
+ TrollDecorator(decorated : Troll)
+ attack()
+ fleeBattle()
+ getAttackPower() : int
}
}
TrollDecorator --> "-decorated" Troll
ClubbedTroll --|> TrollDecorator
SimpleTroll ..|> Troll
TrollDecorator ..|> Troll
@enduml

15
decorator/src/main/java/com/iluwatar/decorator/ClubbedTroll.java
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@ -28,22 +28,29 @@ import org.slf4j.LoggerFactory;
/**
* Decorator that adds a club for the troll
*/
public class ClubbedTroll extends TrollDecorator {
public class ClubbedTroll implements Troll {
private static final Logger LOGGER = LoggerFactory.getLogger(ClubbedTroll.class);
private Troll decorated;
public ClubbedTroll(Troll decorated) {
super(decorated);
this.decorated = decorated;
}
@Override
public void attack() {
super.attack();
decorated.attack();
LOGGER.info("The troll swings at you with a club!");
}
@Override
public int getAttackPower() {
return super.getAttackPower() + 10;
return decorated.getAttackPower() + 10;
}
@Override
public void fleeBattle() {
decorated.fleeBattle();
}
}

53
decorator/src/main/java/com/iluwatar/decorator/TrollDecorator.java
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@ -1,53 +0,0 @@
/**
* The MIT License
* Copyright (c) 2014-2016 Ilkka Seppälä
*
* Permission is hereby granted, free of charge, to any person obtaining a copy
* of this software and associated documentation files (the "Software"), to deal
* in the Software without restriction, including without limitation the rights
* to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
* copies of the Software, and to permit persons to whom the Software is
* furnished to do so, subject to the following conditions:
*
* The above copyright notice and this permission notice shall be included in
* all copies or substantial portions of the Software.
*
* THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
* IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
* FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
* AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
* LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
* OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN
* THE SOFTWARE.
*/
package com.iluwatar.decorator;
/**
* TrollDecorator is a decorator for {@link Troll} objects. The calls to the {@link Troll} interface
* are intercepted and decorated. Finally the calls are delegated to the decorated {@link Troll}
* object.
*
*/
public class TrollDecorator implements Troll {
private Troll decorated;
public TrollDecorator(Troll decorated) {
this.decorated = decorated;
}
@Override
public void attack() {
decorated.attack();
}
@Override
public int getAttackPower() {
return decorated.getAttackPower();
}
@Override
public void fleeBattle() {
decorated.fleeBattle();
}
}

2
decorator/src/test/java/com/iluwatar/decorator/ClubbedTrollTest.java
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@ -34,7 +34,7 @@ import static org.mockito.internal.verification.VerificationModeFactory.times;
public class ClubbedTrollTest {
@Test
public void testSmartHostile() throws Exception {
public void testClubbedTroll() throws Exception {
// Create a normal troll first, but make sure we can spy on it later on.
final Troll simpleTroll = spy(new SimpleTroll());

1
pom.xml
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@ -469,6 +469,7 @@
<param>adapter</param>
<param>bridge</param>
<param>composite</param>
<param>decorator</param>
</skipForProjects>
</configuration>
</plugin>